TY - JOUR
AU - Michon, Gilbert
AB - Abstract The thick, >20 km, crust of the Kerguelen Archipelago formed as the tectonic setting of the Kerguelen Plume changed from an oceanic ridge-centered location at 43 Ma to its present location beneath the Antarctic plate. The uppermost crust is dominantly flood basalt with a thickness of up to 10 km. Inverse isochron 40Ar/39Ar ages for upper and lower lavas in a 630 m section of basalt flows from Mont Bureau are 30.4 and 29.0 Ma; Re–Os isotopic systematics are consistent with this age. Most of the lavas in two stratigraphic sections (Mont Bureau and Mont Rabouillere) from the northern part of the archipelago have Sr, Nd and Pb isotopic characteristics similar to the youngest (Upper Miocene to Pleistocene) lavas erupted in the southeast part of the archipelago, i.e. initial 87Sr/86Sr <0.7050, 143Nd/144Nd <0.5127 and 206Pb/204Pb <18.3. The dominance of this isotopic signature in archipelago lavas for 30 my and its presence in ∼40 Ma gabbros is consistent with the previous interpretation that these are isotopic characteristics of the Kerguelen Plume. Although this component occurs in high (>10%) MgO alkalic lavas in the Southeast Province of the archipelago, in these northern sections it is confined to transitional lavas with <6% MgO. A low plume flux and extensive crustal processing are inferred. In contrast to the plume–derived lavas, ∼15% of the flood basalts in these sections have lower initial 87Sr/86Sr (to 0.70396), higher 143Nd/144Nd (to 0.51289), and they hae some compositional characteristics of plagioclase-rich cumulates, i.e. high Sr/Nd and Ba/Th and positive Eu anomalies. However, plagioclase phenocrysts are absent in most of these lavas; therefore a plagioclase-rich component is required in their source. A plausible interpretation is that plagioclase-rich cumulates formed in the lower oceanic crust when the Southeast Indian Ridge was coincident with the plume at ∼43 Ma; subsequently these cumulates were melted by the plume and the melts contributed to a small proportion of the flood basalts. Previously it was proposed that as the distance between the archipelago and Southeast Indian Ridge increased, there was a systematic decrease in the proportion of mid-ocean ridge basalt (MORB)–related component in the source of archipelago lavas. The new data show that: (1) there is no systematic temporal trend in the proportion of MORB to plume source components and (2) the MORB component was derived from cumulate rocks in the oceanic crust rather than as melts derived directly from the asthenosphere. Finally, there is no evidence of a continental lithosphere component in the source of Kerguelen Archipelago lavas. Introduction The 8500 km2 Kerguelen Archipelago is located on the submarine Kerguelen Plateau within the Antarctic plate in the southern Indian Ocean (Fig. 1). The Kerguelen Plateau and its conjugate Broken Ridge are a very large igneous province that is interpreted to represent voluminous Cretaceous volcanism associated with arrival of the Kerguelen Plume below a newly formed Indian Ocean ( Duncan & Storey, 1992). As a result of the rapid northward movement of the Indian plate over the plume, a 5000 km long, ∼82–38 Ma hotspot track formed the Ninetyeast Ridge. Subsequently, the newly formed Southeast Indian Ridge (SEIR) intersected the plume position, and as this spreading center moved northward the plume became detached from the hotspot track and was isolated beneath the Antarctic plate. From ∼39 to 0.1 Ma, the plume constructed the Kerguelen Archipelago on the nearly stationary, relative to hotspots, Antarctic plate. Thus, there is a very long term, ∼115 my, record of volcanism attributed to the Kerguelen Plume, and the Kerguelen Archipelago reflects the last 39 my of this volcanic activity (Weis et al., 1992). The occurrence of plutonic rocks, ranging from gabbro to syenite and granite, in the archipelago is unusual for an oceanic island, and these plutonic rocks have been studied in detail (e.g. Dosso et al., 1979; Giret & Lameyre, 1983; Weis & Giret, 1994). They range widely in age and size, but the oldest known rocks in the archipelago are gabbros, such as the Val Gabbro in the Southeast Province (Fig. 1). The oldest K–Ar age, 39 ± 3 Ma of the Val Gabbro ( Giret & Lameyre, 1983), is slightly less than the youngest magnetic anomaly in the surrounding oceanic basement, that is, anomaly 18, ∼43 Ma ( Munschy et al., 1994). More than 85% of the archipelago is composed of nearly horizontal ‘flood basalt’ lavas with no obvious vents and few dikes (Fig. 1). Because of a regional dip of 2–5° to the southeast and extensive glacial erosion, the surficial lavas decrease in age from northwest to southeast ( Giret et al., 1992). Locally, the dips of the lavas are affected by nearby intrusions. Especially in the Southeast Province, younger volcanic features have been constructed on the flood basalts; for example, a 6.6–10.2 Ma Upper Miocene Series ranging from basanite to phonolite and the very young Ross volcano, <1 Ma, which ranges from trachybasalt to trachyte. These relatively young lavas have been studied in considerable detail ( Weis et al., 1993, 1998). In contrast, there have been only survey studies of the flood basalts which form the major portion of the archipelago (Fig. 1). Watkins et al., (1974) analyzed major and some trace elements for sequences of lava flows on the Courbet peninsula and lavas from the lower 150 m of Mont Bureau on Foch Island (Fig. 1). An important result was their K/Ar dating, which indicated ages of 24.5–27.3 Ma for these flood basalts. White & Hofmann, (1982) and W. M. White (unpublished data, 1985) obtained Sr, Nd and Pb isotopic data for four of the Mont Bureau lavas studied by Watkins et al., (1974). These results show large isotopic variations within 100 m of the section, and three of the lavas have the lowest 87Sr/86Sr and highest 143Nd/144Nd found in archipelago lavas. Gautier et al., (1990) studied flood basalts and younger lavas from different localities in the Kerguelen Archipelago (2–5 samples from each locality) and Weis et al., (1993) studied a group (six) of Lower Miocene flood basalts from the Southeast Province. These workers concluded that the rock types, age and Sr and Nd isotopic ratios of Kerguelen Archipelago lavas are correlated, with the lowest 87Sr/86Sr ratios in ∼26 Ma lavas from Mont Bureau on Foch Island ( White & Hofmann, 1982) and the highest 87Sr/86Sr ratios in the relatively young, highly alkaline lavas. The relatively high 87Sr/86Sr and low 143Nd/144Nd ratios in young alkalic lavas were confirmed by studies of lavas from the Southeast Province and Mont Ross ( Weis et al., 1993, Gautier et al., (1990) postulated an SEIR mid-ocean ridge basalt (MORB)-Kerguelen Plume interaction model to explain the correlation of isotopic ratios with age. Storey et al., (1988) proposed a similar model, but they also suggested that lavas with low 87Sr/86Sr ratios may contain a component derived from the Kerguelen Plateau lithosphere. Because some of the plutonic rocks in the archipelago are similar to those occurring on continents, initial discussions on the origin of the Kerguelen Archipelago focused on a continental vs oceanic origin (e.g. Watkins et al., 1974; Lameyre et al., 1976). However, the Sr, Nd and Pb isotopic ratios of these plutonic complexes are similar to those of the lavas which were derived from the Kerguelen Plume. None of the plutonic rocks have the extreme isotopic ratios typical of old continental crust; consequently, most recent studies have concluded that a continental origin is unlikely (e.g. Dosso et al., 1979; Weis & Giret, 1994). However, recent Os and Pb isotopic data for a few mantle xenoliths in alkalic lavas from the Southeast Province show continental affinities; that is, relatively low 187Os/188Os ( Hassler & Shimizu, 1995) and high 207Pb/204Pb ( Mattielli et al., 1996). Also, lavas dredged from the east end of Broken Ridge (dredge 8) and recovered from drill Site 738 at the southern end of the Kerguelen Plateau (Fig. 1) have Sr, Nd and Pb isotopic ratios and trace element ratios showing the effects of a continental component ( Mahoney et al., 1995). Therefore, a continental component, perhaps minor in volume and sporadic in location, occurs in this igneous province. In this study we focus on the geochemical characteristics, primarily major and trace element compositions and isotopic ratios of Sr, Nd and Pb, of the flood basalts from stratigraphic sections in the northern part of the archipelago, specifically the 630 m section at Mont Bureau on Foch Island and the 400 m section at Mont Rabouillère on the Joffre Peninsula (Fig. 1). Based on inverse isochrons derived from 40Ar/39Ar data, the age range exposed in the Mont Bureau section ranges from 30.5 ± 0.4 to 29.0 ± 0.5 Ma and a lower flow in the Rabouillère section is 29.0 ± 0.2 Ma ( Nicolaysen et al., 1996). Our goal is to understand the recent history of the Kerguelen Plume by addressing the following questions. How did the geochemical characteristics of the magmas vary with time? Over what time scale did these variations occur? Are variations in major and trace element abundances and isotopic ratios correlated? What is the relative importance of different source compositions such as asthenosphere, plateau lithosphere, continental lithosphere and plume, and how did the relative proportions of these components change with time? Answers to these questions will bear on first-order problems such as: What are the geochemical characteristics of the Kerguelen Plume and how have they changed with time? How did the proportions of source components change in plume-related volcanism as the plume location changed from a small newly opened oceanic basin to a ridge-centered location to its present intraplate oceanic location? Fig. 1. Open in new tabDownload slide Map of the Kerguelen Archipelago showing the major geologic units and the location of Mont Bureau on Foch Island and Mont Rabouillère on the Joffre Peninsula. Inset is a map of the eastern Indian Ocean showing the volcanic structures related to the Kerguelen Plume; i.e. the large igneous province formed by the now separated Kerguelen Plateau and Broken Ridge; the hotspot track formed by the Ninetyeast Ridge, and the 39–0.1 Ma Kerguelen Archipelago on the northern Kerguelen Plateau (●, basement sampling sites). Fig. 2. Open in new tabDownload slide Locations of studied samples in the Mont Bureau and Mont Rabouillère stratigraphic sections. Arrows indicate locations of Mont Bureau samples studied by Watkins et al., (1974) and analyzed for isotopic ratios by White & Hofmann, (1982). Analytical Procedures Twenty-eight lava flows and an ankaramite dike from Mont Bureau and 27 lavas from Mont Rabouillère (Fig. 2) were analyzed for major and trace element abundances (Table 1). Abundances of major elements and V, Cr, Ni, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba, La and Ce were determined by X-ray fluorescence (XRF) at the University of Massachusetts (Rhodes, 1983). Abundances of Sc, Co, Hf, Ta, Th and rare earth elements (REE) were determined by instrumental neutron activation analysis (INAA) at MIT ( Ila & Frey, 1984). The accuracy and precision of these analytical techniques were discussed by Frey et al., (1990). Thirty samples, 20 from Mont Bureau and 10 from Mont Rabouillère, were acid leached following the method of Weis & Frey, (1991) before being analyzed for Sr, Nd and Pb isotopic compositions and abundances of U and Pb at the Brussels Free University (ULB). The analytical procedures and precision were described by Weis et al., (1993). Four samples were analyzed for Os isotopic ratios following standard NiS fire-essay preconcentration techniques ( Hoffman et al., 1978; Hauri & Hart, 1993; Hauri et al., 1996) with isotopic characterization by negative thermal ionization mass spectrometry (N-TIMS) ( Creaser et al., 1991; Volkening et al., 1991) at the Woods Hole Oceanographic Institution. Approximately 20 g of each sample was combined 1:1 with a flux mixture having a blank contribution of 4–8 ng Os and 187Os/186Os of 3.836. Rhenium concentrations were determined on separate sample powder splits by isotope dilution inductively coupled plasma mass spectrometry (ICP-MS) at Oregon State University. Rhenium was purified from ∼1 g acid-dissolved samples aliquots by standard ion-exchange techniques which yield a total procedural blank contribution of ∼2 pg Re ( Hauri & Hart, 1993). Table 1: Abundances of major (wt %) and trace elements (ppm) of Mont Bureau and Mont Rabouillère lavas . Mt Bureau . . . . GM92-59 . GM92-58 . GM92-57 . GM92-56 . GM92-55 . GM92-54 . GM92-53 . GM92-52 . GM92-51 . GM92-50 . Height (m) 40 60 75 80 130 130 130 145 150 180 SiO2 4997 51.08 46.21 51.41 45.92 51.31 44.72 48.08 46.40 47.97 TiO2 2.31 3.29 2.45 3.20 1.55 3.17 3.40 2.38 1.60 2.38 Al2O3 15.46 13.26 16.35 13.18 13.89 13.53 13.73 15.76 13.84 15.81 Fe2O3 11.71 15.08 13.62 14.51 12.58 13.99 13.55 13.30 12.62 13.21 MnO 0.16 0.20 0.19 0.24 0.18 0.20 0.19 0.21 0.18 0.20 MgO 5.69 4.47 7.37 4.64 13.35 4.88 9.32 6.18 12.96 6.48 CaO 11.48 8.29 9.67 8.30 9.57 8.46 10.94 9.87 9.59 9.81 Na2O 2.47 2.94 2.99 2.97 1.91 2.96 1.65 3.11 2.10 3.07 K2O 0.55 0.88 0.69 0.83 0.54 1.05 1.85 0.66 0.54 0.64 P2O5 0.26 0.39 0.37 0.39 0.18 0.38 0.60 0.29 0.19 0.28 Total 100.07 99.87 99.89 99.66 99.66 99.93 99.94 99.82 100.00 99.84 Rb 3.8 8.0 17.3 8.0 11.4 16.7 35.2 11.2 10.5 10.5 Sr 339 339 518 343 274 331 557 374 271 375 Ba 164 251 160 255 158 230 683 161 141 156 Sc 27.8±0.1 28.1 25.1 27.3±0.2 26.5 28.6±0.07 27.9 29.4±0.1 26.1±0.3 29.5 V 226 284 199 277 165 299 250 272 176 278 Cr 155±1 14 80 11±1 693 21±1 217 90±1 655±11 92 Co 40.7±0.3 42.4 52.2 44.5±0.1 66.6 40.3±0.3 52.6 46.5±0.1 64.0±0.3 45.9 Ni 82 40 86 28 380 37 120 74 376 80 Zn 107 136 87 132 86 136 118 97 97 92 Ga 20.7 24.3 18.0 24.0 16.0 22.6 20.3 21.8 17.2 21.4 Y 23.0 34.4 26.8 33.0 16.1 31.6 23.1 27.7 16.8 27.4 Zr 156 240 196 235 94 241 230 184 107 184 Nb 16.6 25.1 13.4 25.6 9.3 24.1 49.9 13.5 10.3 13.7 Hf 3.62±0.04 5.82 4.12 5.56±0.07 2.23 5.57±0.08 5.07 4.10±0.02 2.32±0.01 3.98 Ta 0.92±0.01 1.55 0.81 1.57±0.06 0.66 1.45±0.02 2.86 0.88±0.02 0.58±0.07 0.74 Th 1.70±0.04 2.94 0.87 3.12±0.01 0.57 2.63±0.15 4.00 1.39±0.19 0.89±0.09 1.21 U 0.12 0.37 0.15 0.41 0.45 0.13 0.31 0.34 0.17 0.26 Pb 0.70 1.60 0.84 1.75 1.90 0.37 0.95 1.13 0.40 1.02 La 15.1±0.2 25.9 15.1 24.6±0.6 9.2 22.9±0.2 37.4 15.7±0.3 9.49±0.16 15.2 Ce 35.0±1.9 58.6 35.9 57.6±1.1 20.8 51.6±1.0 70.9 37.1±1.6 20.8±1.7 33.1 Nd 20.1±1.1 35.4 21.4 32.9±3.4 12.5 31.3±3.8 33.1 23.0±1.6 13.5±0.64 19.7 Sm 5.03±0.07 7.65 5.34 7.16±0.28 3.08 7.17±0.13 7.65 5.30±0.12 3.22±0.16 4.97 Eu 1.73±0.05 2.59 1.96 2.43±0.01 1.13 2.38±0.03 2.4 1.86±0.03 1.21±0.05 1.78 Tb 0.81±0.04 1.22 0.87 1.11±0.03 0.51 1.14±0.04 0.84 0.94±0.06 0.58±0.03 0.75 Yb 1.76±0.08 2.92 2.38 2.63±0.31 1.33 2.62±0.01 1.94 2.42±0.07 1.43±0.01 2.28 Lu 0.26±0.01 0.40 0.33 0.39±0.01 0.20 0.39±0.01 0.24 0.36±0.01 0.22±0.01 0.33 . Mt Bureau . . . . GM92-59 . GM92-58 . GM92-57 . GM92-56 . GM92-55 . GM92-54 . GM92-53 . GM92-52 . GM92-51 . GM92-50 . Height (m) 40 60 75 80 130 130 130 145 150 180 SiO2 4997 51.08 46.21 51.41 45.92 51.31 44.72 48.08 46.40 47.97 TiO2 2.31 3.29 2.45 3.20 1.55 3.17 3.40 2.38 1.60 2.38 Al2O3 15.46 13.26 16.35 13.18 13.89 13.53 13.73 15.76 13.84 15.81 Fe2O3 11.71 15.08 13.62 14.51 12.58 13.99 13.55 13.30 12.62 13.21 MnO 0.16 0.20 0.19 0.24 0.18 0.20 0.19 0.21 0.18 0.20 MgO 5.69 4.47 7.37 4.64 13.35 4.88 9.32 6.18 12.96 6.48 CaO 11.48 8.29 9.67 8.30 9.57 8.46 10.94 9.87 9.59 9.81 Na2O 2.47 2.94 2.99 2.97 1.91 2.96 1.65 3.11 2.10 3.07 K2O 0.55 0.88 0.69 0.83 0.54 1.05 1.85 0.66 0.54 0.64 P2O5 0.26 0.39 0.37 0.39 0.18 0.38 0.60 0.29 0.19 0.28 Total 100.07 99.87 99.89 99.66 99.66 99.93 99.94 99.82 100.00 99.84 Rb 3.8 8.0 17.3 8.0 11.4 16.7 35.2 11.2 10.5 10.5 Sr 339 339 518 343 274 331 557 374 271 375 Ba 164 251 160 255 158 230 683 161 141 156 Sc 27.8±0.1 28.1 25.1 27.3±0.2 26.5 28.6±0.07 27.9 29.4±0.1 26.1±0.3 29.5 V 226 284 199 277 165 299 250 272 176 278 Cr 155±1 14 80 11±1 693 21±1 217 90±1 655±11 92 Co 40.7±0.3 42.4 52.2 44.5±0.1 66.6 40.3±0.3 52.6 46.5±0.1 64.0±0.3 45.9 Ni 82 40 86 28 380 37 120 74 376 80 Zn 107 136 87 132 86 136 118 97 97 92 Ga 20.7 24.3 18.0 24.0 16.0 22.6 20.3 21.8 17.2 21.4 Y 23.0 34.4 26.8 33.0 16.1 31.6 23.1 27.7 16.8 27.4 Zr 156 240 196 235 94 241 230 184 107 184 Nb 16.6 25.1 13.4 25.6 9.3 24.1 49.9 13.5 10.3 13.7 Hf 3.62±0.04 5.82 4.12 5.56±0.07 2.23 5.57±0.08 5.07 4.10±0.02 2.32±0.01 3.98 Ta 0.92±0.01 1.55 0.81 1.57±0.06 0.66 1.45±0.02 2.86 0.88±0.02 0.58±0.07 0.74 Th 1.70±0.04 2.94 0.87 3.12±0.01 0.57 2.63±0.15 4.00 1.39±0.19 0.89±0.09 1.21 U 0.12 0.37 0.15 0.41 0.45 0.13 0.31 0.34 0.17 0.26 Pb 0.70 1.60 0.84 1.75 1.90 0.37 0.95 1.13 0.40 1.02 La 15.1±0.2 25.9 15.1 24.6±0.6 9.2 22.9±0.2 37.4 15.7±0.3 9.49±0.16 15.2 Ce 35.0±1.9 58.6 35.9 57.6±1.1 20.8 51.6±1.0 70.9 37.1±1.6 20.8±1.7 33.1 Nd 20.1±1.1 35.4 21.4 32.9±3.4 12.5 31.3±3.8 33.1 23.0±1.6 13.5±0.64 19.7 Sm 5.03±0.07 7.65 5.34 7.16±0.28 3.08 7.17±0.13 7.65 5.30±0.12 3.22±0.16 4.97 Eu 1.73±0.05 2.59 1.96 2.43±0.01 1.13 2.38±0.03 2.4 1.86±0.03 1.21±0.05 1.78 Tb 0.81±0.04 1.22 0.87 1.11±0.03 0.51 1.14±0.04 0.84 0.94±0.06 0.58±0.03 0.75 Yb 1.76±0.08 2.92 2.38 2.63±0.31 1.33 2.62±0.01 1.94 2.42±0.07 1.43±0.01 2.28 Lu 0.26±0.01 0.40 0.33 0.39±0.01 0.20 0.39±0.01 0.24 0.36±0.01 0.22±0.01 0.33 . Mt Bureau . . . . GM92-48 . GM92-46 . GM92-45 . GM92-44 . GM92-43 . GM92-42 . GM92-41 . GM92-40 . GM92-39 . GM92-38 . GM92-37 . Height (m) 250 280 300 320 330 340 350 360 400 410 440 SiO2 47.09 46.36 51.12 51.58 49.10 53.12 50.05 52.46 50.91 49.93 50.59 TiO2 1.44 1.87 3.78 3.81 3.49 3.24 3.37 3.37 3.61 3.66 3.63 Al2O3 16.49 14.91 12.89 13.59 14.28 13.33 13.70 13.14 13.39 13.63 13.31 Fe2O3 11.98 14.17 14.54 13.31 14.10 13.82 14.04 14.49 14.13 14.53 14.56 MnO 0.18 0.21 0.22 0.19 0.19 0.21 0.20 0.24 0.19 0.19 0.22 MgO 8.71 10.18 4.26 4.27 5.12 3.53 4.99 3.37 4.67 4.85 4.70 CaO 10.73 9.43 8.09 8.51 9.91 6.82 9.48 7.13 8.63 8.75 8.44 Na2O 2.50 2.24 2.93 3.10 2.72 3.20 2.88 3.13 2.90 2.97 2.96 K2O 0.43 0.17 1.32 1.04 0.63 1.85 0.77 1.79 1.23 1.02 1.06 P2O5 0.20 0.14 0.53 0.53 0.44 0.60 0.43 0.57 0.44 0.44 0.46 Total 99.74 99.66 99.67 99.92 99.99 99.70 99.91 99.70 100.10 99.98 99.93 Rb 5.8 1.4 22.8 27.1 6.4 46.0 15.2 49.2 25.6 12.3 15.8 Sr 252 226 360 371 382 321 372 322 335 347 355 Ba 98 53 305 301 246 361 257 342 274 280 285 Sc 29.5±0.1 29.3±0.1 26.6 26.3 28.8 24.0 28.2 24.8 27.1±0.4 27.8 27.2±0.2 V 174 211 320 296 296 244 283 263 281 287 299 Cr 271±1 290±3 42 73 81 2 82 6 48±1 48 30±1 Co 51.4±0.3 61.0±1.1 38.3 35.3 42.2 46.7 42.5 34.9 41.3±0.6 43.0 39.2±0.1 Ni 170 241 44 56 55 26 66 33 34 37 38 Zn 75 103 144 131 136 153 133 159 122 125 Ga 17.0 18.5 24.0 24.6 22.9 24.6 24.2 25.4 24.0 23.6 24.3 Y 20.7 21.5 39.2 38.2 33.0 41.8 32.6 44.0 34.3 33.5 34.8 Zr 117 93 330 320 263 461 257 413 259 264 277 Nb 8.3 7.9 34.4 34.0 31.6 39.8 30.5 38.5 29.9 30.7 30.3 Hf 2.52±0.08 2.22±0.01 7.52 7.40 6.32 10.1 5.98 9.39 6.17±0.15 6.25 6.20±0.11 Ta 0.57±0.06 0.42±0.05 1.95 2.16 1.76 2.29 1.81 2.01 1.77±0.02 1.87 1.82±0.08 Th 0.65±0.23 0.19±0.13 3.29 3.45 3.28 5.60 2.91 4.80 3.50±0.25 3.29 3.26±0.08 U 0.08 0.03 0.56 0.14 0.91 Pb 0.27 3.80 2.40 0.87 2.60 La 8.80±0.07 5.85±0.02 33.2 32.4 27.6 40.4 28.1 38.2 27.8±0.4 28.1 28.7±0.1 Ce 21.2±1.1 14.8±1.9 78.5 75.2 64.5 96.6 64.0 90.7 64.3±0.1 66.8 65.8±1.6 Nd 12.9±0.92 10.8±0.01 40.0 40.1 36.1 49.3 36.5 46.2 34.0±3.0 36.3 37.9±3.8 Sm 3.33±0.14 3.19±0.25 8.99 9.78 7.86 10.6 7.61 10.1 7.60±0.21 7.67 8.24±0.11 Eu 1.24±0.02 1.26±0.04 2.94 2.90 2.53 3.24 2.49 3.18 2.54±0.03 2.60 2.66±0.036 Tb 0.63±0.01 0.68±0.01 1.50 1.46 1.21 1.61 1.19 1.50 1.14±0.03 1.21 1.29±0.04 Yb 1.88±0.16 1.92±0.01 3.19 3.22 2.68 3.39 2.55 3.56 2.85±0.23 2.63 2.89±0.07 Lu 0.31±0.02 0.32±0.01 0.47 0.46 0.39 0.48 0.36 0.51 0.39±0.01 0.38 0.43±0.01 . Mt Bureau . . . . GM92-48 . GM92-46 . GM92-45 . GM92-44 . GM92-43 . GM92-42 . GM92-41 . GM92-40 . GM92-39 . GM92-38 . GM92-37 . Height (m) 250 280 300 320 330 340 350 360 400 410 440 SiO2 47.09 46.36 51.12 51.58 49.10 53.12 50.05 52.46 50.91 49.93 50.59 TiO2 1.44 1.87 3.78 3.81 3.49 3.24 3.37 3.37 3.61 3.66 3.63 Al2O3 16.49 14.91 12.89 13.59 14.28 13.33 13.70 13.14 13.39 13.63 13.31 Fe2O3 11.98 14.17 14.54 13.31 14.10 13.82 14.04 14.49 14.13 14.53 14.56 MnO 0.18 0.21 0.22 0.19 0.19 0.21 0.20 0.24 0.19 0.19 0.22 MgO 8.71 10.18 4.26 4.27 5.12 3.53 4.99 3.37 4.67 4.85 4.70 CaO 10.73 9.43 8.09 8.51 9.91 6.82 9.48 7.13 8.63 8.75 8.44 Na2O 2.50 2.24 2.93 3.10 2.72 3.20 2.88 3.13 2.90 2.97 2.96 K2O 0.43 0.17 1.32 1.04 0.63 1.85 0.77 1.79 1.23 1.02 1.06 P2O5 0.20 0.14 0.53 0.53 0.44 0.60 0.43 0.57 0.44 0.44 0.46 Total 99.74 99.66 99.67 99.92 99.99 99.70 99.91 99.70 100.10 99.98 99.93 Rb 5.8 1.4 22.8 27.1 6.4 46.0 15.2 49.2 25.6 12.3 15.8 Sr 252 226 360 371 382 321 372 322 335 347 355 Ba 98 53 305 301 246 361 257 342 274 280 285 Sc 29.5±0.1 29.3±0.1 26.6 26.3 28.8 24.0 28.2 24.8 27.1±0.4 27.8 27.2±0.2 V 174 211 320 296 296 244 283 263 281 287 299 Cr 271±1 290±3 42 73 81 2 82 6 48±1 48 30±1 Co 51.4±0.3 61.0±1.1 38.3 35.3 42.2 46.7 42.5 34.9 41.3±0.6 43.0 39.2±0.1 Ni 170 241 44 56 55 26 66 33 34 37 38 Zn 75 103 144 131 136 153 133 159 122 125 Ga 17.0 18.5 24.0 24.6 22.9 24.6 24.2 25.4 24.0 23.6 24.3 Y 20.7 21.5 39.2 38.2 33.0 41.8 32.6 44.0 34.3 33.5 34.8 Zr 117 93 330 320 263 461 257 413 259 264 277 Nb 8.3 7.9 34.4 34.0 31.6 39.8 30.5 38.5 29.9 30.7 30.3 Hf 2.52±0.08 2.22±0.01 7.52 7.40 6.32 10.1 5.98 9.39 6.17±0.15 6.25 6.20±0.11 Ta 0.57±0.06 0.42±0.05 1.95 2.16 1.76 2.29 1.81 2.01 1.77±0.02 1.87 1.82±0.08 Th 0.65±0.23 0.19±0.13 3.29 3.45 3.28 5.60 2.91 4.80 3.50±0.25 3.29 3.26±0.08 U 0.08 0.03 0.56 0.14 0.91 Pb 0.27 3.80 2.40 0.87 2.60 La 8.80±0.07 5.85±0.02 33.2 32.4 27.6 40.4 28.1 38.2 27.8±0.4 28.1 28.7±0.1 Ce 21.2±1.1 14.8±1.9 78.5 75.2 64.5 96.6 64.0 90.7 64.3±0.1 66.8 65.8±1.6 Nd 12.9±0.92 10.8±0.01 40.0 40.1 36.1 49.3 36.5 46.2 34.0±3.0 36.3 37.9±3.8 Sm 3.33±0.14 3.19±0.25 8.99 9.78 7.86 10.6 7.61 10.1 7.60±0.21 7.67 8.24±0.11 Eu 1.24±0.02 1.26±0.04 2.94 2.90 2.53 3.24 2.49 3.18 2.54±0.03 2.60 2.66±0.036 Tb 0.63±0.01 0.68±0.01 1.50 1.46 1.21 1.61 1.19 1.50 1.14±0.03 1.21 1.29±0.04 Yb 1.88±0.16 1.92±0.01 3.19 3.22 2.68 3.39 2.55 3.56 2.85±0.23 2.63 2.89±0.07 Lu 0.31±0.02 0.32±0.01 0.47 0.46 0.39 0.48 0.36 0.51 0.39±0.01 0.38 0.43±0.01 . Mt Bureau . Mt Rabouillè re . . . . . GM92-36 . GM92-35 . GM92-34 . GM92-33 . GM92-32 . GM92-31 . GM92-30 . GM92-29 . GM92-153 . GM92-152 . GM92-151 . Height (m) 460 460 480 510 530 580 610 620 30 40 50 SiO2 51.66 51.67 51.95 51.05 50.07 50.22 50.92 51.18 50.84 51.52 51.71 TiO2 3.50 3.52 3.34 3.44 3.67 3.62 3.31 3.91 3.85 3.78 3.78 Al2O3 13.09 13.17 13.01 13.53 13.00 13.15 13.46 12.87 13.03 12.85 12.84 Fe2O3 14.36 14.36 14.58 13.84 15.25 15.09 13.60 14.40 14.24 14.50 14.53 MnO 0.22 0.21 0.22 0.17 0.20 0.22 0.21 0.19 0.21 0.21 0.21 MgO 4.08 3.96 3.65 4.95 4.64 4.40 4.84 4.43 4.49 3.95 3.83 CaO 7.47 7.41 7.57 8.74 8.26 8.47 9.24 8.12 8.64 7.76 7.77 Na2O 3.14 3.09 3.15 2.64 2.92 2.89 2.74 2.77 3.13 3.01 3.09 K2O 1.48 1.51 1.40 1.11 1.23 1.25 1.07 1.45 0.82 1.72 1.68 P2O5 0.69 0.69 0.72 0.39 0.49 0.49 0.40 0.53 0.51 0.54 0.54 Total 99.68 99.59 99.58 99.84 99.71 99.78 99.78 99.84 99.78 99.83 99.97 Rb 25.7 36.8 36.2 19.7 25.5 27.8 23.5 28.9 18.3 41.8 41.1 Sr 347 347 335 351 333 340 373 345 335 334 345 Ba 310 303 325 222 267 269 279 315 281 320 308 Sc 25.8 26.0 25.1 30.5 27.7±0.3 27.9 28.1 28.0 27.4 26.5 V 257 255 214 285 311 304 293 322 303 320 319 Cr 10 10 5 56 33±1 41 36 68 43 41 27 Co 35.4 34.7 34.4 43.3 42.7±0.3 44.4 43.3 36.8 42.0 36.9 Ni 33 26 22 55 36 38 47 52 53 34 41 Zn 148 147 154 131 147 148 124 141 149 150 159 Ga 22.9 24.2 25.7 23.3 23.3 22.4 23.2 25.1 23.2 23.1 25.1 Y 40.9 39.5 45.1 28.8 37.0 37.9 33.0 37.8 36.4 37.9 39.1 Zr 312 313 353 242 284 284 257 317 317 330 332 Nb 34.1 34.9 37.5 23.5 30.9 31.2 25.5 32.3 32.0 35.5 35.8 Hf 7.22 7.13 8.05 5.78 6.64±0.12 6.61 6.05 7.30 7.14 7.43 Ta 2.01 2.12 2.19 1.54 1.83±0.03 1.83 1.45 1.93 1.79 2.09 Th 3.99 4.13 4.39 1.99 3.09±0.02 2.70 2.67 3.10 3.32 3.85 U 0.83 0.36 0.55 0.61 0.50 Pb 3.10 1.80 2.30 2.2 2.60 2.10 La 34.1 33.5 37.8 21.0 28.0±1.1 28.6 26.2 31.3 29.7 33.7 28.3 Ce 83.3 79.5 87.2 53.3 69.2±0.5 65.7 62.0 76.4 72.5 75.3 68.5 Nd 43.7 44.8 47.5 32.1 38.5±3.1 36.7 36.0 43.3 37.7 40.6 Sm 9.77 9.78 10.70 7.01 8.51±0.01 8.72 7.89 8.92 8.81 9.28 Eu 3.15 3.10 3.23 2.39 2.75±0.03 2.73 2.43 2.83 2.79 2.96 Tb 1.25 1.44 1.59 1.03 1.22±0.08 1.29 1.25 1.34 1.33 1.40 Yb 3.14 3.20 3.80 2.27 2.88±0.03 3.09 2.69 2.96 3.02 3.08 Lu 0.42 0.45 0.53 0.32 0.43±0.01 0.42 0.39 0.41 0.44 0.45 . Mt Bureau . Mt Rabouillè re . . . . . GM92-36 . GM92-35 . GM92-34 . GM92-33 . GM92-32 . GM92-31 . GM92-30 . GM92-29 . GM92-153 . GM92-152 . GM92-151 . Height (m) 460 460 480 510 530 580 610 620 30 40 50 SiO2 51.66 51.67 51.95 51.05 50.07 50.22 50.92 51.18 50.84 51.52 51.71 TiO2 3.50 3.52 3.34 3.44 3.67 3.62 3.31 3.91 3.85 3.78 3.78 Al2O3 13.09 13.17 13.01 13.53 13.00 13.15 13.46 12.87 13.03 12.85 12.84 Fe2O3 14.36 14.36 14.58 13.84 15.25 15.09 13.60 14.40 14.24 14.50 14.53 MnO 0.22 0.21 0.22 0.17 0.20 0.22 0.21 0.19 0.21 0.21 0.21 MgO 4.08 3.96 3.65 4.95 4.64 4.40 4.84 4.43 4.49 3.95 3.83 CaO 7.47 7.41 7.57 8.74 8.26 8.47 9.24 8.12 8.64 7.76 7.77 Na2O 3.14 3.09 3.15 2.64 2.92 2.89 2.74 2.77 3.13 3.01 3.09 K2O 1.48 1.51 1.40 1.11 1.23 1.25 1.07 1.45 0.82 1.72 1.68 P2O5 0.69 0.69 0.72 0.39 0.49 0.49 0.40 0.53 0.51 0.54 0.54 Total 99.68 99.59 99.58 99.84 99.71 99.78 99.78 99.84 99.78 99.83 99.97 Rb 25.7 36.8 36.2 19.7 25.5 27.8 23.5 28.9 18.3 41.8 41.1 Sr 347 347 335 351 333 340 373 345 335 334 345 Ba 310 303 325 222 267 269 279 315 281 320 308 Sc 25.8 26.0 25.1 30.5 27.7±0.3 27.9 28.1 28.0 27.4 26.5 V 257 255 214 285 311 304 293 322 303 320 319 Cr 10 10 5 56 33±1 41 36 68 43 41 27 Co 35.4 34.7 34.4 43.3 42.7±0.3 44.4 43.3 36.8 42.0 36.9 Ni 33 26 22 55 36 38 47 52 53 34 41 Zn 148 147 154 131 147 148 124 141 149 150 159 Ga 22.9 24.2 25.7 23.3 23.3 22.4 23.2 25.1 23.2 23.1 25.1 Y 40.9 39.5 45.1 28.8 37.0 37.9 33.0 37.8 36.4 37.9 39.1 Zr 312 313 353 242 284 284 257 317 317 330 332 Nb 34.1 34.9 37.5 23.5 30.9 31.2 25.5 32.3 32.0 35.5 35.8 Hf 7.22 7.13 8.05 5.78 6.64±0.12 6.61 6.05 7.30 7.14 7.43 Ta 2.01 2.12 2.19 1.54 1.83±0.03 1.83 1.45 1.93 1.79 2.09 Th 3.99 4.13 4.39 1.99 3.09±0.02 2.70 2.67 3.10 3.32 3.85 U 0.83 0.36 0.55 0.61 0.50 Pb 3.10 1.80 2.30 2.2 2.60 2.10 La 34.1 33.5 37.8 21.0 28.0±1.1 28.6 26.2 31.3 29.7 33.7 28.3 Ce 83.3 79.5 87.2 53.3 69.2±0.5 65.7 62.0 76.4 72.5 75.3 68.5 Nd 43.7 44.8 47.5 32.1 38.5±3.1 36.7 36.0 43.3 37.7 40.6 Sm 9.77 9.78 10.70 7.01 8.51±0.01 8.72 7.89 8.92 8.81 9.28 Eu 3.15 3.10 3.23 2.39 2.75±0.03 2.73 2.43 2.83 2.79 2.96 Tb 1.25 1.44 1.59 1.03 1.22±0.08 1.29 1.25 1.34 1.33 1.40 Yb 3.14 3.20 3.80 2.27 2.88±0.03 3.09 2.69 2.96 3.02 3.08 Lu 0.42 0.45 0.53 0.32 0.43±0.01 0.42 0.39 0.41 0.44 0.45 . Mt Rabouillère . . . . GM92-150 . GM92-149 . GM92-148 . GM92-147 . GM92-146 . GM92-145 . GM92-144 . GM92-143 . GM92-142 . GM92-141 . GM92-140 . GM92-139 . Height (m) 60 70 90 110 120 130 160 170 185 200 210 240 SiO2 50.84 50.77 51.07 51.06 48.87 48.00 49.65 51.17 51.10 50.08 49.88 58.62 TiO2 3.82 3.96 3.58 3.63 1.97 1.75 3.19 3.58 3.55 2.68 3.68 1.82 Fe2O3 13.86 13.67 13.47 15.02 12.10 12.38 14.27 14.28 14.98 13.38 15.90 10.67 MnO 0.20 0.18 0.22 0.22 0.18 0.18 0.21 0.20 0.22 0.17 0.21 0.20 MgO 4.37 4.43 4.61 4.00 5.64 7.08 5.85 4.24 4.09 5.63 4.36 2.49 CaO 8.40 8.99 8.48 7.98 11.20 11.07 9.26 8.14 8.07 9.42 8.33 5.03 Na2O 3.10 3.00 3.00 2.97 2.91 2.81 2.98 3.15 3.27 3.07 2.98 3.88 K2O 1.18 0.90 1.44 1.49 0.34 0.44 0.80 1.37 1.15 0.73 0.94 2.50 P2O5 0.54 0.56 0.50 0.49 0.20 0.18 0.39 0.47 0.62 0.52 0.46 0.68 Total 99.93 99.69 99.69 99.6 99.84 100.08 100.08 99.78 100.13 100.01 99.99 99.96 Rb 15.2 23.8 34.5 36.6 3.9 12.5 10.2 36.6 20.2 6.7 22.1 54.9 Sr 376 359 355 353 276 275 334 346 362 334 345 293 Ba 319 299 282 302 125 121 249 281 318 264 312 516 Sc 25.9 26.8 27.1 27.5 30.5 29.6 28.2 25.5 15.0 V 301 302 309 324 253 228 298 316 271 199 302 59 Cr 56 75 81 11 107 261 176 34 17 144 26 36 Co 35.4 41.3 38.0 40.9 40.9 47.5 45.8 39.8 17.9 Ni 45 69 64 24 70 97 79 35 36 64 21 22 Zn 147 152 151 147 93 95 133 149 148 129 146 145 Ga 24.9 24.2 23.7 23.1 20.9 20.0 22.0 23.6 24.2 22.0 23.3 25.9 Y 42.2 40.6 37.4 39.3 22.6 20.9 31.1 36.1 39.5 34.7 38.9 58.0 Zr 327 340 307 309 125 111 235 288 308 249 298 579 Nb 35.4 36.7 34.4 32.8 11.4 10.7 28.0 32.3 32.9 29.1 31.9 51.5 Hf 7.58 7.41 6.89 6.87 2.88 2.55 5.26 5.51 12.6 Ta 1.94 2.11 2.01 1.88 0.66 0.60 1.77 1.67 2.92 Th 3.61 3.72 3.53 3.19 1.43 0.84 2.82 2.97 6.90 U 0.50 0.10 0.18 0.40 Pb 2.20 0.36 0.67 1.80 La 33.9 33.7 32.0 31.5 10.7 9.58 24.8 28.4 30.4 28.2 32.7 56.1 Ce 79.3 75.9 70.2 74.2 25.1 22.2 58.2 62.8 70.8 66.9 62.6 130 Nd 44.2 40.7 38.0 42.3 14.4 13.0 30.9 35.9 66.4 Sm 9.54 9.38 8.72 9.41 3.90 3.60 7.14 8.32 14.6 Eu 2.99 2.94 2.81 2.80 1.45 1.34 2.26 2.57 3.69 Tb 1.39 1.38 1.35 1.40 0.67 0.62 1.12 1.22 2.04 Yb 3.20 3.18 3.11 3.10 1.86 1.78 2.58 2.64 5.09 Lu 0.46 0.45 0.44 0.43 0.28 0.27 0.36 0.39 0.71 . Mt Rabouillère . . . . GM92-150 . GM92-149 . GM92-148 . GM92-147 . GM92-146 . GM92-145 . GM92-144 . GM92-143 . GM92-142 . GM92-141 . GM92-140 . GM92-139 . Height (m) 60 70 90 110 120 130 160 170 185 200 210 240 SiO2 50.84 50.77 51.07 51.06 48.87 48.00 49.65 51.17 51.10 50.08 49.88 58.62 TiO2 3.82 3.96 3.58 3.63 1.97 1.75 3.19 3.58 3.55 2.68 3.68 1.82 Fe2O3 13.86 13.67 13.47 15.02 12.10 12.38 14.27 14.28 14.98 13.38 15.90 10.67 MnO 0.20 0.18 0.22 0.22 0.18 0.18 0.21 0.20 0.22 0.17 0.21 0.20 MgO 4.37 4.43 4.61 4.00 5.64 7.08 5.85 4.24 4.09 5.63 4.36 2.49 CaO 8.40 8.99 8.48 7.98 11.20 11.07 9.26 8.14 8.07 9.42 8.33 5.03 Na2O 3.10 3.00 3.00 2.97 2.91 2.81 2.98 3.15 3.27 3.07 2.98 3.88 K2O 1.18 0.90 1.44 1.49 0.34 0.44 0.80 1.37 1.15 0.73 0.94 2.50 P2O5 0.54 0.56 0.50 0.49 0.20 0.18 0.39 0.47 0.62 0.52 0.46 0.68 Total 99.93 99.69 99.69 99.6 99.84 100.08 100.08 99.78 100.13 100.01 99.99 99.96 Rb 15.2 23.8 34.5 36.6 3.9 12.5 10.2 36.6 20.2 6.7 22.1 54.9 Sr 376 359 355 353 276 275 334 346 362 334 345 293 Ba 319 299 282 302 125 121 249 281 318 264 312 516 Sc 25.9 26.8 27.1 27.5 30.5 29.6 28.2 25.5 15.0 V 301 302 309 324 253 228 298 316 271 199 302 59 Cr 56 75 81 11 107 261 176 34 17 144 26 36 Co 35.4 41.3 38.0 40.9 40.9 47.5 45.8 39.8 17.9 Ni 45 69 64 24 70 97 79 35 36 64 21 22 Zn 147 152 151 147 93 95 133 149 148 129 146 145 Ga 24.9 24.2 23.7 23.1 20.9 20.0 22.0 23.6 24.2 22.0 23.3 25.9 Y 42.2 40.6 37.4 39.3 22.6 20.9 31.1 36.1 39.5 34.7 38.9 58.0 Zr 327 340 307 309 125 111 235 288 308 249 298 579 Nb 35.4 36.7 34.4 32.8 11.4 10.7 28.0 32.3 32.9 29.1 31.9 51.5 Hf 7.58 7.41 6.89 6.87 2.88 2.55 5.26 5.51 12.6 Ta 1.94 2.11 2.01 1.88 0.66 0.60 1.77 1.67 2.92 Th 3.61 3.72 3.53 3.19 1.43 0.84 2.82 2.97 6.90 U 0.50 0.10 0.18 0.40 Pb 2.20 0.36 0.67 1.80 La 33.9 33.7 32.0 31.5 10.7 9.58 24.8 28.4 30.4 28.2 32.7 56.1 Ce 79.3 75.9 70.2 74.2 25.1 22.2 58.2 62.8 70.8 66.9 62.6 130 Nd 44.2 40.7 38.0 42.3 14.4 13.0 30.9 35.9 66.4 Sm 9.54 9.38 8.72 9.41 3.90 3.60 7.14 8.32 14.6 Eu 2.99 2.94 2.81 2.80 1.45 1.34 2.26 2.57 3.69 Tb 1.39 1.38 1.35 1.40 0.67 0.62 1.12 1.22 2.04 Yb 3.20 3.18 3.11 3.10 1.86 1.78 2.58 2.64 5.09 Lu 0.46 0.45 0.44 0.43 0.28 0.27 0.36 0.39 0.71 . Mt Rabouillè re . . . GM92-138 . GM92-137 . GM92-136 . GM92-135 . GM92-134 . GM92-133 . GM92-132 . GM92-131 . GM92-130 . GM92-129 . GM92-128 . GM92-127 . . Height (m) 260 270 280 300 310 350 355 360 370 380 390 400 SiO2 58.98 52.14 51.75 51.97 51.74 50.03 50.61 50.66 49.72 47.79 50.60 50.61 TiO2 1.81 3.57 3.32 3.36 3.39 3.68 3.61 3.89 3.27 1.60 3.84 3.89 Al2O3 13.87 13.26 13.18 13.33 13.00 13.21 13.04 13.25 13.92 16.11 13.00 12.69 Fe2O3 10.25 14.47 14.25 14.16 14.70 15.18 14.82 14.58 14.30 12.46 14.81 15.20 MnO 0.17 0.19 0.21 0.21 0.21 0.19 0.22 0.20 0.20 0.17 0.22 0.21 MgO 2.67 3.46 4.27 3.97 4.11 4.45 4.38 4.25 5.19 8.37 4.34 4.29 CaO 4.64 7.44 7.82 7.93 8.15 8.45 8.31 8.63 9.47 10.56 8.36 8.05 Na2O 3.83 3.31 2.94 3.10 3.25 2.96 2.97 3.14 2.75 2.53 2.94 2.90 K2O 2.63 1.76 1.51 1.43 0.76 1.26 1.38 0.94 0.71 0.25 1.08 1.42 P2O5 0.69 0.56 0.43 0.44 0.42 0.50 0.47 0.51 0.40 0.14 0.50 0.51 Total 99.53 100.17 99.67 99.89 99.73 99.91 99.82 100.04 99.92 99.98 99.67 99.78 Rb 63.6 54.7 39.0 35.5 26.1 23.7 33.3 26.8 8.8 2.9 16.3 32.2 Sr 282 335 338 346 359 339 323 365 379 214 371 338 Ba 501 311 275 265 262 277 281 256 259 78 332 313 Sc 14.8 26.2 26.4 26.7 27.6 28.4 29.3 27 V 60 295 301 324 302 310 323 308 273 208 329 345 Cr 29 7 17 18 — 26 17 6 23 250 0 15 Co 17.2 33.5 39.4 39.6 42.6 44.7 43.2 49.5 37.6 Ni 29 13 23 24 26 29 35 35 44 98 24 40 Zn 151 155 140 149 149 150 156 159 126 94 151 155 Ga 25.5 25.0 22.5 24.3 25.3 25.3 23.5 24.0 23.1 18.9 22.6 23.5 Y 58.3 40.3 35.7 36.8 36.9 38.8 37.4 39.1 31.5 18.0 42.2 39.0 Zr 573 331 294 296 286 285 293 303 252 84 317 319 Nb 52.0 33.6 29.5 29.4 29.3 30.9 30.9 33.2 25.1 7.3 32.7 33.5 Hf 12.4 7.32 6.54 6.84 6.61 7.24 5.78 2.01 6.84 Ta 2.93 2.12 1.64 1.73 1.80 2.03 1.57 0.50 1.70 Th 7.40 3.54 2.80 3.34 2.95 3.23 2.37 0.46 3.02 U 1.30 0.50 0.50 0.05 0.60 Pb 4.30 2.10 2.10 0.19 2.30 La 56.3 33.7 26.6 29.0 27.6 26.8 25.4 28.9 24.6 6.84 33.2 29.5 Ce 130 78.0 64.6 67.0 63.3 62.2 61.8 69.9 58.4 16.8 72.4 72.2 Nd 66.0 48.4 35.8 39.8 34.7 38.9 33.4 11.0 40.8 Sm 13.6 10.0 8.08 9.06 8.17 9.07 7.58 2.97 8.86 Eu 3.60 3.10 2.55 2.67 2.66 2.99 2.36 1.13 2.66 Tb 2.01 1.53 1.25 1.28 1.22 1.34 1.12 0.66 1.34 Yb 4.85 3.46 2.92 3.09 2.80 2.99 2.56 1.62 2.64 Lu 0.69 0.52 0.41 0.44 0.42 0.43 0.37 0.25 0.42 . Mt Rabouillè re . . . GM92-138 . GM92-137 . GM92-136 . GM92-135 . GM92-134 . GM92-133 . GM92-132 . GM92-131 . GM92-130 . GM92-129 . GM92-128 . GM92-127 . . Height (m) 260 270 280 300 310 350 355 360 370 380 390 400 SiO2 58.98 52.14 51.75 51.97 51.74 50.03 50.61 50.66 49.72 47.79 50.60 50.61 TiO2 1.81 3.57 3.32 3.36 3.39 3.68 3.61 3.89 3.27 1.60 3.84 3.89 Al2O3 13.87 13.26 13.18 13.33 13.00 13.21 13.04 13.25 13.92 16.11 13.00 12.69 Fe2O3 10.25 14.47 14.25 14.16 14.70 15.18 14.82 14.58 14.30 12.46 14.81 15.20 MnO 0.17 0.19 0.21 0.21 0.21 0.19 0.22 0.20 0.20 0.17 0.22 0.21 MgO 2.67 3.46 4.27 3.97 4.11 4.45 4.38 4.25 5.19 8.37 4.34 4.29 CaO 4.64 7.44 7.82 7.93 8.15 8.45 8.31 8.63 9.47 10.56 8.36 8.05 Na2O 3.83 3.31 2.94 3.10 3.25 2.96 2.97 3.14 2.75 2.53 2.94 2.90 K2O 2.63 1.76 1.51 1.43 0.76 1.26 1.38 0.94 0.71 0.25 1.08 1.42 P2O5 0.69 0.56 0.43 0.44 0.42 0.50 0.47 0.51 0.40 0.14 0.50 0.51 Total 99.53 100.17 99.67 99.89 99.73 99.91 99.82 100.04 99.92 99.98 99.67 99.78 Rb 63.6 54.7 39.0 35.5 26.1 23.7 33.3 26.8 8.8 2.9 16.3 32.2 Sr 282 335 338 346 359 339 323 365 379 214 371 338 Ba 501 311 275 265 262 277 281 256 259 78 332 313 Sc 14.8 26.2 26.4 26.7 27.6 28.4 29.3 27 V 60 295 301 324 302 310 323 308 273 208 329 345 Cr 29 7 17 18 — 26 17 6 23 250 0 15 Co 17.2 33.5 39.4 39.6 42.6 44.7 43.2 49.5 37.6 Ni 29 13 23 24 26 29 35 35 44 98 24 40 Zn 151 155 140 149 149 150 156 159 126 94 151 155 Ga 25.5 25.0 22.5 24.3 25.3 25.3 23.5 24.0 23.1 18.9 22.6 23.5 Y 58.3 40.3 35.7 36.8 36.9 38.8 37.4 39.1 31.5 18.0 42.2 39.0 Zr 573 331 294 296 286 285 293 303 252 84 317 319 Nb 52.0 33.6 29.5 29.4 29.3 30.9 30.9 33.2 25.1 7.3 32.7 33.5 Hf 12.4 7.32 6.54 6.84 6.61 7.24 5.78 2.01 6.84 Ta 2.93 2.12 1.64 1.73 1.80 2.03 1.57 0.50 1.70 Th 7.40 3.54 2.80 3.34 2.95 3.23 2.37 0.46 3.02 U 1.30 0.50 0.50 0.05 0.60 Pb 4.30 2.10 2.10 0.19 2.30 La 56.3 33.7 26.6 29.0 27.6 26.8 25.4 28.9 24.6 6.84 33.2 29.5 Ce 130 78.0 64.6 67.0 63.3 62.2 61.8 69.9 58.4 16.8 72.4 72.2 Nd 66.0 48.4 35.8 39.8 34.7 38.9 33.4 11.0 40.8 Sm 13.6 10.0 8.08 9.06 8.17 9.07 7.58 2.97 8.86 Eu 3.60 3.10 2.55 2.67 2.66 2.99 2.36 1.13 2.66 Tb 2.01 1.53 1.25 1.28 1.22 1.34 1.12 0.66 1.34 Yb 4.85 3.46 2.92 3.09 2.80 2.99 2.56 1.62 2.64 Lu 0.69 0.52 0.41 0.44 0.42 0.43 0.37 0.25 0.42 Major oxides and the trace elements Rb, Sr, Ba, V, Ni, Zn, Ga, Y, Zr and Nb determined by duplicate analyses using X-ray fluorescence (XRF). Abundances of U and Pb determined by isotope dilution mass spectrometry. Abundances of Sc, Cr, Hf, Ta, Th and rare earth elements La to Lu determined by instrumental neutron activation analysis (INAA). The mean is indicated for 10 duplicate INAA analyses (see text for discussion of accuracy and precision). Some samples were not analyzed by INAA (those lacking Sc, Co and complete REE data); in these cases, Cr, La and Ce data are by XRF. Open in new tab Table 1: Abundances of major (wt %) and trace elements (ppm) of Mont Bureau and Mont Rabouillère lavas . Mt Bureau . . . . GM92-59 . GM92-58 . GM92-57 . GM92-56 . GM92-55 . GM92-54 . GM92-53 . GM92-52 . GM92-51 . GM92-50 . Height (m) 40 60 75 80 130 130 130 145 150 180 SiO2 4997 51.08 46.21 51.41 45.92 51.31 44.72 48.08 46.40 47.97 TiO2 2.31 3.29 2.45 3.20 1.55 3.17 3.40 2.38 1.60 2.38 Al2O3 15.46 13.26 16.35 13.18 13.89 13.53 13.73 15.76 13.84 15.81 Fe2O3 11.71 15.08 13.62 14.51 12.58 13.99 13.55 13.30 12.62 13.21 MnO 0.16 0.20 0.19 0.24 0.18 0.20 0.19 0.21 0.18 0.20 MgO 5.69 4.47 7.37 4.64 13.35 4.88 9.32 6.18 12.96 6.48 CaO 11.48 8.29 9.67 8.30 9.57 8.46 10.94 9.87 9.59 9.81 Na2O 2.47 2.94 2.99 2.97 1.91 2.96 1.65 3.11 2.10 3.07 K2O 0.55 0.88 0.69 0.83 0.54 1.05 1.85 0.66 0.54 0.64 P2O5 0.26 0.39 0.37 0.39 0.18 0.38 0.60 0.29 0.19 0.28 Total 100.07 99.87 99.89 99.66 99.66 99.93 99.94 99.82 100.00 99.84 Rb 3.8 8.0 17.3 8.0 11.4 16.7 35.2 11.2 10.5 10.5 Sr 339 339 518 343 274 331 557 374 271 375 Ba 164 251 160 255 158 230 683 161 141 156 Sc 27.8±0.1 28.1 25.1 27.3±0.2 26.5 28.6±0.07 27.9 29.4±0.1 26.1±0.3 29.5 V 226 284 199 277 165 299 250 272 176 278 Cr 155±1 14 80 11±1 693 21±1 217 90±1 655±11 92 Co 40.7±0.3 42.4 52.2 44.5±0.1 66.6 40.3±0.3 52.6 46.5±0.1 64.0±0.3 45.9 Ni 82 40 86 28 380 37 120 74 376 80 Zn 107 136 87 132 86 136 118 97 97 92 Ga 20.7 24.3 18.0 24.0 16.0 22.6 20.3 21.8 17.2 21.4 Y 23.0 34.4 26.8 33.0 16.1 31.6 23.1 27.7 16.8 27.4 Zr 156 240 196 235 94 241 230 184 107 184 Nb 16.6 25.1 13.4 25.6 9.3 24.1 49.9 13.5 10.3 13.7 Hf 3.62±0.04 5.82 4.12 5.56±0.07 2.23 5.57±0.08 5.07 4.10±0.02 2.32±0.01 3.98 Ta 0.92±0.01 1.55 0.81 1.57±0.06 0.66 1.45±0.02 2.86 0.88±0.02 0.58±0.07 0.74 Th 1.70±0.04 2.94 0.87 3.12±0.01 0.57 2.63±0.15 4.00 1.39±0.19 0.89±0.09 1.21 U 0.12 0.37 0.15 0.41 0.45 0.13 0.31 0.34 0.17 0.26 Pb 0.70 1.60 0.84 1.75 1.90 0.37 0.95 1.13 0.40 1.02 La 15.1±0.2 25.9 15.1 24.6±0.6 9.2 22.9±0.2 37.4 15.7±0.3 9.49±0.16 15.2 Ce 35.0±1.9 58.6 35.9 57.6±1.1 20.8 51.6±1.0 70.9 37.1±1.6 20.8±1.7 33.1 Nd 20.1±1.1 35.4 21.4 32.9±3.4 12.5 31.3±3.8 33.1 23.0±1.6 13.5±0.64 19.7 Sm 5.03±0.07 7.65 5.34 7.16±0.28 3.08 7.17±0.13 7.65 5.30±0.12 3.22±0.16 4.97 Eu 1.73±0.05 2.59 1.96 2.43±0.01 1.13 2.38±0.03 2.4 1.86±0.03 1.21±0.05 1.78 Tb 0.81±0.04 1.22 0.87 1.11±0.03 0.51 1.14±0.04 0.84 0.94±0.06 0.58±0.03 0.75 Yb 1.76±0.08 2.92 2.38 2.63±0.31 1.33 2.62±0.01 1.94 2.42±0.07 1.43±0.01 2.28 Lu 0.26±0.01 0.40 0.33 0.39±0.01 0.20 0.39±0.01 0.24 0.36±0.01 0.22±0.01 0.33 . Mt Bureau . . . . GM92-59 . GM92-58 . GM92-57 . GM92-56 . GM92-55 . GM92-54 . GM92-53 . GM92-52 . GM92-51 . GM92-50 . Height (m) 40 60 75 80 130 130 130 145 150 180 SiO2 4997 51.08 46.21 51.41 45.92 51.31 44.72 48.08 46.40 47.97 TiO2 2.31 3.29 2.45 3.20 1.55 3.17 3.40 2.38 1.60 2.38 Al2O3 15.46 13.26 16.35 13.18 13.89 13.53 13.73 15.76 13.84 15.81 Fe2O3 11.71 15.08 13.62 14.51 12.58 13.99 13.55 13.30 12.62 13.21 MnO 0.16 0.20 0.19 0.24 0.18 0.20 0.19 0.21 0.18 0.20 MgO 5.69 4.47 7.37 4.64 13.35 4.88 9.32 6.18 12.96 6.48 CaO 11.48 8.29 9.67 8.30 9.57 8.46 10.94 9.87 9.59 9.81 Na2O 2.47 2.94 2.99 2.97 1.91 2.96 1.65 3.11 2.10 3.07 K2O 0.55 0.88 0.69 0.83 0.54 1.05 1.85 0.66 0.54 0.64 P2O5 0.26 0.39 0.37 0.39 0.18 0.38 0.60 0.29 0.19 0.28 Total 100.07 99.87 99.89 99.66 99.66 99.93 99.94 99.82 100.00 99.84 Rb 3.8 8.0 17.3 8.0 11.4 16.7 35.2 11.2 10.5 10.5 Sr 339 339 518 343 274 331 557 374 271 375 Ba 164 251 160 255 158 230 683 161 141 156 Sc 27.8±0.1 28.1 25.1 27.3±0.2 26.5 28.6±0.07 27.9 29.4±0.1 26.1±0.3 29.5 V 226 284 199 277 165 299 250 272 176 278 Cr 155±1 14 80 11±1 693 21±1 217 90±1 655±11 92 Co 40.7±0.3 42.4 52.2 44.5±0.1 66.6 40.3±0.3 52.6 46.5±0.1 64.0±0.3 45.9 Ni 82 40 86 28 380 37 120 74 376 80 Zn 107 136 87 132 86 136 118 97 97 92 Ga 20.7 24.3 18.0 24.0 16.0 22.6 20.3 21.8 17.2 21.4 Y 23.0 34.4 26.8 33.0 16.1 31.6 23.1 27.7 16.8 27.4 Zr 156 240 196 235 94 241 230 184 107 184 Nb 16.6 25.1 13.4 25.6 9.3 24.1 49.9 13.5 10.3 13.7 Hf 3.62±0.04 5.82 4.12 5.56±0.07 2.23 5.57±0.08 5.07 4.10±0.02 2.32±0.01 3.98 Ta 0.92±0.01 1.55 0.81 1.57±0.06 0.66 1.45±0.02 2.86 0.88±0.02 0.58±0.07 0.74 Th 1.70±0.04 2.94 0.87 3.12±0.01 0.57 2.63±0.15 4.00 1.39±0.19 0.89±0.09 1.21 U 0.12 0.37 0.15 0.41 0.45 0.13 0.31 0.34 0.17 0.26 Pb 0.70 1.60 0.84 1.75 1.90 0.37 0.95 1.13 0.40 1.02 La 15.1±0.2 25.9 15.1 24.6±0.6 9.2 22.9±0.2 37.4 15.7±0.3 9.49±0.16 15.2 Ce 35.0±1.9 58.6 35.9 57.6±1.1 20.8 51.6±1.0 70.9 37.1±1.6 20.8±1.7 33.1 Nd 20.1±1.1 35.4 21.4 32.9±3.4 12.5 31.3±3.8 33.1 23.0±1.6 13.5±0.64 19.7 Sm 5.03±0.07 7.65 5.34 7.16±0.28 3.08 7.17±0.13 7.65 5.30±0.12 3.22±0.16 4.97 Eu 1.73±0.05 2.59 1.96 2.43±0.01 1.13 2.38±0.03 2.4 1.86±0.03 1.21±0.05 1.78 Tb 0.81±0.04 1.22 0.87 1.11±0.03 0.51 1.14±0.04 0.84 0.94±0.06 0.58±0.03 0.75 Yb 1.76±0.08 2.92 2.38 2.63±0.31 1.33 2.62±0.01 1.94 2.42±0.07 1.43±0.01 2.28 Lu 0.26±0.01 0.40 0.33 0.39±0.01 0.20 0.39±0.01 0.24 0.36±0.01 0.22±0.01 0.33 . Mt Bureau . . . . GM92-48 . GM92-46 . GM92-45 . GM92-44 . GM92-43 . GM92-42 . GM92-41 . GM92-40 . GM92-39 . GM92-38 . GM92-37 . Height (m) 250 280 300 320 330 340 350 360 400 410 440 SiO2 47.09 46.36 51.12 51.58 49.10 53.12 50.05 52.46 50.91 49.93 50.59 TiO2 1.44 1.87 3.78 3.81 3.49 3.24 3.37 3.37 3.61 3.66 3.63 Al2O3 16.49 14.91 12.89 13.59 14.28 13.33 13.70 13.14 13.39 13.63 13.31 Fe2O3 11.98 14.17 14.54 13.31 14.10 13.82 14.04 14.49 14.13 14.53 14.56 MnO 0.18 0.21 0.22 0.19 0.19 0.21 0.20 0.24 0.19 0.19 0.22 MgO 8.71 10.18 4.26 4.27 5.12 3.53 4.99 3.37 4.67 4.85 4.70 CaO 10.73 9.43 8.09 8.51 9.91 6.82 9.48 7.13 8.63 8.75 8.44 Na2O 2.50 2.24 2.93 3.10 2.72 3.20 2.88 3.13 2.90 2.97 2.96 K2O 0.43 0.17 1.32 1.04 0.63 1.85 0.77 1.79 1.23 1.02 1.06 P2O5 0.20 0.14 0.53 0.53 0.44 0.60 0.43 0.57 0.44 0.44 0.46 Total 99.74 99.66 99.67 99.92 99.99 99.70 99.91 99.70 100.10 99.98 99.93 Rb 5.8 1.4 22.8 27.1 6.4 46.0 15.2 49.2 25.6 12.3 15.8 Sr 252 226 360 371 382 321 372 322 335 347 355 Ba 98 53 305 301 246 361 257 342 274 280 285 Sc 29.5±0.1 29.3±0.1 26.6 26.3 28.8 24.0 28.2 24.8 27.1±0.4 27.8 27.2±0.2 V 174 211 320 296 296 244 283 263 281 287 299 Cr 271±1 290±3 42 73 81 2 82 6 48±1 48 30±1 Co 51.4±0.3 61.0±1.1 38.3 35.3 42.2 46.7 42.5 34.9 41.3±0.6 43.0 39.2±0.1 Ni 170 241 44 56 55 26 66 33 34 37 38 Zn 75 103 144 131 136 153 133 159 122 125 Ga 17.0 18.5 24.0 24.6 22.9 24.6 24.2 25.4 24.0 23.6 24.3 Y 20.7 21.5 39.2 38.2 33.0 41.8 32.6 44.0 34.3 33.5 34.8 Zr 117 93 330 320 263 461 257 413 259 264 277 Nb 8.3 7.9 34.4 34.0 31.6 39.8 30.5 38.5 29.9 30.7 30.3 Hf 2.52±0.08 2.22±0.01 7.52 7.40 6.32 10.1 5.98 9.39 6.17±0.15 6.25 6.20±0.11 Ta 0.57±0.06 0.42±0.05 1.95 2.16 1.76 2.29 1.81 2.01 1.77±0.02 1.87 1.82±0.08 Th 0.65±0.23 0.19±0.13 3.29 3.45 3.28 5.60 2.91 4.80 3.50±0.25 3.29 3.26±0.08 U 0.08 0.03 0.56 0.14 0.91 Pb 0.27 3.80 2.40 0.87 2.60 La 8.80±0.07 5.85±0.02 33.2 32.4 27.6 40.4 28.1 38.2 27.8±0.4 28.1 28.7±0.1 Ce 21.2±1.1 14.8±1.9 78.5 75.2 64.5 96.6 64.0 90.7 64.3±0.1 66.8 65.8±1.6 Nd 12.9±0.92 10.8±0.01 40.0 40.1 36.1 49.3 36.5 46.2 34.0±3.0 36.3 37.9±3.8 Sm 3.33±0.14 3.19±0.25 8.99 9.78 7.86 10.6 7.61 10.1 7.60±0.21 7.67 8.24±0.11 Eu 1.24±0.02 1.26±0.04 2.94 2.90 2.53 3.24 2.49 3.18 2.54±0.03 2.60 2.66±0.036 Tb 0.63±0.01 0.68±0.01 1.50 1.46 1.21 1.61 1.19 1.50 1.14±0.03 1.21 1.29±0.04 Yb 1.88±0.16 1.92±0.01 3.19 3.22 2.68 3.39 2.55 3.56 2.85±0.23 2.63 2.89±0.07 Lu 0.31±0.02 0.32±0.01 0.47 0.46 0.39 0.48 0.36 0.51 0.39±0.01 0.38 0.43±0.01 . Mt Bureau . . . . GM92-48 . GM92-46 . GM92-45 . GM92-44 . GM92-43 . GM92-42 . GM92-41 . GM92-40 . GM92-39 . GM92-38 . GM92-37 . Height (m) 250 280 300 320 330 340 350 360 400 410 440 SiO2 47.09 46.36 51.12 51.58 49.10 53.12 50.05 52.46 50.91 49.93 50.59 TiO2 1.44 1.87 3.78 3.81 3.49 3.24 3.37 3.37 3.61 3.66 3.63 Al2O3 16.49 14.91 12.89 13.59 14.28 13.33 13.70 13.14 13.39 13.63 13.31 Fe2O3 11.98 14.17 14.54 13.31 14.10 13.82 14.04 14.49 14.13 14.53 14.56 MnO 0.18 0.21 0.22 0.19 0.19 0.21 0.20 0.24 0.19 0.19 0.22 MgO 8.71 10.18 4.26 4.27 5.12 3.53 4.99 3.37 4.67 4.85 4.70 CaO 10.73 9.43 8.09 8.51 9.91 6.82 9.48 7.13 8.63 8.75 8.44 Na2O 2.50 2.24 2.93 3.10 2.72 3.20 2.88 3.13 2.90 2.97 2.96 K2O 0.43 0.17 1.32 1.04 0.63 1.85 0.77 1.79 1.23 1.02 1.06 P2O5 0.20 0.14 0.53 0.53 0.44 0.60 0.43 0.57 0.44 0.44 0.46 Total 99.74 99.66 99.67 99.92 99.99 99.70 99.91 99.70 100.10 99.98 99.93 Rb 5.8 1.4 22.8 27.1 6.4 46.0 15.2 49.2 25.6 12.3 15.8 Sr 252 226 360 371 382 321 372 322 335 347 355 Ba 98 53 305 301 246 361 257 342 274 280 285 Sc 29.5±0.1 29.3±0.1 26.6 26.3 28.8 24.0 28.2 24.8 27.1±0.4 27.8 27.2±0.2 V 174 211 320 296 296 244 283 263 281 287 299 Cr 271±1 290±3 42 73 81 2 82 6 48±1 48 30±1 Co 51.4±0.3 61.0±1.1 38.3 35.3 42.2 46.7 42.5 34.9 41.3±0.6 43.0 39.2±0.1 Ni 170 241 44 56 55 26 66 33 34 37 38 Zn 75 103 144 131 136 153 133 159 122 125 Ga 17.0 18.5 24.0 24.6 22.9 24.6 24.2 25.4 24.0 23.6 24.3 Y 20.7 21.5 39.2 38.2 33.0 41.8 32.6 44.0 34.3 33.5 34.8 Zr 117 93 330 320 263 461 257 413 259 264 277 Nb 8.3 7.9 34.4 34.0 31.6 39.8 30.5 38.5 29.9 30.7 30.3 Hf 2.52±0.08 2.22±0.01 7.52 7.40 6.32 10.1 5.98 9.39 6.17±0.15 6.25 6.20±0.11 Ta 0.57±0.06 0.42±0.05 1.95 2.16 1.76 2.29 1.81 2.01 1.77±0.02 1.87 1.82±0.08 Th 0.65±0.23 0.19±0.13 3.29 3.45 3.28 5.60 2.91 4.80 3.50±0.25 3.29 3.26±0.08 U 0.08 0.03 0.56 0.14 0.91 Pb 0.27 3.80 2.40 0.87 2.60 La 8.80±0.07 5.85±0.02 33.2 32.4 27.6 40.4 28.1 38.2 27.8±0.4 28.1 28.7±0.1 Ce 21.2±1.1 14.8±1.9 78.5 75.2 64.5 96.6 64.0 90.7 64.3±0.1 66.8 65.8±1.6 Nd 12.9±0.92 10.8±0.01 40.0 40.1 36.1 49.3 36.5 46.2 34.0±3.0 36.3 37.9±3.8 Sm 3.33±0.14 3.19±0.25 8.99 9.78 7.86 10.6 7.61 10.1 7.60±0.21 7.67 8.24±0.11 Eu 1.24±0.02 1.26±0.04 2.94 2.90 2.53 3.24 2.49 3.18 2.54±0.03 2.60 2.66±0.036 Tb 0.63±0.01 0.68±0.01 1.50 1.46 1.21 1.61 1.19 1.50 1.14±0.03 1.21 1.29±0.04 Yb 1.88±0.16 1.92±0.01 3.19 3.22 2.68 3.39 2.55 3.56 2.85±0.23 2.63 2.89±0.07 Lu 0.31±0.02 0.32±0.01 0.47 0.46 0.39 0.48 0.36 0.51 0.39±0.01 0.38 0.43±0.01 . Mt Bureau . Mt Rabouillè re . . . . . GM92-36 . GM92-35 . GM92-34 . GM92-33 . GM92-32 . GM92-31 . GM92-30 . GM92-29 . GM92-153 . GM92-152 . GM92-151 . Height (m) 460 460 480 510 530 580 610 620 30 40 50 SiO2 51.66 51.67 51.95 51.05 50.07 50.22 50.92 51.18 50.84 51.52 51.71 TiO2 3.50 3.52 3.34 3.44 3.67 3.62 3.31 3.91 3.85 3.78 3.78 Al2O3 13.09 13.17 13.01 13.53 13.00 13.15 13.46 12.87 13.03 12.85 12.84 Fe2O3 14.36 14.36 14.58 13.84 15.25 15.09 13.60 14.40 14.24 14.50 14.53 MnO 0.22 0.21 0.22 0.17 0.20 0.22 0.21 0.19 0.21 0.21 0.21 MgO 4.08 3.96 3.65 4.95 4.64 4.40 4.84 4.43 4.49 3.95 3.83 CaO 7.47 7.41 7.57 8.74 8.26 8.47 9.24 8.12 8.64 7.76 7.77 Na2O 3.14 3.09 3.15 2.64 2.92 2.89 2.74 2.77 3.13 3.01 3.09 K2O 1.48 1.51 1.40 1.11 1.23 1.25 1.07 1.45 0.82 1.72 1.68 P2O5 0.69 0.69 0.72 0.39 0.49 0.49 0.40 0.53 0.51 0.54 0.54 Total 99.68 99.59 99.58 99.84 99.71 99.78 99.78 99.84 99.78 99.83 99.97 Rb 25.7 36.8 36.2 19.7 25.5 27.8 23.5 28.9 18.3 41.8 41.1 Sr 347 347 335 351 333 340 373 345 335 334 345 Ba 310 303 325 222 267 269 279 315 281 320 308 Sc 25.8 26.0 25.1 30.5 27.7±0.3 27.9 28.1 28.0 27.4 26.5 V 257 255 214 285 311 304 293 322 303 320 319 Cr 10 10 5 56 33±1 41 36 68 43 41 27 Co 35.4 34.7 34.4 43.3 42.7±0.3 44.4 43.3 36.8 42.0 36.9 Ni 33 26 22 55 36 38 47 52 53 34 41 Zn 148 147 154 131 147 148 124 141 149 150 159 Ga 22.9 24.2 25.7 23.3 23.3 22.4 23.2 25.1 23.2 23.1 25.1 Y 40.9 39.5 45.1 28.8 37.0 37.9 33.0 37.8 36.4 37.9 39.1 Zr 312 313 353 242 284 284 257 317 317 330 332 Nb 34.1 34.9 37.5 23.5 30.9 31.2 25.5 32.3 32.0 35.5 35.8 Hf 7.22 7.13 8.05 5.78 6.64±0.12 6.61 6.05 7.30 7.14 7.43 Ta 2.01 2.12 2.19 1.54 1.83±0.03 1.83 1.45 1.93 1.79 2.09 Th 3.99 4.13 4.39 1.99 3.09±0.02 2.70 2.67 3.10 3.32 3.85 U 0.83 0.36 0.55 0.61 0.50 Pb 3.10 1.80 2.30 2.2 2.60 2.10 La 34.1 33.5 37.8 21.0 28.0±1.1 28.6 26.2 31.3 29.7 33.7 28.3 Ce 83.3 79.5 87.2 53.3 69.2±0.5 65.7 62.0 76.4 72.5 75.3 68.5 Nd 43.7 44.8 47.5 32.1 38.5±3.1 36.7 36.0 43.3 37.7 40.6 Sm 9.77 9.78 10.70 7.01 8.51±0.01 8.72 7.89 8.92 8.81 9.28 Eu 3.15 3.10 3.23 2.39 2.75±0.03 2.73 2.43 2.83 2.79 2.96 Tb 1.25 1.44 1.59 1.03 1.22±0.08 1.29 1.25 1.34 1.33 1.40 Yb 3.14 3.20 3.80 2.27 2.88±0.03 3.09 2.69 2.96 3.02 3.08 Lu 0.42 0.45 0.53 0.32 0.43±0.01 0.42 0.39 0.41 0.44 0.45 . Mt Bureau . Mt Rabouillè re . . . . . GM92-36 . GM92-35 . GM92-34 . GM92-33 . GM92-32 . GM92-31 . GM92-30 . GM92-29 . GM92-153 . GM92-152 . GM92-151 . Height (m) 460 460 480 510 530 580 610 620 30 40 50 SiO2 51.66 51.67 51.95 51.05 50.07 50.22 50.92 51.18 50.84 51.52 51.71 TiO2 3.50 3.52 3.34 3.44 3.67 3.62 3.31 3.91 3.85 3.78 3.78 Al2O3 13.09 13.17 13.01 13.53 13.00 13.15 13.46 12.87 13.03 12.85 12.84 Fe2O3 14.36 14.36 14.58 13.84 15.25 15.09 13.60 14.40 14.24 14.50 14.53 MnO 0.22 0.21 0.22 0.17 0.20 0.22 0.21 0.19 0.21 0.21 0.21 MgO 4.08 3.96 3.65 4.95 4.64 4.40 4.84 4.43 4.49 3.95 3.83 CaO 7.47 7.41 7.57 8.74 8.26 8.47 9.24 8.12 8.64 7.76 7.77 Na2O 3.14 3.09 3.15 2.64 2.92 2.89 2.74 2.77 3.13 3.01 3.09 K2O 1.48 1.51 1.40 1.11 1.23 1.25 1.07 1.45 0.82 1.72 1.68 P2O5 0.69 0.69 0.72 0.39 0.49 0.49 0.40 0.53 0.51 0.54 0.54 Total 99.68 99.59 99.58 99.84 99.71 99.78 99.78 99.84 99.78 99.83 99.97 Rb 25.7 36.8 36.2 19.7 25.5 27.8 23.5 28.9 18.3 41.8 41.1 Sr 347 347 335 351 333 340 373 345 335 334 345 Ba 310 303 325 222 267 269 279 315 281 320 308 Sc 25.8 26.0 25.1 30.5 27.7±0.3 27.9 28.1 28.0 27.4 26.5 V 257 255 214 285 311 304 293 322 303 320 319 Cr 10 10 5 56 33±1 41 36 68 43 41 27 Co 35.4 34.7 34.4 43.3 42.7±0.3 44.4 43.3 36.8 42.0 36.9 Ni 33 26 22 55 36 38 47 52 53 34 41 Zn 148 147 154 131 147 148 124 141 149 150 159 Ga 22.9 24.2 25.7 23.3 23.3 22.4 23.2 25.1 23.2 23.1 25.1 Y 40.9 39.5 45.1 28.8 37.0 37.9 33.0 37.8 36.4 37.9 39.1 Zr 312 313 353 242 284 284 257 317 317 330 332 Nb 34.1 34.9 37.5 23.5 30.9 31.2 25.5 32.3 32.0 35.5 35.8 Hf 7.22 7.13 8.05 5.78 6.64±0.12 6.61 6.05 7.30 7.14 7.43 Ta 2.01 2.12 2.19 1.54 1.83±0.03 1.83 1.45 1.93 1.79 2.09 Th 3.99 4.13 4.39 1.99 3.09±0.02 2.70 2.67 3.10 3.32 3.85 U 0.83 0.36 0.55 0.61 0.50 Pb 3.10 1.80 2.30 2.2 2.60 2.10 La 34.1 33.5 37.8 21.0 28.0±1.1 28.6 26.2 31.3 29.7 33.7 28.3 Ce 83.3 79.5 87.2 53.3 69.2±0.5 65.7 62.0 76.4 72.5 75.3 68.5 Nd 43.7 44.8 47.5 32.1 38.5±3.1 36.7 36.0 43.3 37.7 40.6 Sm 9.77 9.78 10.70 7.01 8.51±0.01 8.72 7.89 8.92 8.81 9.28 Eu 3.15 3.10 3.23 2.39 2.75±0.03 2.73 2.43 2.83 2.79 2.96 Tb 1.25 1.44 1.59 1.03 1.22±0.08 1.29 1.25 1.34 1.33 1.40 Yb 3.14 3.20 3.80 2.27 2.88±0.03 3.09 2.69 2.96 3.02 3.08 Lu 0.42 0.45 0.53 0.32 0.43±0.01 0.42 0.39 0.41 0.44 0.45 . Mt Rabouillère . . . . GM92-150 . GM92-149 . GM92-148 . GM92-147 . GM92-146 . GM92-145 . GM92-144 . GM92-143 . GM92-142 . GM92-141 . GM92-140 . GM92-139 . Height (m) 60 70 90 110 120 130 160 170 185 200 210 240 SiO2 50.84 50.77 51.07 51.06 48.87 48.00 49.65 51.17 51.10 50.08 49.88 58.62 TiO2 3.82 3.96 3.58 3.63 1.97 1.75 3.19 3.58 3.55 2.68 3.68 1.82 Fe2O3 13.86 13.67 13.47 15.02 12.10 12.38 14.27 14.28 14.98 13.38 15.90 10.67 MnO 0.20 0.18 0.22 0.22 0.18 0.18 0.21 0.20 0.22 0.17 0.21 0.20 MgO 4.37 4.43 4.61 4.00 5.64 7.08 5.85 4.24 4.09 5.63 4.36 2.49 CaO 8.40 8.99 8.48 7.98 11.20 11.07 9.26 8.14 8.07 9.42 8.33 5.03 Na2O 3.10 3.00 3.00 2.97 2.91 2.81 2.98 3.15 3.27 3.07 2.98 3.88 K2O 1.18 0.90 1.44 1.49 0.34 0.44 0.80 1.37 1.15 0.73 0.94 2.50 P2O5 0.54 0.56 0.50 0.49 0.20 0.18 0.39 0.47 0.62 0.52 0.46 0.68 Total 99.93 99.69 99.69 99.6 99.84 100.08 100.08 99.78 100.13 100.01 99.99 99.96 Rb 15.2 23.8 34.5 36.6 3.9 12.5 10.2 36.6 20.2 6.7 22.1 54.9 Sr 376 359 355 353 276 275 334 346 362 334 345 293 Ba 319 299 282 302 125 121 249 281 318 264 312 516 Sc 25.9 26.8 27.1 27.5 30.5 29.6 28.2 25.5 15.0 V 301 302 309 324 253 228 298 316 271 199 302 59 Cr 56 75 81 11 107 261 176 34 17 144 26 36 Co 35.4 41.3 38.0 40.9 40.9 47.5 45.8 39.8 17.9 Ni 45 69 64 24 70 97 79 35 36 64 21 22 Zn 147 152 151 147 93 95 133 149 148 129 146 145 Ga 24.9 24.2 23.7 23.1 20.9 20.0 22.0 23.6 24.2 22.0 23.3 25.9 Y 42.2 40.6 37.4 39.3 22.6 20.9 31.1 36.1 39.5 34.7 38.9 58.0 Zr 327 340 307 309 125 111 235 288 308 249 298 579 Nb 35.4 36.7 34.4 32.8 11.4 10.7 28.0 32.3 32.9 29.1 31.9 51.5 Hf 7.58 7.41 6.89 6.87 2.88 2.55 5.26 5.51 12.6 Ta 1.94 2.11 2.01 1.88 0.66 0.60 1.77 1.67 2.92 Th 3.61 3.72 3.53 3.19 1.43 0.84 2.82 2.97 6.90 U 0.50 0.10 0.18 0.40 Pb 2.20 0.36 0.67 1.80 La 33.9 33.7 32.0 31.5 10.7 9.58 24.8 28.4 30.4 28.2 32.7 56.1 Ce 79.3 75.9 70.2 74.2 25.1 22.2 58.2 62.8 70.8 66.9 62.6 130 Nd 44.2 40.7 38.0 42.3 14.4 13.0 30.9 35.9 66.4 Sm 9.54 9.38 8.72 9.41 3.90 3.60 7.14 8.32 14.6 Eu 2.99 2.94 2.81 2.80 1.45 1.34 2.26 2.57 3.69 Tb 1.39 1.38 1.35 1.40 0.67 0.62 1.12 1.22 2.04 Yb 3.20 3.18 3.11 3.10 1.86 1.78 2.58 2.64 5.09 Lu 0.46 0.45 0.44 0.43 0.28 0.27 0.36 0.39 0.71 . Mt Rabouillère . . . . GM92-150 . GM92-149 . GM92-148 . GM92-147 . GM92-146 . GM92-145 . GM92-144 . GM92-143 . GM92-142 . GM92-141 . GM92-140 . GM92-139 . Height (m) 60 70 90 110 120 130 160 170 185 200 210 240 SiO2 50.84 50.77 51.07 51.06 48.87 48.00 49.65 51.17 51.10 50.08 49.88 58.62 TiO2 3.82 3.96 3.58 3.63 1.97 1.75 3.19 3.58 3.55 2.68 3.68 1.82 Fe2O3 13.86 13.67 13.47 15.02 12.10 12.38 14.27 14.28 14.98 13.38 15.90 10.67 MnO 0.20 0.18 0.22 0.22 0.18 0.18 0.21 0.20 0.22 0.17 0.21 0.20 MgO 4.37 4.43 4.61 4.00 5.64 7.08 5.85 4.24 4.09 5.63 4.36 2.49 CaO 8.40 8.99 8.48 7.98 11.20 11.07 9.26 8.14 8.07 9.42 8.33 5.03 Na2O 3.10 3.00 3.00 2.97 2.91 2.81 2.98 3.15 3.27 3.07 2.98 3.88 K2O 1.18 0.90 1.44 1.49 0.34 0.44 0.80 1.37 1.15 0.73 0.94 2.50 P2O5 0.54 0.56 0.50 0.49 0.20 0.18 0.39 0.47 0.62 0.52 0.46 0.68 Total 99.93 99.69 99.69 99.6 99.84 100.08 100.08 99.78 100.13 100.01 99.99 99.96 Rb 15.2 23.8 34.5 36.6 3.9 12.5 10.2 36.6 20.2 6.7 22.1 54.9 Sr 376 359 355 353 276 275 334 346 362 334 345 293 Ba 319 299 282 302 125 121 249 281 318 264 312 516 Sc 25.9 26.8 27.1 27.5 30.5 29.6 28.2 25.5 15.0 V 301 302 309 324 253 228 298 316 271 199 302 59 Cr 56 75 81 11 107 261 176 34 17 144 26 36 Co 35.4 41.3 38.0 40.9 40.9 47.5 45.8 39.8 17.9 Ni 45 69 64 24 70 97 79 35 36 64 21 22 Zn 147 152 151 147 93 95 133 149 148 129 146 145 Ga 24.9 24.2 23.7 23.1 20.9 20.0 22.0 23.6 24.2 22.0 23.3 25.9 Y 42.2 40.6 37.4 39.3 22.6 20.9 31.1 36.1 39.5 34.7 38.9 58.0 Zr 327 340 307 309 125 111 235 288 308 249 298 579 Nb 35.4 36.7 34.4 32.8 11.4 10.7 28.0 32.3 32.9 29.1 31.9 51.5 Hf 7.58 7.41 6.89 6.87 2.88 2.55 5.26 5.51 12.6 Ta 1.94 2.11 2.01 1.88 0.66 0.60 1.77 1.67 2.92 Th 3.61 3.72 3.53 3.19 1.43 0.84 2.82 2.97 6.90 U 0.50 0.10 0.18 0.40 Pb 2.20 0.36 0.67 1.80 La 33.9 33.7 32.0 31.5 10.7 9.58 24.8 28.4 30.4 28.2 32.7 56.1 Ce 79.3 75.9 70.2 74.2 25.1 22.2 58.2 62.8 70.8 66.9 62.6 130 Nd 44.2 40.7 38.0 42.3 14.4 13.0 30.9 35.9 66.4 Sm 9.54 9.38 8.72 9.41 3.90 3.60 7.14 8.32 14.6 Eu 2.99 2.94 2.81 2.80 1.45 1.34 2.26 2.57 3.69 Tb 1.39 1.38 1.35 1.40 0.67 0.62 1.12 1.22 2.04 Yb 3.20 3.18 3.11 3.10 1.86 1.78 2.58 2.64 5.09 Lu 0.46 0.45 0.44 0.43 0.28 0.27 0.36 0.39 0.71 . Mt Rabouillè re . . . GM92-138 . GM92-137 . GM92-136 . GM92-135 . GM92-134 . GM92-133 . GM92-132 . GM92-131 . GM92-130 . GM92-129 . GM92-128 . GM92-127 . . Height (m) 260 270 280 300 310 350 355 360 370 380 390 400 SiO2 58.98 52.14 51.75 51.97 51.74 50.03 50.61 50.66 49.72 47.79 50.60 50.61 TiO2 1.81 3.57 3.32 3.36 3.39 3.68 3.61 3.89 3.27 1.60 3.84 3.89 Al2O3 13.87 13.26 13.18 13.33 13.00 13.21 13.04 13.25 13.92 16.11 13.00 12.69 Fe2O3 10.25 14.47 14.25 14.16 14.70 15.18 14.82 14.58 14.30 12.46 14.81 15.20 MnO 0.17 0.19 0.21 0.21 0.21 0.19 0.22 0.20 0.20 0.17 0.22 0.21 MgO 2.67 3.46 4.27 3.97 4.11 4.45 4.38 4.25 5.19 8.37 4.34 4.29 CaO 4.64 7.44 7.82 7.93 8.15 8.45 8.31 8.63 9.47 10.56 8.36 8.05 Na2O 3.83 3.31 2.94 3.10 3.25 2.96 2.97 3.14 2.75 2.53 2.94 2.90 K2O 2.63 1.76 1.51 1.43 0.76 1.26 1.38 0.94 0.71 0.25 1.08 1.42 P2O5 0.69 0.56 0.43 0.44 0.42 0.50 0.47 0.51 0.40 0.14 0.50 0.51 Total 99.53 100.17 99.67 99.89 99.73 99.91 99.82 100.04 99.92 99.98 99.67 99.78 Rb 63.6 54.7 39.0 35.5 26.1 23.7 33.3 26.8 8.8 2.9 16.3 32.2 Sr 282 335 338 346 359 339 323 365 379 214 371 338 Ba 501 311 275 265 262 277 281 256 259 78 332 313 Sc 14.8 26.2 26.4 26.7 27.6 28.4 29.3 27 V 60 295 301 324 302 310 323 308 273 208 329 345 Cr 29 7 17 18 — 26 17 6 23 250 0 15 Co 17.2 33.5 39.4 39.6 42.6 44.7 43.2 49.5 37.6 Ni 29 13 23 24 26 29 35 35 44 98 24 40 Zn 151 155 140 149 149 150 156 159 126 94 151 155 Ga 25.5 25.0 22.5 24.3 25.3 25.3 23.5 24.0 23.1 18.9 22.6 23.5 Y 58.3 40.3 35.7 36.8 36.9 38.8 37.4 39.1 31.5 18.0 42.2 39.0 Zr 573 331 294 296 286 285 293 303 252 84 317 319 Nb 52.0 33.6 29.5 29.4 29.3 30.9 30.9 33.2 25.1 7.3 32.7 33.5 Hf 12.4 7.32 6.54 6.84 6.61 7.24 5.78 2.01 6.84 Ta 2.93 2.12 1.64 1.73 1.80 2.03 1.57 0.50 1.70 Th 7.40 3.54 2.80 3.34 2.95 3.23 2.37 0.46 3.02 U 1.30 0.50 0.50 0.05 0.60 Pb 4.30 2.10 2.10 0.19 2.30 La 56.3 33.7 26.6 29.0 27.6 26.8 25.4 28.9 24.6 6.84 33.2 29.5 Ce 130 78.0 64.6 67.0 63.3 62.2 61.8 69.9 58.4 16.8 72.4 72.2 Nd 66.0 48.4 35.8 39.8 34.7 38.9 33.4 11.0 40.8 Sm 13.6 10.0 8.08 9.06 8.17 9.07 7.58 2.97 8.86 Eu 3.60 3.10 2.55 2.67 2.66 2.99 2.36 1.13 2.66 Tb 2.01 1.53 1.25 1.28 1.22 1.34 1.12 0.66 1.34 Yb 4.85 3.46 2.92 3.09 2.80 2.99 2.56 1.62 2.64 Lu 0.69 0.52 0.41 0.44 0.42 0.43 0.37 0.25 0.42 . Mt Rabouillè re . . . GM92-138 . GM92-137 . GM92-136 . GM92-135 . GM92-134 . GM92-133 . GM92-132 . GM92-131 . GM92-130 . GM92-129 . GM92-128 . GM92-127 . . Height (m) 260 270 280 300 310 350 355 360 370 380 390 400 SiO2 58.98 52.14 51.75 51.97 51.74 50.03 50.61 50.66 49.72 47.79 50.60 50.61 TiO2 1.81 3.57 3.32 3.36 3.39 3.68 3.61 3.89 3.27 1.60 3.84 3.89 Al2O3 13.87 13.26 13.18 13.33 13.00 13.21 13.04 13.25 13.92 16.11 13.00 12.69 Fe2O3 10.25 14.47 14.25 14.16 14.70 15.18 14.82 14.58 14.30 12.46 14.81 15.20 MnO 0.17 0.19 0.21 0.21 0.21 0.19 0.22 0.20 0.20 0.17 0.22 0.21 MgO 2.67 3.46 4.27 3.97 4.11 4.45 4.38 4.25 5.19 8.37 4.34 4.29 CaO 4.64 7.44 7.82 7.93 8.15 8.45 8.31 8.63 9.47 10.56 8.36 8.05 Na2O 3.83 3.31 2.94 3.10 3.25 2.96 2.97 3.14 2.75 2.53 2.94 2.90 K2O 2.63 1.76 1.51 1.43 0.76 1.26 1.38 0.94 0.71 0.25 1.08 1.42 P2O5 0.69 0.56 0.43 0.44 0.42 0.50 0.47 0.51 0.40 0.14 0.50 0.51 Total 99.53 100.17 99.67 99.89 99.73 99.91 99.82 100.04 99.92 99.98 99.67 99.78 Rb 63.6 54.7 39.0 35.5 26.1 23.7 33.3 26.8 8.8 2.9 16.3 32.2 Sr 282 335 338 346 359 339 323 365 379 214 371 338 Ba 501 311 275 265 262 277 281 256 259 78 332 313 Sc 14.8 26.2 26.4 26.7 27.6 28.4 29.3 27 V 60 295 301 324 302 310 323 308 273 208 329 345 Cr 29 7 17 18 — 26 17 6 23 250 0 15 Co 17.2 33.5 39.4 39.6 42.6 44.7 43.2 49.5 37.6 Ni 29 13 23 24 26 29 35 35 44 98 24 40 Zn 151 155 140 149 149 150 156 159 126 94 151 155 Ga 25.5 25.0 22.5 24.3 25.3 25.3 23.5 24.0 23.1 18.9 22.6 23.5 Y 58.3 40.3 35.7 36.8 36.9 38.8 37.4 39.1 31.5 18.0 42.2 39.0 Zr 573 331 294 296 286 285 293 303 252 84 317 319 Nb 52.0 33.6 29.5 29.4 29.3 30.9 30.9 33.2 25.1 7.3 32.7 33.5 Hf 12.4 7.32 6.54 6.84 6.61 7.24 5.78 2.01 6.84 Ta 2.93 2.12 1.64 1.73 1.80 2.03 1.57 0.50 1.70 Th 7.40 3.54 2.80 3.34 2.95 3.23 2.37 0.46 3.02 U 1.30 0.50 0.50 0.05 0.60 Pb 4.30 2.10 2.10 0.19 2.30 La 56.3 33.7 26.6 29.0 27.6 26.8 25.4 28.9 24.6 6.84 33.2 29.5 Ce 130 78.0 64.6 67.0 63.3 62.2 61.8 69.9 58.4 16.8 72.4 72.2 Nd 66.0 48.4 35.8 39.8 34.7 38.9 33.4 11.0 40.8 Sm 13.6 10.0 8.08 9.06 8.17 9.07 7.58 2.97 8.86 Eu 3.60 3.10 2.55 2.67 2.66 2.99 2.36 1.13 2.66 Tb 2.01 1.53 1.25 1.28 1.22 1.34 1.12 0.66 1.34 Yb 4.85 3.46 2.92 3.09 2.80 2.99 2.56 1.62 2.64 Lu 0.69 0.52 0.41 0.44 0.42 0.43 0.37 0.25 0.42 Major oxides and the trace elements Rb, Sr, Ba, V, Ni, Zn, Ga, Y, Zr and Nb determined by duplicate analyses using X-ray fluorescence (XRF). Abundances of U and Pb determined by isotope dilution mass spectrometry. Abundances of Sc, Cr, Hf, Ta, Th and rare earth elements La to Lu determined by instrumental neutron activation analysis (INAA). The mean is indicated for 10 duplicate INAA analyses (see text for discussion of accuracy and precision). Some samples were not analyzed by INAA (those lacking Sc, Co and complete REE data); in these cases, Cr, La and Ce data are by XRF. Open in new tab Results Petrography The textures of Mont Bureau and Mont Rabouillère samples are similar, ranging from aphyric to moderately phyric typically with groundmasses (<0.1 mm) of fine-grained plagioclase, clinopyroxene and opaques plus olivine in some samples. Six samples (GM92–46, -51, -55, -129, -145 and -146) have coarser groundmasses (<0.3 mm). Most plagioclase phenocrysts (>0.7 mm) are slightly resorbed and some are zoned. In general, olivine and clinopyroxene phenocrysts or microphenocrysts (0.1–0.7 mm) are subhedral and many olivine phenocrysts or microphenocrysts are altered. All Mont Bureau and Mont Rabouillere samples contain secondary minerals, dominantly zeolites, formed after eruption ( Giret et al., 1992). Mont Bureau Fifteen of the 29 Mont Bureau samples are sparsely phyric with <2% plagioclase phenocrysts in a groundmass of subequal amounts of plagioclase laths and fine-grained clinopyroxene, opaques and olivine (<5%). Some of these samples also contain sparse and altered olivine microphenocrysts, and five samples (GM92–34, -35, -36, -40 and -42) contain abundant, ∼20%, opaque minerals. Two samples (GM92–40 and -42) also contain fine-grained apatite. Five samples (GM92–29, -30, -41, -43 and -44) have ∼10% phenocrysts and microphenocrysts in a groundmass similar to that of the sparsely phyric samples; except for sample GM92–29, plagioclase is the dominant phenocryst and microphenocryst phase (Table 2) samples (GM92–46, -48, -50, -51, -52, -55, -57 and -59) contain >15% phenocrysts and microphenocrysts (Table 2). Sample GM92–59 has ∼10% plagioclase phenocrysts. Plagioclase (∼6%) accompanied by olivine is also the dominant phenocryst in three samples (GM92–48, -50 and -52) whereas olivine accompanied by clinopyroxene is the main phenocryst in the other four samples; plagioclase phenocrysts are rare in these samples. An ankaramite dike (GM92–53) contains ∼16% clinopyroxene and ∼5% olivine phenocrysts. Mont Rabouillère Five lavas (GM92–153, −147, −139, −138 and −135) are aphyric, and nine (GM92–133, −134, −136, −140, −142, −143, −144, −148, and −152), are sparsely plagioclase phyric ( ≤ 2%). Three other sparsely phyric samples (GM92–151, −137 and −128) have olivine and plagioclase phenocrysts. Similar to five of the Mont Bureau lavas, four of these Mont Rabouillère lavas (GM92–135, −137, −151 and −152) have large amounts, ∼20%, of opaque minerals. Ten samples contain >2% phenocrysts (Table 2). All of these samples contain olivine and plagioclase phenocrysts or microphenocrysts, and four also have small amounts of clinopyroxene phenocrysts. Plagioclase is the main phenocryst or microphenocryst phase in seven samples (GM92–127, −130, −131, −132, −145, −146 and −150) with the largest amount (17.2%) in GM92–146. Olivine is the dominant phenocryst in three samples (GM92–129, −141 and −149). Major element abundances Most of the samples have basaltic compositions; the two most evolved lavas, GM92–138 and GM92–139 with ∼59% SiO2 and 2.5% MgO, are in the Mont Rabouillère section (Table 1). If FeO is assumed to be 85% of total iron, all of the lavas are hypersthene normative. Previously, lavas from Mont Bureau and the Loranchet Peninsula in the northwest of the archipelago (Fig. 1) were classed as tholeiitic lavas ( Watkins et al., 1974; Storey et al., 1988). However, based on a silica–total alkalis diagram, all of these lavas and our new analyses of Mont Bureau and Mont Rabouillère lavas lie close to the dividing line between alkalic and tholeiitic basalts (Fig. 3). Therefore, we classify these flood basalts as ‘transitional’ in composition. Abundance of MgO does not vary systematically with stratigraphic height, but in both sections most of the lavas have 4–5% MgO (Fig. 4a and b). At Mont Bureau lavas from the upper 330 m are surprisingly uniform in composition with MgO ranging only from 5.12 to 3.37%, whereas those from the lower 300 m range in MgO content from 13.35 to 4.47% (Fig. 4a). Lavas from both sections overlap in MgO variation plots, and the evolved lavas with <6% MgO form well-defined trends; for example, as MgO content decreases the abundance of CaO decreases whereas Al2O3/CaO and abundances of SiO2, Na2O, K2O and P2O5 increase. The abundance of TiO2 peaks at 4.4% MgO (Fig. 5). Table 2: Modes for Mont Bureau and Mont Rabouillère samples Sample . Group* . olivine . plagioclase . clinopyroxene . opaque . AP† . GM‡ . . . . . . . . . . . Ph§ . Mph§ . Ph . Mph . Ph . Mph . Mph . . . Mont Bureau GM92-29 P 6.6 2.5 — 0.2 2.7 0.6 — — 87.4 GM92-30 P 2.4 (1.1) 0.9 (0.3) 2.8 3.6 0.7 0.6 — — 89.0 GM92-41 P 1.5 (1.5) 0.2 (0.1) 2.6 3.1 — 0.5 — 7.5 84.6 GM92-43 P 0.2 (0.2) — 2.9 3.9 — 0.7 — 11.5 80.6 GM92-44 P 0.3 (0.2) 0.8 2.3 7.1 — — — 8.5 81.0 GM92-46 D 14.3 (3.3) 7.9 (0.8) — 0.3 2.0 1.3 — 8.1 66.1 GM92-48 D 5.0 6.1 (1.6) 6.3 8.5 — — 0.1 13.8 60.2 GM92-50 D 0.9 (0.2) — 5.9 9.0 0.2 — — 0.4 83.6 GM92-51 D 9.8 (1.1) 7.4 (1.2) — — — 1.4 — 9.5 71.9 GM92-52 D 2.6 (1.0) 0.9 7.0 10.1 0.2 — 0.2 0.4 78.6 GM92-53 dike 4.9 (3.5) 3.1 (1.9) — 0.3 16.8 2.5 — 3.6 68.8 GM92-55 D 7.5 (2.6) 14.4 (2.0) 0.7 1.0 0.3 2.0 0.8 9.9 63.4 GM92-57 D 6.8 7.1 — — 1.2 — 0.3 0.9 83.7 GM92-59 P 1.1 (1.1) 0.1 9.7 8.8 — 1.9 — 1.0 77.4 Mont Rabouillè re GM92-127 P 0.1 0.3 1.3 2.4 0.1 0.5 — 0.2 95.1 GM92-129 D 5.9 (0.1) 1.7 2.3 0.4 2.9 0.8 — 27.1 58.9 GM92-130 P 1.9 (1.1) 0.7 (0.4) 3.6 5.8 0.4 0.8 — 0.5 86.3 GM92-131 P — 0.3 — 8.3 — 0.2 0.2 3.4 87.6 GM92-132 P 0.4 0.3 (0.1) 1.8 1.7 0.7 0.1 — 0.9 94.1 GM92-141 P 4.3 (4.3)¶ 0.3 (0.3) 3.3 1.1 — — 0.1 0.1 90.8 GM92-145 D 0.3 (0.3) 1.7 (0.5) 2.6 3.1 — — — 3.2 89.1 GM92-146 D 0.2 (0.2) — 7.3 9.9 — — — 11.5 71.1 GM92-149 P 5.5 (5.0) 0.7 (0.3) 1.5 1.3 — — 0.4 10.0 80.6 GN92-150 P 0.6 (0.3) 0.6 (0.2) 3.1 6.6 — 0.2 — 1.8 87.1 Sample . Group* . olivine . plagioclase . clinopyroxene . opaque . AP† . GM‡ . . . . . . . . . . . Ph§ . Mph§ . Ph . Mph . Ph . Mph . Mph . . . Mont Bureau GM92-29 P 6.6 2.5 — 0.2 2.7 0.6 — — 87.4 GM92-30 P 2.4 (1.1) 0.9 (0.3) 2.8 3.6 0.7 0.6 — — 89.0 GM92-41 P 1.5 (1.5) 0.2 (0.1) 2.6 3.1 — 0.5 — 7.5 84.6 GM92-43 P 0.2 (0.2) — 2.9 3.9 — 0.7 — 11.5 80.6 GM92-44 P 0.3 (0.2) 0.8 2.3 7.1 — — — 8.5 81.0 GM92-46 D 14.3 (3.3) 7.9 (0.8) — 0.3 2.0 1.3 — 8.1 66.1 GM92-48 D 5.0 6.1 (1.6) 6.3 8.5 — — 0.1 13.8 60.2 GM92-50 D 0.9 (0.2) — 5.9 9.0 0.2 — — 0.4 83.6 GM92-51 D 9.8 (1.1) 7.4 (1.2) — — — 1.4 — 9.5 71.9 GM92-52 D 2.6 (1.0) 0.9 7.0 10.1 0.2 — 0.2 0.4 78.6 GM92-53 dike 4.9 (3.5) 3.1 (1.9) — 0.3 16.8 2.5 — 3.6 68.8 GM92-55 D 7.5 (2.6) 14.4 (2.0) 0.7 1.0 0.3 2.0 0.8 9.9 63.4 GM92-57 D 6.8 7.1 — — 1.2 — 0.3 0.9 83.7 GM92-59 P 1.1 (1.1) 0.1 9.7 8.8 — 1.9 — 1.0 77.4 Mont Rabouillè re GM92-127 P 0.1 0.3 1.3 2.4 0.1 0.5 — 0.2 95.1 GM92-129 D 5.9 (0.1) 1.7 2.3 0.4 2.9 0.8 — 27.1 58.9 GM92-130 P 1.9 (1.1) 0.7 (0.4) 3.6 5.8 0.4 0.8 — 0.5 86.3 GM92-131 P — 0.3 — 8.3 — 0.2 0.2 3.4 87.6 GM92-132 P 0.4 0.3 (0.1) 1.8 1.7 0.7 0.1 — 0.9 94.1 GM92-141 P 4.3 (4.3)¶ 0.3 (0.3) 3.3 1.1 — — 0.1 0.1 90.8 GM92-145 D 0.3 (0.3) 1.7 (0.5) 2.6 3.1 — — — 3.2 89.1 GM92-146 D 0.2 (0.2) — 7.3 9.9 — — — 11.5 71.1 GM92-149 P 5.5 (5.0) 0.7 (0.3) 1.5 1.3 — — 0.4 10.0 80.6 GN92-150 P 0.6 (0.3) 0.6 (0.2) 3.1 6.6 — 0.2 — 1.8 87.1 Modes are based on 1300–1700 counts per sample. * See text for grouping samples. † AP indicates alteration products (>0.1 mm) whose original minerals cannot be identified. ‡ GM indicates groundmass. Except for GM92-46, GM92-48, GM92-51, GM92-55, GM92-129, GM92-145, and GM92-146 whose groundmasses are coarser, <0.3 mm, other samples have groundmasses <0.1 mm. § Ph and Mph indicate phenocryst (>0.7 mm) and microphenocryst (0.3–0.7 mm for seven samples indicated in previous footnote, and 0.1–0.7 mm for other samples), respectively. ¶ Number in parentheses is the proportion of altered olivine. Open in new tab Table 2: Modes for Mont Bureau and Mont Rabouillère samples Sample . Group* . olivine . plagioclase . clinopyroxene . opaque . AP† . GM‡ . . . . . . . . . . . Ph§ . Mph§ . Ph . Mph . Ph . Mph . Mph . . . Mont Bureau GM92-29 P 6.6 2.5 — 0.2 2.7 0.6 — — 87.4 GM92-30 P 2.4 (1.1) 0.9 (0.3) 2.8 3.6 0.7 0.6 — — 89.0 GM92-41 P 1.5 (1.5) 0.2 (0.1) 2.6 3.1 — 0.5 — 7.5 84.6 GM92-43 P 0.2 (0.2) — 2.9 3.9 — 0.7 — 11.5 80.6 GM92-44 P 0.3 (0.2) 0.8 2.3 7.1 — — — 8.5 81.0 GM92-46 D 14.3 (3.3) 7.9 (0.8) — 0.3 2.0 1.3 — 8.1 66.1 GM92-48 D 5.0 6.1 (1.6) 6.3 8.5 — — 0.1 13.8 60.2 GM92-50 D 0.9 (0.2) — 5.9 9.0 0.2 — — 0.4 83.6 GM92-51 D 9.8 (1.1) 7.4 (1.2) — — — 1.4 — 9.5 71.9 GM92-52 D 2.6 (1.0) 0.9 7.0 10.1 0.2 — 0.2 0.4 78.6 GM92-53 dike 4.9 (3.5) 3.1 (1.9) — 0.3 16.8 2.5 — 3.6 68.8 GM92-55 D 7.5 (2.6) 14.4 (2.0) 0.7 1.0 0.3 2.0 0.8 9.9 63.4 GM92-57 D 6.8 7.1 — — 1.2 — 0.3 0.9 83.7 GM92-59 P 1.1 (1.1) 0.1 9.7 8.8 — 1.9 — 1.0 77.4 Mont Rabouillè re GM92-127 P 0.1 0.3 1.3 2.4 0.1 0.5 — 0.2 95.1 GM92-129 D 5.9 (0.1) 1.7 2.3 0.4 2.9 0.8 — 27.1 58.9 GM92-130 P 1.9 (1.1) 0.7 (0.4) 3.6 5.8 0.4 0.8 — 0.5 86.3 GM92-131 P — 0.3 — 8.3 — 0.2 0.2 3.4 87.6 GM92-132 P 0.4 0.3 (0.1) 1.8 1.7 0.7 0.1 — 0.9 94.1 GM92-141 P 4.3 (4.3)¶ 0.3 (0.3) 3.3 1.1 — — 0.1 0.1 90.8 GM92-145 D 0.3 (0.3) 1.7 (0.5) 2.6 3.1 — — — 3.2 89.1 GM92-146 D 0.2 (0.2) — 7.3 9.9 — — — 11.5 71.1 GM92-149 P 5.5 (5.0) 0.7 (0.3) 1.5 1.3 — — 0.4 10.0 80.6 GN92-150 P 0.6 (0.3) 0.6 (0.2) 3.1 6.6 — 0.2 — 1.8 87.1 Sample . Group* . olivine . plagioclase . clinopyroxene . opaque . AP† . GM‡ . . . . . . . . . . . Ph§ . Mph§ . Ph . Mph . Ph . Mph . Mph . . . Mont Bureau GM92-29 P 6.6 2.5 — 0.2 2.7 0.6 — — 87.4 GM92-30 P 2.4 (1.1) 0.9 (0.3) 2.8 3.6 0.7 0.6 — — 89.0 GM92-41 P 1.5 (1.5) 0.2 (0.1) 2.6 3.1 — 0.5 — 7.5 84.6 GM92-43 P 0.2 (0.2) — 2.9 3.9 — 0.7 — 11.5 80.6 GM92-44 P 0.3 (0.2) 0.8 2.3 7.1 — — — 8.5 81.0 GM92-46 D 14.3 (3.3) 7.9 (0.8) — 0.3 2.0 1.3 — 8.1 66.1 GM92-48 D 5.0 6.1 (1.6) 6.3 8.5 — — 0.1 13.8 60.2 GM92-50 D 0.9 (0.2) — 5.9 9.0 0.2 — — 0.4 83.6 GM92-51 D 9.8 (1.1) 7.4 (1.2) — — — 1.4 — 9.5 71.9 GM92-52 D 2.6 (1.0) 0.9 7.0 10.1 0.2 — 0.2 0.4 78.6 GM92-53 dike 4.9 (3.5) 3.1 (1.9) — 0.3 16.8 2.5 — 3.6 68.8 GM92-55 D 7.5 (2.6) 14.4 (2.0) 0.7 1.0 0.3 2.0 0.8 9.9 63.4 GM92-57 D 6.8 7.1 — — 1.2 — 0.3 0.9 83.7 GM92-59 P 1.1 (1.1) 0.1 9.7 8.8 — 1.9 — 1.0 77.4 Mont Rabouillè re GM92-127 P 0.1 0.3 1.3 2.4 0.1 0.5 — 0.2 95.1 GM92-129 D 5.9 (0.1) 1.7 2.3 0.4 2.9 0.8 — 27.1 58.9 GM92-130 P 1.9 (1.1) 0.7 (0.4) 3.6 5.8 0.4 0.8 — 0.5 86.3 GM92-131 P — 0.3 — 8.3 — 0.2 0.2 3.4 87.6 GM92-132 P 0.4 0.3 (0.1) 1.8 1.7 0.7 0.1 — 0.9 94.1 GM92-141 P 4.3 (4.3)¶ 0.3 (0.3) 3.3 1.1 — — 0.1 0.1 90.8 GM92-145 D 0.3 (0.3) 1.7 (0.5) 2.6 3.1 — — — 3.2 89.1 GM92-146 D 0.2 (0.2) — 7.3 9.9 — — — 11.5 71.1 GM92-149 P 5.5 (5.0) 0.7 (0.3) 1.5 1.3 — — 0.4 10.0 80.6 GN92-150 P 0.6 (0.3) 0.6 (0.2) 3.1 6.6 — 0.2 — 1.8 87.1 Modes are based on 1300–1700 counts per sample. * See text for grouping samples. † AP indicates alteration products (>0.1 mm) whose original minerals cannot be identified. ‡ GM indicates groundmass. Except for GM92-46, GM92-48, GM92-51, GM92-55, GM92-129, GM92-145, and GM92-146 whose groundmasses are coarser, <0.3 mm, other samples have groundmasses <0.1 mm. § Ph and Mph indicate phenocryst (>0.7 mm) and microphenocryst (0.3–0.7 mm for seven samples indicated in previous footnote, and 0.1–0.7 mm for other samples), respectively. ¶ Number in parentheses is the proportion of altered olivine. Open in new tab Trace element abundances First series transition metals All samples define positive correlations of Ni and Cr abundances with MgO content (Fig. 6), but like MgO content, Ni abundances were apparently buffered; e.g. most lavas have 20–40 ppm Ni (Fig. 4b). As in MgO–major oxide plots, lavas with MgO < 6% define the most coherent MgO–Sc variation, with Sc being depleted in the most evolved lavas (Fig. 6). Lavas in this group also define an inflected V–MgO trend peaking at ∼4% MgO (Fig. 6); i.e. similar to the TiO2–MgO trend (Fig. 5). Fig. 3. Open in new tabDownload slide SiO2 vs Na2O + K2O (all in wt %). Ferrous iron calculated as 85% of total iron. The tholeiitic–alkalic dividing line is from Macdonald & Katsura, (1964). In (a) data points are for Mont Bureau and Mont Rabouillère samples; shown for comparison are data fields for an Upper Miocene basanite to phonolite suite ( Weis et al., 1993), the Pleistocene alkalic basalt to trachyte suite forming Mont Ross ( Weis et al., 1998), a Lower Miocene alkalic basalt to trachyte suite from the Southeast flood basalt province ( Weis et al., 1993), and samples from the Loranchet Peninsula in the northwest ( Storey et al., 1988). In (b) the data points for Mont Bureau samples are divided into the groups defined in Fig. 14 (also see text). The continuous lines connect measured data for altered samples and adjusted compositions whose K2O contents were increased so that K/Nb ratio was 375. Incompatible elements Abundances of immobile incompatible elements such as Th, high field strength elements and REE are highly correlated in these flood basalts; even Ba, which is often mobile during late-stage alteration, defines a good correlation with Nb (Fig. 7). In contrast, Sr contents are nearly constant in lavas with 3.4–5.4% MgO, i.e. Sr ranges only from 321 to 382 ppm over a nearly two-foldincrease in Nb with the lowest Sr contents in the two highly evolved (∼2.5% MgO) lavas from Mont Rabouillère (Fig. 7). Sample GM92–57 from Mont Bureau has an anomalously high Sr content (Fig. 7) and Al2O3/CaO ratio (Fig. 5) possibly indicating plagioclase accumulation, but this sample does not contain plagioclase phenocrysts (Table 2). When normalized to primitive mantle the most conspicuous feature of the low-MgO (<5.8%) lavas is relative depletion in Sr (Fig. 8a). Despite this relative depletion in Sr, only the two most evolved lavas (GM92–138 and-139 from Mont Rabouillère) are relatively depletedin Eu (Fig. 8b). In contrast, the lavas with relatively high MgO contents (>6.1%) have (Sr/Nd)N = 1.0–1.6 (N indicates normalized to primitive mantle) and most(eight of 10) have small but obvious positive Eu anomalies (Fig. 8b). These high-MgO lavas have relatively low contents of incompatible elements, and they have conspicuous relative deficiencies in Th (Fig. 8a). An obvious complexity is the crossing REE pattern of GM92–46 (Fig. 8b). Fig. 4. Open in new tabDownload slide (a) MgO (wt %) and initial 87Sr/86Sr variations with stratigraphic height in meters. Symbol size is larger than the 2σ uncertainty. In the Mont Bureau section 40Ar/39Ar ages range from 30.4 Ma (lower flow) to 29.0 Ma (upper flow) ( Nicolaysen et al., 1996). (b) MgO and Ni histograms for Mont Bureau and Mont Rabouillère sections. The effects of post-magmatic alteration (zeolitization) on chemical compositions Many of the flood basalts contain zeolites. Giret et al., (1992) defined a vertical zoneography for flood basalts in the archipelago by identifying five zeolite zonesrepresenting different temperature ranges. Theuppermost zone contains phillipsite and chabazite, which are stable at 40–80°C, and the lowest zone contains laumontite and chlorite, which formed at 170–240°C. This zoneography indicates a geothermal gradient of 70–90°C/km, comparable with that, 100–110°C/km, inferred from zeolite zones in the Tertiary basalts of Iceland ( Walker, 1960). The lava sections at Mont Bureau and Mont Rabouillère contain zeolite zones 3 (100–140°C) and 4 (130–180°C). Giret et al., (1992) inferred that zeolitization was associated with 12–15 Ma plutonism. What compositional changes were caused by formation of zeolites and other alteration phases? Abundances of four normally incompatible elements, K, Rb, U and Pb, are poorly correlated with relatively immobile incompatible elements, such as Nb (Fig. 9). Because it is well established that these four elements are mobile during post-magmatic alteration of ocean island basalts (OIB; e.g. Wood et al., 1976; Kennedy et al., 1991), deviations from the general trends in Fig. 9 are interpreted in terms of loss or gain of these elements. Based on deviations from the K2O and Rb vs Nb trends, 20 of the 56 flood basalt samples have lost K2O and Rb (Fig. 9). The most significant effect was Rb loss. Like young ocean island basalt, most of the archipelago lavas have Ba/Rb < 20 ( Hofmann & White, 1983), but in the 13 most altered samples Ba/Rb and K/Rb increase from 20 to 43 and from 550 to 1200, respectively. Obviously, mobility of K2O is important in classifying lavas as tholeiitic, alkalic or transitional. If K2O contents are adjusted to a K/Nb ratio of 375 which is typical of the unaltered lavas, the normative quartz decreases from 2.24 to 0.05% for the Mont Bureau lava with the lowest K/Nb ratio, GM92–43, and from 5.13 to 2.65% for the Mont Rabouillère lava with the lowest K/Nb, GM92–149. In the silica–alkalis diagram, loss of K2O moves a sample toward the tholeiitic field. For example, with the adjusted K2O contents, Mont Bureau samples GM92–41, GM92–43 and GM92–44 move to near the alkalic–tholeiitic boundary (Fig. 3b), and nine of the 12 Mont Rabouillère samples with anomalously low K2O contents move from the tholeiitic to the alkalic field. Despite these shifts the lavas are best described as transitional basalts; i.e. they are not tholeiitic, and they are not as alkaline as Lower Miocene and recent archipelago lavas (Fig. 3a). Fig. 5. Open in new tabDownload slide Abundances of major oxides and Al2O3/CaO vs MgO content (all in wt %). Fig. 6. Open in new tabDownload slide Abundance of MgO (%) vs Sc, V, Ni and Cr contents (ppm). Fig. 7. Open in new tabDownload slide Abundances of Ba, Th, La, and Sr vs Nb abundance (all in ppm). Fig. 8. Open in new tabDownload slide Open in new tabDownload slide (a) Incompatible element abundances in representative samples from both sections normalized to primary mantle estimates ( Sun & McDonough, 1989). For each section the upper legend is for group P samples (2.5–5.8% MgO) and the lower legend is for group D (5.6–13.0% MgO) samples [groups are defined in text and Fig. 14 (below)]. (b) Chondrite-normalized REE patterns of representative lavas. Most of the group D samples have small positive Eu anomalies. Chondritic values are from Sun & McDonough, (1989). Fig. 9. Open in new tabDownload slide Abundance of Nb vs abundances of the mobile elements, K, Rb, U and Pb (all in ppm). Some altered samples are labeled and groups are as defined in Fig. 14 (below). In the Nb–K panel, the continuous line indicates K/Nb = 375, which is typical for the relatively unaltered lavas. The post-magmatic mobility of Pb and U is moredifficult to evaluate because the samples were acid-leached before analyses for Pb and U. In general, abundances of Pb and U are positively correlated with Nb contents, but the offset of Mont Bureau samples GM92–55 and GM92–46 to higher U and Pb probably reflects addition of these elements during late-stage alteration (Fig. 9). Sr, Nd, Pb and Os isotopes The measured isotopic ratios of Sr, Nd and Pb were corrected for in situ Rb, Sm, U and Th decay since 28 Ma (Table 3Figs 10, 11 and 12), which is an estimate of the mean age for lavas in these sections ( Nicolaysen et al., 1996). For the Sr and Nd isotopic systems the age corrections use parent/daughter abundance ratios measured by XRF or INAA techniques on unleached powders. However, 206Pb/204Pb and 207Pb/204Pb ratios were corrected using Pb and U concentrations measured by isotope dilution on acid-leached samples. Twenty-nine samples were analyzed for U and Pb; the 238U/204Pb ratio ranges from 10 to 27 in 28 of these samples and from only 10.9 to 19.3 in 23 of the 29 samples, including all 10 samples from the Mont Rabouillère section (Table 3). We find that the age-corrected 206Pb/204Pb and 207Pb/204Pb ratios for the six Mont Bureau samples with 238U/204Pb outside this range form a tighter cluster, if we use the average 238U/204Pb of 15.1 in the other 13 Mont Bureau samples, rather than the extreme measured ratios in these six samples (Table 3). Age corrections for 208Pb/204Pb ratios are more complex because Th data are for unleached whole rocks. Not surprisingly, in the 30 samples 232Th/204Pb ratios range widely from 4.8 to 278, but in 26 samples the range is from 67.7 to 163. Again we find a tighter cluster of age-corrected data, if the average 232Th/204Pb ratio of 89.7 is used for the samples with extreme 232Th/204Pb. The greater coherency obtained by using the average parent/daughter abundance ratios for samples with outlier 238U/204Pb and 232Th/204Pb is shown in Fig. 11a. With this approach the maximum age corrections of 0.45% for 206Pb/204Pb and 0.57% for 208Pb/204Pb are less than the measured variations in 206Pb/204Pb (1.9%) and 208Pb/204Pb (1.5%). The age correction for 207Pb/204Pb is insignificant because of the low abundance of 235U. In all subsequent discussion we use these initial ratios for Sr, Nd and Pb isotopic ratios. The flood basalts from Mont Bureau and Mont Rabouillère define an inverse correlation between initial 87Sr/86Sr and 143Nd/144Nd, which range from ratios typical of enriched Indian Ocean MORB to 87Sr/86Sr greater than the bulk Earth estimate (Fig. 10). Although there are no systematic variations in isotopic ratios with stratigraphic height (Fig. 4a), the flood basalts form distinct isotopic groups. The eight uppermost lavas in the Mont Bureau section, from 300 to 620 m, and seven of the 10 analyzed Mont Rabouillère lavas span a narrow range of initial 87Sr/86Sr (0.70499–0.70550 and 0.70504–0.70520, respectively, Fig. 4a). This range overlaps with that of lavas from the Pleistocene Mont Ross volcano and the Lower Miocene flood basalts from the Southeast Province of the Kerguelen Archipelago (Fig. 10). Among these flood basalts, three samples from the bottom of the Mont Bureau section, GM92–59, GM92–58 and GM92–56, have the highest initial 87Sr/86Sr ratios (0.70539–0.70550); these ratios overlap with those of the Upper Miocene samples from the Southeast Province (Fig. 10), which were suggested to be the representative of the Kerguelen Plume ( Weis et al., 1993). Four other samples from low in the Mont Bureau section have much lower initial 87Sr/86Sr (0.70399–0.70409, Fig. 10). A group of six lavas, three from each section, has intermediate initial 87Sr/86Sr (0.70431–0.70459), but the three lavas from Mont Rabouillère have lower initial 143Nd/144Nd (Fig. 10). As a group, the Sr and Nd isotopic ratios in these ∼28–30 Ma flood basalts encompass nearly the entire range previously found in archipelago lavas of widely varying age (Fig. 10). This is an important result because previous isotopic data for lavas of varying age from different parts of the archipelago led to the conclusion that the isotopic ratios are correlated with eruption age and degree of alkalinity ( Storey et al., 1988; Gautier et al., 1990; Weis et al., 1993). These flood basalt data show that large isotopic variations occurred on a short time scale and are not correlated with alkalinity as measured in a silica–total alkalis plot (Fig. 3). Table 3: Sr, Nd and Pb isotopic ratios and parent/daughter abundance ratios of Mont Bureau and Mont Rabouillère . Mt Bureau . . . . 87Rb/86Sr . (87Sr/86Sr)m . 2σ . (87Sr/86Sr)i . 147Sm/143Nd . (143Nd/144Nd)m . 2σ . (143Nd/144Nd)i . . GM92-59 0.0324 0.705511 8 0.70550 0.1513 0.512577 16 0.51255 GM92-58 0.0683 0.705507 9 0.70548 0.1306 0.512593 16 0.51257 GM92-57 0.0966 0.704017 8 0.70398 0.1509 0.512887 10 0.51286 GM92-56 0.0675 0.705416 7 0.70539 0.1316 0.512539 8 0.51251 GM92-55 0.1204 0.704394 6 0.70435 0.1490 0.512801 38 0.51277 GM92-54 0.1460 0.705282 5 0.70522 0.1385 0.512598 14 0.51257 GM92-53 0.1829 0.704911 8 0.70484 0.1397 0.512659 15 0.51263 GM92-52 0.0867 0.704094 7 0.70406 0.1393 0.512787 7 0.51276 GM92-51 0.1121 0.704411 6 0.70437 0.1442 0.512770 23 0.51274 GM92-50 0.0810 0.704070 7 0.70404 0.1525 0.512809 7 0.51278 GM92-48 0.0666 0.703965 8 0.70394 0.1561 0.512879 11 0.51285 GM92-46 0.0179 0.704587 7 0.70458 0.1786 0.512775 16 0.51274 GM92-45 0.1833 0.705203 6 0.70513 0.1359 0.512619 14 0.51259 GM92-43 0.0485 0.705037 6 0.70502 0.1316 0.512596 21 0.51257 GM92-40 0.4422 0.705267 7 0.70509 0.1322 0.512613 60 0.51259 GM92-34 0.3127 0.705114 5 0.70499 0.1362 0.512603 9 0.51258 GM92-33 0.1624 0.705191 6 0.70513 0.1320 0.512631 10 0.51261 GM92-31 0.2366 0.705133 7 0.70504 0.1436 0.512627 21 0.51260 GM92-30 0.1823 0.705288 6 0.70522 0.1325 0.512577 42 0.51255 GM92-29 0.2424 0.705276 8 0.70518 0.1245 0.512618 8 0.51260 Mt Bureau 238U/204Pb 235U/204Pb 232Th/204Pb (206Pb/204Pb)m (207Pb/204Pb)m (208Pb/204Pb)m (206Pb/204Pb)i (207Pb/204Pb)i (208Pb/204Pb)i GM92-59 10.92 0.0792 162.7 18.226 15.536 38.94 18.18 15.53 38.71 GM92-58 14.75 0.1070 121.1 18.283 15.536 38.97 18.22 15.53 38.80 GM92-57 11.29 0.0819 67.69 18.222 15.509 38.45 18.17 15.51 38.36 GM92-56 14.96 0.1085 117.7 18.338 15.546 39.02 18.27 15.54 38.86 GM92-55 22.31 0.1618 101.1 18.270 15.536 38.68 18.20 15.53 38.54 GM92-54 15.12 0.1097 94.81 18.372 15.541 38.98 18.31 15.54 38.85 GM92-53 20.85 0.1512 277.9 18.397 15.590 38.93 18.33 15.59 38.81 GM92-52 19.21 0.1393 72.97 18.482 15.556 38.83 18.40 15.55 38.73 GM92-51 26.90 0.1951 134.0 18.229 15.478 38.52 18.16 15.47 38.40 GM92-50 16.25 0.1179 78.16 18.467 15.539 38.78 18.40 15.54 38.67 GM92-48 18.81 0.1364 116.6 18.419 15.500 38.55 18.34 15.50 38.39 GM92-46 0.500 0.0036 4.820 18.257 15.491 38.49 18.19 15.49 38.37 GM92-45 14.83 0.1076 90.03 18.205 15.527 38.81 18.14 15.52 38.69 GM92-43 10.27 0.0745 248.6 18.353 15.542 38.95 18.29 15.54 38.83 GM92-40 22.28 0.1616 121.4 13.297 15.510 38.86 18.23 15.51 38.74 GM92-34 17.04 0.1236 93.10 18.279 15.513 38.83 18.20 15.51 38.70 GM92-33 12.67 0.0919 72.37 18.131 15.510 38.66 18.08 15.51 38.56 GM92-31 15.20 0.1102 86.24 18.248 15.512 38.79 18.18 15.51 38.67 GM92-30 — — 79.57 18.127 15.521 38.77 18.06 15.52 38.66 GM92-29 14.90 0.1080 78.22 18.129 15.537 38.80 18.06 15.53 38.69 Mt Rabouillère 87Rb/86Sr (87Sr/86Sr)m 2σ (87Sr/86Sr)i 147Sm/143Nd (143Nd/144Nd)m 2σ (143Nd/144Nd)i GM92-127 0.2758 0.705293 7 0.70520 0.1313 0.512573 9 0.512549 GM92-129 0.0389 0.704516 8 0.70450 0.1632 0.512715 7 0.512686 GM92-131 0.2124 0.705190 13 0.70511 0.1410 0.512649 8 0.512624 GM92-136 0.3336 0.705216 7 0.70511 0.1364 0.512635 9 0.512610 GM92-138 0.6538 0.705287 8 0.70504 0.1246 0.512563 9 0.512541 GM92-144 0.0884 0.705104 7 0.70507 0.1397 0.512609 9 0.512584 GM92-145 0.1318 0.704296 7 0.70431 0.1674 0.512679 17 0.512649 GM92-146 0.0410 0.704456 6 0.70444 0.1637 0.512681 9 0.512652 GM92-150 0.1166 0.705170 8 0.70512 0.1305 0.512603 9 0.512580 GM92-153 0.1583 0.705153 7 0.70514 0.1413 0.512603 8 0.512578 Mt Rabouillère 238U/204Pb 235U/204Pb 232Th/204Pb (206Pb/204Pb)m (207Pb/204Pb)m (208Pb/204Pb)m (206Pb/204Pb)i (207Pb/204Pb)i (208Pb/204Pb)i GM92-127 16.62 0.1205 86.42 18.182 15.578 38.94 18.11 15.57 38.82 GM92-129 16.69 0.1211 158.7 18.202 15.547 38.65 18.13 15.54 38.43 GM92-131 15.20 0.1102 101.4 18.416 15.568 38.88 18.35 15.56 38.74 GM92-136 15.18 0.1101 87.87 18.272 15.574 38.95 18.21 15.57 38.83 GM92-138 19.30 0.1400 113.5 18.334 15.568 38.97 18.25 15.56 38.82 GM92-144 14.24 0.1033 103.7 18.438 15.581 39.12 18.38 15.58 38.98 GM92-145 17.13 0.1242 82.61 18.398 15.558 38.83 18.32 15.55 38.72 GM92-146 17.74 0.1287 262.2 18.384 15.586 38.94 18.31 15.58 38.82 GM92-150 14.47 0.1050 108.0 18.250 15.553 38.89 18.19 15.55 38.74 GM92-153 15.19 0.1102 104.2 18.288 15.574 38.97 18.22 15.57 38.83 . Mt Bureau . . . . 87Rb/86Sr . (87Sr/86Sr)m . 2σ . (87Sr/86Sr)i . 147Sm/143Nd . (143Nd/144Nd)m . 2σ . (143Nd/144Nd)i . . GM92-59 0.0324 0.705511 8 0.70550 0.1513 0.512577 16 0.51255 GM92-58 0.0683 0.705507 9 0.70548 0.1306 0.512593 16 0.51257 GM92-57 0.0966 0.704017 8 0.70398 0.1509 0.512887 10 0.51286 GM92-56 0.0675 0.705416 7 0.70539 0.1316 0.512539 8 0.51251 GM92-55 0.1204 0.704394 6 0.70435 0.1490 0.512801 38 0.51277 GM92-54 0.1460 0.705282 5 0.70522 0.1385 0.512598 14 0.51257 GM92-53 0.1829 0.704911 8 0.70484 0.1397 0.512659 15 0.51263 GM92-52 0.0867 0.704094 7 0.70406 0.1393 0.512787 7 0.51276 GM92-51 0.1121 0.704411 6 0.70437 0.1442 0.512770 23 0.51274 GM92-50 0.0810 0.704070 7 0.70404 0.1525 0.512809 7 0.51278 GM92-48 0.0666 0.703965 8 0.70394 0.1561 0.512879 11 0.51285 GM92-46 0.0179 0.704587 7 0.70458 0.1786 0.512775 16 0.51274 GM92-45 0.1833 0.705203 6 0.70513 0.1359 0.512619 14 0.51259 GM92-43 0.0485 0.705037 6 0.70502 0.1316 0.512596 21 0.51257 GM92-40 0.4422 0.705267 7 0.70509 0.1322 0.512613 60 0.51259 GM92-34 0.3127 0.705114 5 0.70499 0.1362 0.512603 9 0.51258 GM92-33 0.1624 0.705191 6 0.70513 0.1320 0.512631 10 0.51261 GM92-31 0.2366 0.705133 7 0.70504 0.1436 0.512627 21 0.51260 GM92-30 0.1823 0.705288 6 0.70522 0.1325 0.512577 42 0.51255 GM92-29 0.2424 0.705276 8 0.70518 0.1245 0.512618 8 0.51260 Mt Bureau 238U/204Pb 235U/204Pb 232Th/204Pb (206Pb/204Pb)m (207Pb/204Pb)m (208Pb/204Pb)m (206Pb/204Pb)i (207Pb/204Pb)i (208Pb/204Pb)i GM92-59 10.92 0.0792 162.7 18.226 15.536 38.94 18.18 15.53 38.71 GM92-58 14.75 0.1070 121.1 18.283 15.536 38.97 18.22 15.53 38.80 GM92-57 11.29 0.0819 67.69 18.222 15.509 38.45 18.17 15.51 38.36 GM92-56 14.96 0.1085 117.7 18.338 15.546 39.02 18.27 15.54 38.86 GM92-55 22.31 0.1618 101.1 18.270 15.536 38.68 18.20 15.53 38.54 GM92-54 15.12 0.1097 94.81 18.372 15.541 38.98 18.31 15.54 38.85 GM92-53 20.85 0.1512 277.9 18.397 15.590 38.93 18.33 15.59 38.81 GM92-52 19.21 0.1393 72.97 18.482 15.556 38.83 18.40 15.55 38.73 GM92-51 26.90 0.1951 134.0 18.229 15.478 38.52 18.16 15.47 38.40 GM92-50 16.25 0.1179 78.16 18.467 15.539 38.78 18.40 15.54 38.67 GM92-48 18.81 0.1364 116.6 18.419 15.500 38.55 18.34 15.50 38.39 GM92-46 0.500 0.0036 4.820 18.257 15.491 38.49 18.19 15.49 38.37 GM92-45 14.83 0.1076 90.03 18.205 15.527 38.81 18.14 15.52 38.69 GM92-43 10.27 0.0745 248.6 18.353 15.542 38.95 18.29 15.54 38.83 GM92-40 22.28 0.1616 121.4 13.297 15.510 38.86 18.23 15.51 38.74 GM92-34 17.04 0.1236 93.10 18.279 15.513 38.83 18.20 15.51 38.70 GM92-33 12.67 0.0919 72.37 18.131 15.510 38.66 18.08 15.51 38.56 GM92-31 15.20 0.1102 86.24 18.248 15.512 38.79 18.18 15.51 38.67 GM92-30 — — 79.57 18.127 15.521 38.77 18.06 15.52 38.66 GM92-29 14.90 0.1080 78.22 18.129 15.537 38.80 18.06 15.53 38.69 Mt Rabouillère 87Rb/86Sr (87Sr/86Sr)m 2σ (87Sr/86Sr)i 147Sm/143Nd (143Nd/144Nd)m 2σ (143Nd/144Nd)i GM92-127 0.2758 0.705293 7 0.70520 0.1313 0.512573 9 0.512549 GM92-129 0.0389 0.704516 8 0.70450 0.1632 0.512715 7 0.512686 GM92-131 0.2124 0.705190 13 0.70511 0.1410 0.512649 8 0.512624 GM92-136 0.3336 0.705216 7 0.70511 0.1364 0.512635 9 0.512610 GM92-138 0.6538 0.705287 8 0.70504 0.1246 0.512563 9 0.512541 GM92-144 0.0884 0.705104 7 0.70507 0.1397 0.512609 9 0.512584 GM92-145 0.1318 0.704296 7 0.70431 0.1674 0.512679 17 0.512649 GM92-146 0.0410 0.704456 6 0.70444 0.1637 0.512681 9 0.512652 GM92-150 0.1166 0.705170 8 0.70512 0.1305 0.512603 9 0.512580 GM92-153 0.1583 0.705153 7 0.70514 0.1413 0.512603 8 0.512578 Mt Rabouillère 238U/204Pb 235U/204Pb 232Th/204Pb (206Pb/204Pb)m (207Pb/204Pb)m (208Pb/204Pb)m (206Pb/204Pb)i (207Pb/204Pb)i (208Pb/204Pb)i GM92-127 16.62 0.1205 86.42 18.182 15.578 38.94 18.11 15.57 38.82 GM92-129 16.69 0.1211 158.7 18.202 15.547 38.65 18.13 15.54 38.43 GM92-131 15.20 0.1102 101.4 18.416 15.568 38.88 18.35 15.56 38.74 GM92-136 15.18 0.1101 87.87 18.272 15.574 38.95 18.21 15.57 38.83 GM92-138 19.30 0.1400 113.5 18.334 15.568 38.97 18.25 15.56 38.82 GM92-144 14.24 0.1033 103.7 18.438 15.581 39.12 18.38 15.58 38.98 GM92-145 17.13 0.1242 82.61 18.398 15.558 38.83 18.32 15.55 38.72 GM92-146 17.74 0.1287 262.2 18.384 15.586 38.94 18.31 15.58 38.82 GM92-150 14.47 0.1050 108.0 18.250 15.553 38.89 18.19 15.55 38.74 GM92-153 15.19 0.1102 104.2 18.288 15.574 38.97 18.22 15.57 38.83 The 2σ uncertainties for (87Sr/86Sr)m and (143Nd/144Nd)m (m indicates measured) indicate variation in last significant digits. Initial ratios (subscript i) are calculated for 28 Ma. Parent/daughter abundance ratios are calculated from data in Table 1. These ratios were used to calculate the initial isotopic ratios except that a 238U/204Pb of 15.1 was used for the seven samples with measured 238U/204Pb <10.92 or >19.30 and a 232Th/M204Pb of 89.7 was used for the six samples with measured 232Th/204Pb <67.69 or >162.7 (see discussion in text). Open in new tab Table 3: Sr, Nd and Pb isotopic ratios and parent/daughter abundance ratios of Mont Bureau and Mont Rabouillère . Mt Bureau . . . . 87Rb/86Sr . (87Sr/86Sr)m . 2σ . (87Sr/86Sr)i . 147Sm/143Nd . (143Nd/144Nd)m . 2σ . (143Nd/144Nd)i . . GM92-59 0.0324 0.705511 8 0.70550 0.1513 0.512577 16 0.51255 GM92-58 0.0683 0.705507 9 0.70548 0.1306 0.512593 16 0.51257 GM92-57 0.0966 0.704017 8 0.70398 0.1509 0.512887 10 0.51286 GM92-56 0.0675 0.705416 7 0.70539 0.1316 0.512539 8 0.51251 GM92-55 0.1204 0.704394 6 0.70435 0.1490 0.512801 38 0.51277 GM92-54 0.1460 0.705282 5 0.70522 0.1385 0.512598 14 0.51257 GM92-53 0.1829 0.704911 8 0.70484 0.1397 0.512659 15 0.51263 GM92-52 0.0867 0.704094 7 0.70406 0.1393 0.512787 7 0.51276 GM92-51 0.1121 0.704411 6 0.70437 0.1442 0.512770 23 0.51274 GM92-50 0.0810 0.704070 7 0.70404 0.1525 0.512809 7 0.51278 GM92-48 0.0666 0.703965 8 0.70394 0.1561 0.512879 11 0.51285 GM92-46 0.0179 0.704587 7 0.70458 0.1786 0.512775 16 0.51274 GM92-45 0.1833 0.705203 6 0.70513 0.1359 0.512619 14 0.51259 GM92-43 0.0485 0.705037 6 0.70502 0.1316 0.512596 21 0.51257 GM92-40 0.4422 0.705267 7 0.70509 0.1322 0.512613 60 0.51259 GM92-34 0.3127 0.705114 5 0.70499 0.1362 0.512603 9 0.51258 GM92-33 0.1624 0.705191 6 0.70513 0.1320 0.512631 10 0.51261 GM92-31 0.2366 0.705133 7 0.70504 0.1436 0.512627 21 0.51260 GM92-30 0.1823 0.705288 6 0.70522 0.1325 0.512577 42 0.51255 GM92-29 0.2424 0.705276 8 0.70518 0.1245 0.512618 8 0.51260 Mt Bureau 238U/204Pb 235U/204Pb 232Th/204Pb (206Pb/204Pb)m (207Pb/204Pb)m (208Pb/204Pb)m (206Pb/204Pb)i (207Pb/204Pb)i (208Pb/204Pb)i GM92-59 10.92 0.0792 162.7 18.226 15.536 38.94 18.18 15.53 38.71 GM92-58 14.75 0.1070 121.1 18.283 15.536 38.97 18.22 15.53 38.80 GM92-57 11.29 0.0819 67.69 18.222 15.509 38.45 18.17 15.51 38.36 GM92-56 14.96 0.1085 117.7 18.338 15.546 39.02 18.27 15.54 38.86 GM92-55 22.31 0.1618 101.1 18.270 15.536 38.68 18.20 15.53 38.54 GM92-54 15.12 0.1097 94.81 18.372 15.541 38.98 18.31 15.54 38.85 GM92-53 20.85 0.1512 277.9 18.397 15.590 38.93 18.33 15.59 38.81 GM92-52 19.21 0.1393 72.97 18.482 15.556 38.83 18.40 15.55 38.73 GM92-51 26.90 0.1951 134.0 18.229 15.478 38.52 18.16 15.47 38.40 GM92-50 16.25 0.1179 78.16 18.467 15.539 38.78 18.40 15.54 38.67 GM92-48 18.81 0.1364 116.6 18.419 15.500 38.55 18.34 15.50 38.39 GM92-46 0.500 0.0036 4.820 18.257 15.491 38.49 18.19 15.49 38.37 GM92-45 14.83 0.1076 90.03 18.205 15.527 38.81 18.14 15.52 38.69 GM92-43 10.27 0.0745 248.6 18.353 15.542 38.95 18.29 15.54 38.83 GM92-40 22.28 0.1616 121.4 13.297 15.510 38.86 18.23 15.51 38.74 GM92-34 17.04 0.1236 93.10 18.279 15.513 38.83 18.20 15.51 38.70 GM92-33 12.67 0.0919 72.37 18.131 15.510 38.66 18.08 15.51 38.56 GM92-31 15.20 0.1102 86.24 18.248 15.512 38.79 18.18 15.51 38.67 GM92-30 — — 79.57 18.127 15.521 38.77 18.06 15.52 38.66 GM92-29 14.90 0.1080 78.22 18.129 15.537 38.80 18.06 15.53 38.69 Mt Rabouillère 87Rb/86Sr (87Sr/86Sr)m 2σ (87Sr/86Sr)i 147Sm/143Nd (143Nd/144Nd)m 2σ (143Nd/144Nd)i GM92-127 0.2758 0.705293 7 0.70520 0.1313 0.512573 9 0.512549 GM92-129 0.0389 0.704516 8 0.70450 0.1632 0.512715 7 0.512686 GM92-131 0.2124 0.705190 13 0.70511 0.1410 0.512649 8 0.512624 GM92-136 0.3336 0.705216 7 0.70511 0.1364 0.512635 9 0.512610 GM92-138 0.6538 0.705287 8 0.70504 0.1246 0.512563 9 0.512541 GM92-144 0.0884 0.705104 7 0.70507 0.1397 0.512609 9 0.512584 GM92-145 0.1318 0.704296 7 0.70431 0.1674 0.512679 17 0.512649 GM92-146 0.0410 0.704456 6 0.70444 0.1637 0.512681 9 0.512652 GM92-150 0.1166 0.705170 8 0.70512 0.1305 0.512603 9 0.512580 GM92-153 0.1583 0.705153 7 0.70514 0.1413 0.512603 8 0.512578 Mt Rabouillère 238U/204Pb 235U/204Pb 232Th/204Pb (206Pb/204Pb)m (207Pb/204Pb)m (208Pb/204Pb)m (206Pb/204Pb)i (207Pb/204Pb)i (208Pb/204Pb)i GM92-127 16.62 0.1205 86.42 18.182 15.578 38.94 18.11 15.57 38.82 GM92-129 16.69 0.1211 158.7 18.202 15.547 38.65 18.13 15.54 38.43 GM92-131 15.20 0.1102 101.4 18.416 15.568 38.88 18.35 15.56 38.74 GM92-136 15.18 0.1101 87.87 18.272 15.574 38.95 18.21 15.57 38.83 GM92-138 19.30 0.1400 113.5 18.334 15.568 38.97 18.25 15.56 38.82 GM92-144 14.24 0.1033 103.7 18.438 15.581 39.12 18.38 15.58 38.98 GM92-145 17.13 0.1242 82.61 18.398 15.558 38.83 18.32 15.55 38.72 GM92-146 17.74 0.1287 262.2 18.384 15.586 38.94 18.31 15.58 38.82 GM92-150 14.47 0.1050 108.0 18.250 15.553 38.89 18.19 15.55 38.74 GM92-153 15.19 0.1102 104.2 18.288 15.574 38.97 18.22 15.57 38.83 . Mt Bureau . . . . 87Rb/86Sr . (87Sr/86Sr)m . 2σ . (87Sr/86Sr)i . 147Sm/143Nd . (143Nd/144Nd)m . 2σ . (143Nd/144Nd)i . . GM92-59 0.0324 0.705511 8 0.70550 0.1513 0.512577 16 0.51255 GM92-58 0.0683 0.705507 9 0.70548 0.1306 0.512593 16 0.51257 GM92-57 0.0966 0.704017 8 0.70398 0.1509 0.512887 10 0.51286 GM92-56 0.0675 0.705416 7 0.70539 0.1316 0.512539 8 0.51251 GM92-55 0.1204 0.704394 6 0.70435 0.1490 0.512801 38 0.51277 GM92-54 0.1460 0.705282 5 0.70522 0.1385 0.512598 14 0.51257 GM92-53 0.1829 0.704911 8 0.70484 0.1397 0.512659 15 0.51263 GM92-52 0.0867 0.704094 7 0.70406 0.1393 0.512787 7 0.51276 GM92-51 0.1121 0.704411 6 0.70437 0.1442 0.512770 23 0.51274 GM92-50 0.0810 0.704070 7 0.70404 0.1525 0.512809 7 0.51278 GM92-48 0.0666 0.703965 8 0.70394 0.1561 0.512879 11 0.51285 GM92-46 0.0179 0.704587 7 0.70458 0.1786 0.512775 16 0.51274 GM92-45 0.1833 0.705203 6 0.70513 0.1359 0.512619 14 0.51259 GM92-43 0.0485 0.705037 6 0.70502 0.1316 0.512596 21 0.51257 GM92-40 0.4422 0.705267 7 0.70509 0.1322 0.512613 60 0.51259 GM92-34 0.3127 0.705114 5 0.70499 0.1362 0.512603 9 0.51258 GM92-33 0.1624 0.705191 6 0.70513 0.1320 0.512631 10 0.51261 GM92-31 0.2366 0.705133 7 0.70504 0.1436 0.512627 21 0.51260 GM92-30 0.1823 0.705288 6 0.70522 0.1325 0.512577 42 0.51255 GM92-29 0.2424 0.705276 8 0.70518 0.1245 0.512618 8 0.51260 Mt Bureau 238U/204Pb 235U/204Pb 232Th/204Pb (206Pb/204Pb)m (207Pb/204Pb)m (208Pb/204Pb)m (206Pb/204Pb)i (207Pb/204Pb)i (208Pb/204Pb)i GM92-59 10.92 0.0792 162.7 18.226 15.536 38.94 18.18 15.53 38.71 GM92-58 14.75 0.1070 121.1 18.283 15.536 38.97 18.22 15.53 38.80 GM92-57 11.29 0.0819 67.69 18.222 15.509 38.45 18.17 15.51 38.36 GM92-56 14.96 0.1085 117.7 18.338 15.546 39.02 18.27 15.54 38.86 GM92-55 22.31 0.1618 101.1 18.270 15.536 38.68 18.20 15.53 38.54 GM92-54 15.12 0.1097 94.81 18.372 15.541 38.98 18.31 15.54 38.85 GM92-53 20.85 0.1512 277.9 18.397 15.590 38.93 18.33 15.59 38.81 GM92-52 19.21 0.1393 72.97 18.482 15.556 38.83 18.40 15.55 38.73 GM92-51 26.90 0.1951 134.0 18.229 15.478 38.52 18.16 15.47 38.40 GM92-50 16.25 0.1179 78.16 18.467 15.539 38.78 18.40 15.54 38.67 GM92-48 18.81 0.1364 116.6 18.419 15.500 38.55 18.34 15.50 38.39 GM92-46 0.500 0.0036 4.820 18.257 15.491 38.49 18.19 15.49 38.37 GM92-45 14.83 0.1076 90.03 18.205 15.527 38.81 18.14 15.52 38.69 GM92-43 10.27 0.0745 248.6 18.353 15.542 38.95 18.29 15.54 38.83 GM92-40 22.28 0.1616 121.4 13.297 15.510 38.86 18.23 15.51 38.74 GM92-34 17.04 0.1236 93.10 18.279 15.513 38.83 18.20 15.51 38.70 GM92-33 12.67 0.0919 72.37 18.131 15.510 38.66 18.08 15.51 38.56 GM92-31 15.20 0.1102 86.24 18.248 15.512 38.79 18.18 15.51 38.67 GM92-30 — — 79.57 18.127 15.521 38.77 18.06 15.52 38.66 GM92-29 14.90 0.1080 78.22 18.129 15.537 38.80 18.06 15.53 38.69 Mt Rabouillère 87Rb/86Sr (87Sr/86Sr)m 2σ (87Sr/86Sr)i 147Sm/143Nd (143Nd/144Nd)m 2σ (143Nd/144Nd)i GM92-127 0.2758 0.705293 7 0.70520 0.1313 0.512573 9 0.512549 GM92-129 0.0389 0.704516 8 0.70450 0.1632 0.512715 7 0.512686 GM92-131 0.2124 0.705190 13 0.70511 0.1410 0.512649 8 0.512624 GM92-136 0.3336 0.705216 7 0.70511 0.1364 0.512635 9 0.512610 GM92-138 0.6538 0.705287 8 0.70504 0.1246 0.512563 9 0.512541 GM92-144 0.0884 0.705104 7 0.70507 0.1397 0.512609 9 0.512584 GM92-145 0.1318 0.704296 7 0.70431 0.1674 0.512679 17 0.512649 GM92-146 0.0410 0.704456 6 0.70444 0.1637 0.512681 9 0.512652 GM92-150 0.1166 0.705170 8 0.70512 0.1305 0.512603 9 0.512580 GM92-153 0.1583 0.705153 7 0.70514 0.1413 0.512603 8 0.512578 Mt Rabouillère 238U/204Pb 235U/204Pb 232Th/204Pb (206Pb/204Pb)m (207Pb/204Pb)m (208Pb/204Pb)m (206Pb/204Pb)i (207Pb/204Pb)i (208Pb/204Pb)i GM92-127 16.62 0.1205 86.42 18.182 15.578 38.94 18.11 15.57 38.82 GM92-129 16.69 0.1211 158.7 18.202 15.547 38.65 18.13 15.54 38.43 GM92-131 15.20 0.1102 101.4 18.416 15.568 38.88 18.35 15.56 38.74 GM92-136 15.18 0.1101 87.87 18.272 15.574 38.95 18.21 15.57 38.83 GM92-138 19.30 0.1400 113.5 18.334 15.568 38.97 18.25 15.56 38.82 GM92-144 14.24 0.1033 103.7 18.438 15.581 39.12 18.38 15.58 38.98 GM92-145 17.13 0.1242 82.61 18.398 15.558 38.83 18.32 15.55 38.72 GM92-146 17.74 0.1287 262.2 18.384 15.586 38.94 18.31 15.58 38.82 GM92-150 14.47 0.1050 108.0 18.250 15.553 38.89 18.19 15.55 38.74 GM92-153 15.19 0.1102 104.2 18.288 15.574 38.97 18.22 15.57 38.83 The 2σ uncertainties for (87Sr/86Sr)m and (143Nd/144Nd)m (m indicates measured) indicate variation in last significant digits. Initial ratios (subscript i) are calculated for 28 Ma. Parent/daughter abundance ratios are calculated from data in Table 1. These ratios were used to calculate the initial isotopic ratios except that a 238U/204Pb of 15.1 was used for the seven samples with measured 238U/204Pb <10.92 or >19.30 and a 232Th/M204Pb of 89.7 was used for the six samples with measured 232Th/204Pb <67.69 or >162.7 (see discussion in text). Open in new tab Fig. 10. Open in new tabDownload slide Initial 87Sr/86Sr vs initial 143Nd/144Nd for flood basalt lavas from Mont Bureau and Mont Rabouillère divided into the groups shown in Fig. 14 (below). □, ankaramite dike in the Bureau section; ×, four Mont Bureau samples analyzed by White & Hofmann, (1982). SE LMS field is for Lower Miocene flood basalts from the Southeast Province ( Weis et al., 1993). Shown for comparison are fields for the youngest archipelago lavas; i.e. for Mont Ross (Pleistocene) and the Southeast Upper Miocene Series (SE UMS) ( Weis et al., 1993, 1998). Field for SEIR MORB from Hamelin et al., (1986), Michard et al., (1986), Dosso et al., (1988) and J. J. Mahoney (unpublished data, 1996). The stippled sub-field for SEIR MORB is for 29 samples distant from the St Paul–Amsterdam Island platform, whereas the larger field extending to higher Sr and lower Nd isotopic ratios includes 14 samples dredged from the SEIR near these islands [see fig. 1 of Dosso et al., (1988)]. Inset shows fields for mafic and ultramafic xenoliths found in recent archipelago lavas ( Mattielli, 1996; Mattielli et al., 1996) compared with field for all archipelago lavas. In Pb–Pb plots, the flood basalts are offset from theIndian MORB field to high 207Pb/204Pb and 208Pb/204Pb (Fig. 11b and c). Most of the samples, 21 of 30, have 206Pb/204Pb ratios within a narrow range, 18.06–18.29, similar to those of the 6.6–10.2 Ma Upper Miocene lavas from the Southeast Province; thus compared with the ∼22 Ma Lower Miocene flood basalts from the Southeast Province, most of the ∼28–30 Ma flood basalt lavas have lower 206Pb/204Pb ratios (Figs 11b and c and 12). Therefore, as with 87Sr/86Sr and 143Nd/144Nd, archipelago lavas do not show a systematic temporal variation in 206Pb/204Pb ratio. There is no correlation between 206Pb/204Pb and 87Sr/86Sr ratios (Fig. 12). In detail, lavas from the Mont Bureau and Mont Rabouillère sections overlap in 206Pb/204Pb vs 208Pb/204Pb (Fig. 11c), but lavas from the Mont Bureau section have lower 207Pb/204Pb at a given 206Pb/204Pb ratio (Fig. 11b). In summary, the Sr, Nd and Pb isotopic data show that: (1) most of the flood basalts have relatively high 87Sr/86Sr, >0.70500, and low 206Pb/204Pb, <18.3, and some of the oldest flood basalts have isotopic ratios similar to those of the much younger, Upper Miocene to Pleistocene, lavas erupted in the southeast part of the archipelago; and (2) there are no systematic correlations between isotopic ratios and eruption age or the alkalinity of the lavas. Three lavas and an ankaramite dike with relatively high MgO (8.7–13.4%) and Ni (120–380 ppm) contents were selected for Os isotopic analyses (Table 4). Three samples are from the Mont Bureau section and one is a flood basalt from the southeast part of the archipelago. These samples cover a substantial range in Os and Re concentration (9.2–360 ppt and 125–559 ppt, respectively, Table 4). If these new data are considered with previous Os isotopic data for archipelago lavas ( Reisberg et al., 1993), the data show a trend to more radiogenic and more variable 187Os/186Os with decreasing Os content as observed in other oceanic island suites ( Reisberg et al., 1993; Marcantonio et al., 1995; Roy-Barman & Allègre, 1995; Widom & Shirey, 1996). Present-day 187Os/186Os values for these samples vary from 1.268 to 1.990 (187Os/188Os from 0.152 to 0.239), and age corrections (Table 4) indicate initial 187Os/186Os compositions between 1.24 and 1.44 (187Os/188Os from 0.149 to 0.173). The two samples with the highest Os concentrations (>60 ppt), are least likely to have been affected by shallow-level crustal contamination or alteration, and they are nearly isotopically indistinguishable with initial 187Os/186Os values of 1.240 and 1.253 (187Os/188Os = 0.149 and 0.151). As a group, the Re–Os variations loosely define an ‘isochron’ with an estimated age of 29.6 Ma and intercept of 187Os/186Os = 1.301 ± 0.045 (Fig. 13). Given that these samples are from three different sections of the flood basalt (Table 4 and Fig. 1), this coherence is remarkable and suggests that the Re–Os systematics have not been significantly perturbed by secondary processes. Furthermore, this Re–Os age estimate is consistent with 40Ar–39Ar ages for the Mont Bureau lavas ( Nicolaysen et al., 1996), thereby supporting the contention that flood basalt formation was not a short-lived episode associated with coincidence of the Kerguelen Plume and the Southeast Indian Ridge at ∼43 Ma. Table 4: Osmium isotopic systematics of lavas from the Kerguelen Archipelago Location . Sample . (87Sr/86Sr)i* . (187Os/186Os)m† . ± . (187Os/186Os)c† . (187Os/188Os)c† . Mt Bureau OB93-334‡ 0.70484 2.029 0.012 1.990 0.239 Mt Bureau GM92-48 0.70394 1.693 0.006 1.671 0.201 Mt Bureau GM92-55 0.70435 1.263 0.003 1.262 0.152 SE Province LVLK-88 0.70522 1.286 0.005 1.278 0.154 Mt Bureau KG4-1§ 0.70407 5.580 — — 0.672 Courbet KG11-1§ 0.70526 1.542 0.013 — 0.186 Peninsula Location . Sample . (87Sr/86Sr)i* . (187Os/186Os)m† . ± . (187Os/186Os)c† . (187Os/188Os)c† . Mt Bureau OB93-334‡ 0.70484 2.029 0.012 1.990 0.239 Mt Bureau GM92-48 0.70394 1.693 0.006 1.671 0.201 Mt Bureau GM92-55 0.70435 1.263 0.003 1.262 0.152 SE Province LVLK-88 0.70522 1.286 0.005 1.278 0.154 Mt Bureau KG4-1§ 0.70407 5.580 — — 0.672 Courbet KG11-1§ 0.70526 1.542 0.013 — 0.186 Peninsula . Sample . Age . Initial . Os . ± . Re . ± . Re/Os . ± . 187Re/186Os . ± . Location . . (Ma) . 187Os/186Os . (ppt) . . (ppt) . . . . . . Mt Bureau OB93-334‡ ? — 9.2 0.04 559 28 60.8 3.05 2497 125 Mt Bureau GM92-48 29 1.435 19.6 0.1 238 12 12.1 0.61 497 25 Mt Bureau GM92-55 29 1.253 360 9.0 171 9 0.48 0.03 19.3 1 SE Province LVLK-88 22 1.240 64.1 0.5 125 6 2.0 0.10 79.3 4 Mt Bureau KG4-1§ 29 1.386 3.6 0.11 733 37 203.6 11.93 8826 517 Courbet KG11-1§ 25 1.329 15.6 0.5 199 10 12.8 0.76 521 31 Peninsula . Sample . Age . Initial . Os . ± . Re . ± . Re/Os . ± . 187Re/186Os . ± . Location . . (Ma) . 187Os/186Os . (ppt) . . (ppt) . . . . . . Mt Bureau OB93-334‡ ? — 9.2 0.04 559 28 60.8 3.05 2497 125 Mt Bureau GM92-48 29 1.435 19.6 0.1 238 12 12.1 0.61 497 25 Mt Bureau GM92-55 29 1.253 360 9.0 171 9 0.48 0.03 19.3 1 SE Province LVLK-88 22 1.240 64.1 0.5 125 6 2.0 0.10 79.3 4 Mt Bureau KG4-1§ 29 1.386 3.6 0.11 733 37 203.6 11.93 8826 517 Courbet KG11-1§ 25 1.329 15.6 0.5 199 10 12.8 0.76 521 31 Peninsula * These samples encompass the wide range of (87Sr/86Sr)i found in archipelago lavas. † Subscripts m and c indicate measured and blank corrected ratios, respectively. ‡ OB93-334 is from the same dike as GM92-53 (Fig. 2 and Tables 1 and 2). § Data from Reisburg et al. (1993). Sr isotopic ratios are measured data. Open in new tab Table 4: Osmium isotopic systematics of lavas from the Kerguelen Archipelago Location . Sample . (87Sr/86Sr)i* . (187Os/186Os)m† . ± . (187Os/186Os)c† . (187Os/188Os)c† . Mt Bureau OB93-334‡ 0.70484 2.029 0.012 1.990 0.239 Mt Bureau GM92-48 0.70394 1.693 0.006 1.671 0.201 Mt Bureau GM92-55 0.70435 1.263 0.003 1.262 0.152 SE Province LVLK-88 0.70522 1.286 0.005 1.278 0.154 Mt Bureau KG4-1§ 0.70407 5.580 — — 0.672 Courbet KG11-1§ 0.70526 1.542 0.013 — 0.186 Peninsula Location . Sample . (87Sr/86Sr)i* . (187Os/186Os)m† . ± . (187Os/186Os)c† . (187Os/188Os)c† . Mt Bureau OB93-334‡ 0.70484 2.029 0.012 1.990 0.239 Mt Bureau GM92-48 0.70394 1.693 0.006 1.671 0.201 Mt Bureau GM92-55 0.70435 1.263 0.003 1.262 0.152 SE Province LVLK-88 0.70522 1.286 0.005 1.278 0.154 Mt Bureau KG4-1§ 0.70407 5.580 — — 0.672 Courbet KG11-1§ 0.70526 1.542 0.013 — 0.186 Peninsula . Sample . Age . Initial . Os . ± . Re . ± . Re/Os . ± . 187Re/186Os . ± . Location . . (Ma) . 187Os/186Os . (ppt) . . (ppt) . . . . . . Mt Bureau OB93-334‡ ? — 9.2 0.04 559 28 60.8 3.05 2497 125 Mt Bureau GM92-48 29 1.435 19.6 0.1 238 12 12.1 0.61 497 25 Mt Bureau GM92-55 29 1.253 360 9.0 171 9 0.48 0.03 19.3 1 SE Province LVLK-88 22 1.240 64.1 0.5 125 6 2.0 0.10 79.3 4 Mt Bureau KG4-1§ 29 1.386 3.6 0.11 733 37 203.6 11.93 8826 517 Courbet KG11-1§ 25 1.329 15.6 0.5 199 10 12.8 0.76 521 31 Peninsula . Sample . Age . Initial . Os . ± . Re . ± . Re/Os . ± . 187Re/186Os . ± . Location . . (Ma) . 187Os/186Os . (ppt) . . (ppt) . . . . . . Mt Bureau OB93-334‡ ? — 9.2 0.04 559 28 60.8 3.05 2497 125 Mt Bureau GM92-48 29 1.435 19.6 0.1 238 12 12.1 0.61 497 25 Mt Bureau GM92-55 29 1.253 360 9.0 171 9 0.48 0.03 19.3 1 SE Province LVLK-88 22 1.240 64.1 0.5 125 6 2.0 0.10 79.3 4 Mt Bureau KG4-1§ 29 1.386 3.6 0.11 733 37 203.6 11.93 8826 517 Courbet KG11-1§ 25 1.329 15.6 0.5 199 10 12.8 0.76 521 31 Peninsula * These samples encompass the wide range of (87Sr/86Sr)i found in archipelago lavas. † Subscripts m and c indicate measured and blank corrected ratios, respectively. ‡ OB93-334 is from the same dike as GM92-53 (Fig. 2 and Tables 1 and 2). § Data from Reisburg et al. (1993). Sr isotopic ratios are measured data. Open in new tab Based on these limited data, the mantle-derived magmas forming the Kerguelen Archipelago apparently had a rather uniform initial 187Os/186Os despite a widerange in Sr, Nd and Pb isotopic ratios (Table 4 and Figs 10- 13). Similar Os isotopic uniformity coupled with Sr, Nd and Pb isotopic variability has been found at São Miguel in the Azores ( Widom & Shirey, 1996). The Os isotopic composition of the Kerguelen Archipelago lavas is typical of that for other plume-derived oceanic island basalt provinces ( Hauri & Hart, 1993; Reisberg et al., 1993; Martin et al., 1994; Marcantonio et al., 1995; Roy-Barman & Allègre, 1995; Widom & Shirey, 1996); however, the high 187Os/186Os and low 206Pb/204Pb are distinct from HIMU OIB, i.e. the Kerguelen Archipelago lavas are more similar to EM1-type Pitcairn compositions ( Woodhead & McCulloch, 1989; Reisberg et al., 1993). In contrast to the initial 187Os/186Os of ∼1.24–1.30 in the lavas (Table 4 and Fig. 13), harzburgite xenoliths from the northeast part of the archipelago indicate that the underlying lithospheric mantle is less radiogenic; specifically, the 187Os/186Os of these xenoliths (0.989–1.150) range from values associated with depleted, sub-cratonic mantle lithosphere or orogenic massifs to oceanic upper mantle to typical oceanic island plume sources ( Hassler & Shimizu, 1995), but the higher 187Os/186Os compositions of the flood basalts require an additional radiogenic contribution. Grouping of samples In MgO vs 87Sr/86Sr and 143Nd/144Nd plots, lavas from Mont Bureau and Mont Rabouillère can be divided into two general groups. The 19 lavas designated as group P (P indicating plume-derived) have <6% MgO, relatively high 87Sr/86Sr and low 143Nd/144Nd (Fig. 14). These lavas have 87Sr/86Sr, 143Nd/144Nd and 206Pb/204Pb ratios which overlap with those of the Upper Miocene to Pleistocene lavas erupted from volcanic centers constructed on the flood basalts in the Southeast Province ( 11 and 12). In the Bureau and Rabouillère sections the lavas forming group P occur throughout the sections; that is, they include the oldest and youngest samples. The other 10 samples are a more heterogeneous population designated as group D (D indicating relatively depleted); they range to higher MgO contents, lower 87Sr/86Sr and higher 143Nd/144Nd (Fig. 14). In particular, four samples, GM92–48, GM92–50, GM92–52 and GM92–57, from the lower 250 m of the Mont Bureau section are within or close to the alkalic field (Fig. 3b), but among these flood basalts they have the lowest 87Sr/86Sr and range to highest 143Nd/144Nd (Table 3 and Fig. 14). Despite the association of relatively high 87Sr/86Sr with low MgO contents, there is no simple trend for increasing 87Sr/86Sr and decreasing 143Nd/144Nd with decreasing MgO content (Fig. 14). Although there is no correlation of Pb isotopic ratios with MgO content (Fig. 14), at a given 206Pb/204Pb, group P lavas have higher 208Pb/204Pb ratios than group D lavas (Fig. 11a). Fig. 11. Open in new tabDownload slide Open in new tabDownload slide Initial 206Pb/204Pb vs 208Pb/204Pb and 207Pb/204Pb for Mont Bureau and Mont Rabouillere samples. (a) Effects of age corrections using measured and adjusted 238U/204Pb and 232Th/204Pb ratios (see text and Table 2). Groups P and D as defined in Fig. 14 (below). Small fields with three data points in lower right of Mont Bureau panel are data from W. M. White (unpublished, 1985). The most important point is that for the two lava types at Mont Bureau (P and D), the field defined by the dashed line is smaller than the fields defined by the continuous lines. (b) and (c), age-corrected ratios for archipelago flood basalts compared with fields for SEIR MORB (divided into two sub-fields as explained in Fig. 10 caption), Ninetyeast Ridge, Amsterdam–St Paul Islands and young Kerguelen Archipelago lavas. Data for Ocean Drilling Program (ODP) Sites (738, 747, 748, 749, 750) on the Kerguelen Plateau from Salters et al., (1992) and Mahoney et al., (1995) and for dredge sites (●) from Weis et al., (1989). Other fields and data sources are those described in Fig. 10 caption. Discussion Geochemical variations in group P lavas from Mont Bureau and Mont Rabouillère The role of crystal fractionation All group P samples have <6% MgO, and it is likely that their compositions were strongly affected by crustal processes. Most of these lavas have similar isotopic ratios, e.g. 16 of the 19 analyzed group P lavas range in initial 87Sr/86Sr from 0.70499 to 0.70529, but the lowermost group P lavas at Mont Bureau have higher initial 87Sr/86Sr of 0.70539–0.70550 (Fig. 4). In detail, there are complexities to the compositional trends which reflect the combined effects of differences in the melting process, crustal fractionation processes and extent of post-magmatic alteration. Nevertheless, the major and trace element compositional trends require that plagioclase and clinopyroxene were major fractionating phases. For example, the relatively low Al2O3 contents (∼13%) and relative depletion in Sr (Fig. 8a) are consistent with plagioclase fractionation. All group P samples have similar Sr contents but Nb contents vary from 25 to 37 ppm (Fig. 7), indicating that the partition coefficient of Sr (DSr) between the fractionating mineral assemblage and residual liquid was about unity. Qualitatively, this requires a major proportion (∼40–50%) of plagioclase in the fractionating assemblage ( Blundy & Wood, 1991). The inverse correlation of MgO–Al2O3/CaO and positive trends of MgO–CaO (Fig. 5) and MgO–Sc (Fig. 6) require segregation of clinopyroxene. The decrease of Sc with increasing Nb in group P samples indicates that DSc between fractionating phases and residual liquid was greater than unity. Qualitatively, this result requires that clinopyroxene was also a major (>30 to 50%) fractionating phase ( Green, 1994). Fe–Ti oxide minerals occur in all group P lavas, and it is inferred that they fractionated and controlled abundances of TiO2 and V in the most evolved, <4% MgO, lavas which typically contain ∼20% Fe–Ti oxides. Relative to the general negative MgO–TiO2 and MgO–V trend, these lavas are offset to lower TiO2 and V contents ( Figs 5 and 6) indicating that the bulk DTi and DV were greater than unity during formation of these lavas. This requires 10–20% of Fe–Ti oxides in the fractionating assemblage ( Green, 1994). The inferred fractionating phase assemblage, initially dominant plagioclase and clinopyroxene which are joined by a significant proportion of Fe–Ti oxides with decreasing temperature, is consistent with the experimental results and calculations of Toplis & Carroll, (1995, 1996). The most evolved Mont Bureau lavas (GM92–42 and -40, with MgO ∼3.5%) contain abundant apatite, and they have lower P2O5 contents than Mont Bureau lavas with slightly higher MgO contents of 3.7–4.1% (Fig. 5). Fig. 12. Open in new tabDownload slide Initial 87Sr/86Sr vs 206Pb/204Pb for Mont Bureau and Mont Rabouillère samples. Fields and data sources are those described in Fig. 10 caption. Fig. 13. Open in new tabDownload slide 187Re/186Os vs 187Os/186Os for six archipelago lavas (flood basalts from Mont Bureau, Courbet Peninsula in northeast and Southeast Province and an ankaramite dike, Table 3). The basaltic lavas (i.e. excluding the dike) define a trend with a slope corresponding to an age of 29.6 Ma (decay constant 1.638 × 10−11; Linder et al., 1989) and an initial 187Os/186Os of 1.301 ± 0.045 (standard error). The low-Os, high-Re sample (KG4–1; Reisberg et al., 1993) is not blank corrected although it significantly affects the slope and therefore age of the trend. Blank correction would presumably lower the slope so we believe the integrated age estimate probably represents a maximum age of eruption. The intercept is clearly controlled by the high-Os, low-Re samples and is indistinguishable from these samples within the error of estimate. A more constrained assessment of the age significance of these preliminary data requires further Re–Os analyses. Fig. 14. Open in new tabDownload slide MgO content (wt %) vs initial 87Sr/86Sr, 143Nd/144Nd and 206Pb/204Pb, showing that the flood basalts can be divided into two distinct isotopic groups. Group P (P for plume) lavas have the isotopic characteristics proposed for the Kerguelen Plume by Weis et al., (1993, 1998). Although group P lavas from Mont Bureau and Mont Rabouillère have <6% MgO, Lower Miocene flood basalt lavas from the Southeast Province range to higher MgO contents, possibly as a result of clinopyroxene and olivine accumulation ( Weis et al., 1993). Group D (D for relatively depleted) have lower 87Sr/86Sr, higher 143Nd/144Nd and generally higher MgO contents ( ≥ 6%). In general, the varying abundances of trace elements in group P lavas are consistent with control by a fractionating assemblage of plagioclase, clinopyroxene, Fe–Ti oxides and apatite. Using plots such as Fig. 7 ( Hanson, 1989), the order of compatibility during evolution of the most mafic group P samples is Th < La < Ce < Nb < Ta ∼ Zr < Hf < Nd < Ba < Sm < Ti ∼ Eu < Tb < Yb ∼ Y < Lu < Na < Sr < Sc; however, in the seven most evolved samples, the compatibility of Ti was increased to be between that of Na and Sr. The mobile elements, K, Rb, U and Pb are not considered, as they were affected by alteration. This order of compatibility differs from that for mantle melting in that Ba and Sr are more compatible than light REE (LREE), and Nb and Ta are more compatible than La, reflecting the role of plagioclase and Fe–Ti oxide fractionation, respectively. Consequently, with decreasing MgO content, Sr/Nd and Ba/Th decrease because of plagioclase fractionation, Zr/Nb and La/Nb increase because of the relative compatibility of Nb in fractionating Fe–Ti oxides, P/Ce decreases and Hf/Sm increases in the most evolved lavas because of the relative compatibility of REE in fractionating apatite, and Zr/Hf and La/Yb increase because of abundant clinopyroxene in the fractionating assemblage (Fig. 15). Finally, the low Ni abundances in these lavas (Fig. 4b) reflects an early role for olivine fractionation which is a phenocryst phase in the most MgO-rich group P lavas. In summary, the compositional variations of group P lavas with MgO contents ranging from 4.3 to 5.9% show that fractionation of plagioclase and augite was dominant, and at <4.4% MgO in the magmas, Fe–Ti oxides and minor apatite were also important fractionating phases. However, the group P lavas with the highest MgO contents are not suitable parental lavas for the low-MgO lavas. That is, sample GM92–59 (5.69% MgO) from Mont Bureau has anomalously low contents of TiO2 and Fe2O3 and high contents of CaO and Al2O3 (Fig. 5), which probably reflect accumulation of plagioclase (see mode in Table 4); and sample GM92–144 (5.85% MgO) from Mont Rabouillère is too enriched in Na2O (Fig. 5), which is probably a result of post-magmatic alteration, as this sample has anomalously low Rb/Nb and K/Nb (Fig. 9). Compositional variations with stratigraphic height The temporal compositional variations within group P lavas suggest episodic magma input into a fractionating magmatic system. For example, at Mont Bureau all samples from the upper 325 m belong to group P (Fig. 4). In the interval from 300 to 400 m there are abrupt oscillations between the MgO extremes (5.12–3.37%). From 410 to 480 m, there are five lavas that define a trend of increasing abundances of incompatible elements and decreasing MgO, CaO and TiO2 contents (Fig. 4 and Table 1). Overlying this sequence from 510 to 620 m, the lavas range in MgO over a narrow range (4.40–4.95%). The uppermost lava, GM92–29, has a relatively high iron content and the highest TiO2 and V contents ( Figs 5 and 6); thus it defines the composition where further fractionation results in an important role for Fe–Ti oxide segregation. Within the Mont Rabouillère section from 200 to 240 m there is also an upsection decrease in MgO (5.63–2.49%) and CaO contents accompanied by increasing abundances of incompatible elements (Fig. 4 and Table 1). Above and below this sequence most of the group P lavas range in MgO from 3.8 to 4.5%. Within both sections the relatively minor compositional variations of group P lavas were interrupted by extrusion of the isotopically distinct and relatively mafic lavas forming group D; at Mont Bureau such lavas occur only in the lower 280 m but a group D lava occurs high in the Mont Rabouillère section (Fig. 4). The homogeneity of group P lavas Although there are geochemical variations within group P lavas, what is most surprising about these lavas is their long-term compositional homogeneity (e.g. the 21 group P flows from Mont Bureau vary only from 3.37 to 5.69% MgO, and 19 of the 24 group P lavas from Mont Rabouillère range only from 3.46 to 4.61% MgO); also, abundances of Ni range from only 20 to 50 ppm in 33 of the 45 group P lavas (Fig. 4b). This homogeneity persisted for more than 1 my [Fig. 4 and Nicolaysen et al., (1996)]. In fact, a dominance of evolved, nearly aphyric lavas is not unusual for ocean islands. For example, Reynolds & Geist, (1995) found that Sierra Negra volcano in the Galapagos Islands is composed dominantly of lavas with 5–6% MgO. Neither the Sierra Negra or group P Kerguelen Archipelago lavas are characterized by relatively low magma density. Reynolds & Geist, (1995) inferred that 'some property other than buoyancy produced by low pressure fractionation controls eruptibility in the hotspot setting’. Fig. 15. Open in new tabDownload slide Abundance ratios of various incompatible elements vs MgO content (%). Within group P these ratios vary because of crystal fractionation. In contrast, the higher Sr/Nd and Ba/Th of group D lavas is interpreted as a source characteristic. Arrows indicate ratios of primitive mantle ( Sun & McDonough, 1989). Two types of processes have been proposed for buffering magma compositions. The conventional interpretation is that the dominance of nearly homogeneous evolved lavas reflects achievement of near steady-state compositions in a magma chamber that is subjected to periodic eruptions and replenishment ( O'Hara & Mathews, 1981; Albarède, 1985; Cox, 1988; O'Hara & Fry, 1996a, 1996b). The temporal compositional variations shown by some group P lavas are consistent with an evolving magma chamber (Fig. 4). A significantly different interpretation for buffered magma compositions is based on the recognition that magma chambers are likely to be dominated by crystal mushes ( Sinton & Detrick, 1992; Bergantz, 1995; Marsh, 1996). Bedard, (1993) proposed that in an oceanic ridge setting ascending primary magmas equilibrate with gabbroic cumulates. Also a role for magma–cumulate interactions has been recently emphasized for oceanic island lavas [ Clague et al., (1995) for Hawaii, and Albarède et al., (1997) for Reunion]. In particular, Albarède et al., (1997) proposed that a nearly uniform evolved magma composition reflects buffering in a cumulate-dominated environment. The structure of the Kerguelen Archipelago ( Recq et al., 1990) together with the range of mantle to crustal xenoliths found in dikes of alkaline rocks ( Grégoire et al., 1994, 1995, 1997) provide evidence for a thick section of cumulate rocks. The xenoliths can be divided into three main types ( Grégoire et al., 1995; Mattielli et al., 1996): (1) mantle harzburgite and dunite; (2) metamorphosed (granulite facies) ultrabasic to basic rocks which are dominantly two-pyroxene plus spinel assemblages including wehrlites, websterite, metagabbros and anorthosites, although there is also a group of clinopyroxene–ilmenite–spinel rocks and ilmenite metagabbros; many of these xenoliths have relict igneous textures; (3) hornblende- and biotite-bearing rocks interpreted as cumulates from alkalic magmas. If the composition of group P lavas was controlled by reactions with a cumulate pile, some of these xenolith types may have equilibrated with group P lavas. The mineral assemblages of Type 2 xenoliths include the phases plagioclase, clinopyroxene and ilmenite, which were important fractionating phases in creating group P lavas. Moreover, the relict cumulate texture in some xenoliths, and the presence of positive Eu anomalies, shows that many of these xenoliths originated as cumulate rocks in the crust ( Grégoire et al., 1994, 1995; Mattielli et al., 1996). Most of the Type 2 xenoliths are not isotopically similar to group P lavas; for example, they have lower 87Sr/86Sr and higher 143Nd/144Nd, but the two analyzed samples of one sub-type (2b, Grégoire et al., 1995; Mattielli, 1996) ranging from clinopyroxenites to ilmenite-bearing metagabbros have Sr and Nd isotopic ratios which overlap with those of group P lavas (see inset of Fig. 10). The Type 1 xenoliths also have Sr, Nd and Pb isotopic ratios similar to those of group P lavas [Fig. 10 and Mattielli et al., (1996)]. Although Type 1 xenoliths are highly depleted in basaltic constituents; i.e. <5% modal clinopyroxene, the occurrence of poikilitic pyroxenes and U-shaped chondrite-normalized REE patterns ( Mattielli et al., 1996; Grégoire et al., 1997) and the presence of incompatible element-rich melt inclusions in olivine ( Schiano et al., 1994) indicate that Type 1 xenoliths are metasomatized upper mantle. These xenoliths provide unambiguous evidence for melt–rock reaction; however, the high alkalinity of the reacting melt (Grégoire et al., 1996; Mattielli et al., 1996) shows that the migrating melt was not similar in composition to the group P lavas, which are not highly alkaline. In addition, the Mg-rich mineral compositions of Type 1 peridotite xenoliths, e.g. olivine Fo content >88, show that these xenoliths are not cumulates related to group P lavas, but they could have formed as residues from the partial melting process that created the parental magmas of group P lavas. In summary, extensive sequences of nearly aphyric and compositionally uniform evolved lavas may reflect a steady-state composition in a replenished and fractionating magma chamber or reaction of melts with a thick pile of cumulates. The thick 20 km crust of the archipelago ( Recq et al., 1990) is a setting where extensive reaction between melts and cumulates is expected. The Type 2 and 3 xenoliths in alkaline archipelago lavas document the presence of cumulate rocks in the deep crust, and Type 1 xenoliths show the importance of melt–rock reactions. Some of the Type 2 xenoliths have the geochemical characteristics required of cumulates related to group P lavas. These hypotheses for the generation of nearly compositionally uniform lavas do not explain the differences in steady-state compositions from volcano to volcano; e.g. from the moderately high MgO content (6–8%) of three-phase saturated Hawaiian shield lavas such as at Mauna Loa ( Rhodes, 1988) to the high Fe–Ti lavas at some Galapagos and Icelandic volcanoes (Sierra Negra, Reynolds & Geist, 1995; Katla, Jakobsson, 1979) to the low MgO (3.4–5.1%) group P lavas of the Kerguelen Archipelago (Fig. 4). Magma supply is an important controlling parameter; that is, plumes are characterized by uniform magma supply for a significant time period (1–2 my), but the magnitude of this supply rate varies from plume to plume and at individual plumes on longer time scales. When the magma supply rate is high the magma composition is buffered at moderately high MgO content and the cumulate pile is maintained at high temperature and is olivine rich (dunite). Low magma supply results in a more evolved steady-state magma composition and a cool cumulate pile (gabbro). The latter case is consistent with these group P archipelago lavas. The origin of isotopic diversity in the flood basalts Summary of temporal isotopic trends in the Kerguelen Archipelago Plutonic and volcanic rocks in the archipelago range widely in initial isotopic ratios of Sr, Nd and Pb ( Dosso et al., 1979; Dosso & Murthy, 1980; Gautier et al., 1990; Weis et al., 1993, 1998; Weis & Giret, 1994). The studied volcanic record extends from ∼0.1 Ma at Mont Ross ( Weis et al., 1998) to the oldest Mont Bureau lava, 30 Ma; the plutonic rocks range in age from 5 to 40 Ma [K/Ar and Rb–Sr isochron ages, summarized by Giret, (1990) and Weis & Giret, (1994)]. Therefore, combining the isotopic data sets for plutonic and volcanic rocks allows evaluation of temporal isotopic trends throughout formation of the archipelago (Fig. 16). A gradual increase in mean 87Sr/86Sr with decreasing age has been emphasized ( Storey et al., 1988; Gautier et al., 1990; Weis et al., 1993; Weis & Giret, 1994), but lavas and plutonic rocks with 87Sr/86Sr >0.7050 have persisted throughout growth of the archipelago (Fig. 16). The major temporal change is that from ∼40 to 28 Ma there was considerable isotopic heterogeneity in coeval samples; for example, in the lava sections from Mont Bureau and Mont Rabouillère and the three ∼40 Ma gabbroic plutons (Fig. 16). The trend for mean 87Sr/86Sr to increase with decreasing age of archipelago samples results from the absence of low (<0.7050) 87Sr/86Sr in <22 Ma rocks. Fig. 16. Open in new tabDownload slide Initial 87Sr/86Sr vs age for plutonic and volcanic rocks of the Kerguelen Archipelago. The oldest, ∼40 Ma, rocks are three gabbroic plutons. Like flood basalts from Mont Bureau and Mont Rabouillère, the gabbros range widely in initial 87Sr/86Sr, from ∼0.7045 to ∼0.7055. In contrast, none of the <28 Ma samples have relatively low initial 87Sr/86Sr; they range from 0.70483 to 0.70573. Data for plutonic rocks from Weis & Giret, (1994); data for volcanic rocks from this paper and Weis et al., (1993, 1998); ×, four Mont Bureau samples analyzed by White & Hofmann, (1982). Evaluation of previous models Storey et al., (1988) and Gautier et al., (1990) used isotopic and compositional data for archipelago lavas from diverse locations to conclude that there was a transition from older tholeiitic to younger alkalic volcanism that was accompanied by increasing 87Sr/86Sr and decreasing 143Nd/144Nd. Although Hawaiian volcanoes also evolve from tholeiitic shield lavas to post-shield alkalic lavas, this inferred temporal isotopic trend of Kerguelen Archipelago lavas is opposite to the trend for individual Hawaiian volcanoes ( Weis & Frey, 1996). The archipelago trend was interpreted as resulting from the change in tectonic setting from a ridge-centered plume at ∼40 Ma to the present intraplate location; that is, the low 87Sr/86Sr and high 143Nd/144Nd of the older tholeiitic rocks reflect a larger proportion of a MORB component than in the younger, plume-dominated alkalic lavas. Weis et al., (1993, 1998) studied the youngest archipelago lavas and emphasized that these define the high 87Sr/86Sr, low 143Nd/144Nd and low 206Pb/204Pb extremes for archipelago lavas ( Figs 10, 11 and 12). Given the present intraplate environment, they proposed that this extreme is characteristic of the Kerguelen Plume. This interpretation has been debated by Class et al., (1993, Frey & Weis, (1995, 1996). Several aspects of the new data for the flood basalts bear significantly on these interpretations: Isotopic variations in lavas from the sections at Mont Bureau and Mont Rabouillère are not correlated with alkalinity of the lavas; the complete isotopic range found in archipelago lavas occurs in the transitional Mont Bureau lavas. Lavas with relatively high 87Sr/86Sr, low 143Nd/144Nd and low 206Pb/204Pb occur near the base of the exposed sections and throughout the lava pile (Fig. 4). The abrupt isotopic changes, e.g. 87Sr/86Sr typically steps from ≥ 0.7052 to ≤ 0.7045 (Fig. 4), require rapid changes in source components or their proportions, but the absence of a systematic temporal trend from ∼40 to 22 Ma (Fig. 16) is inconsistent with a simple MORB–plume mixing model whereby the MORB proportion systematically decreases as the SEIR migrated away from the plume ( Storey et al., 1988; Gautier et al., 1990). The isotopic characteristics of the oldest group P flood basalts, the lowermost Mont Bureau lavas GM92–58 and GM92–59, are similar to those of the youngest alkalic lavas erupted in the archipelago. For example, they are at the high 87Sr/86Sr and low 143Nd/144Nd extremes of archipelago lavas, and they also have the low 206Pb/204Pb that is characteristic of the youngest archipelago lavas ( Figs 10, 11 and 12); however, an important difference is that Mont Bureau lavas have lower 207Pb/204Pb than the Upper Miocene to Pleistocene lavas erupted in the Southeast Province (Fig. 11b). Although group P lavas from the three studied regions of flood basalt overlap in 87Sr/86Sr and 143Nd/144Nd (Fig. 10), each region has distinctive Pb isotopic ratios; that is, the similar age, ∼28–30 Ma, lavas from the northern part of the archipelago (Mont Bureau and Mont Rabouillère) differ in 207Pb/204Pb and the younger, ∼22 Ma, Lower Miocene flood basalt lavas from the southeast have higher 206Pb/204Pb (Fig. 11b and c). In summary, a component with relatively high 87Sr/86Sr and low 143Nd/144Nd is volumetrically significant in the flood basalts forming the Kerguelen Archipelago; it has persisted throughout evolution of the archipelago; and it is also present in the ∼81 Ma basalts erupted at drill Site 216 on the Ninetyeast Ridge ( Frey & Weis, 1995). We concur with previous interpretations that this pervasive and long-lived high 87Sr/86Sr and low 143Nd/144Nd is a signature of the Kerguelen Plume. In contrast, the local variability of Pb isotopic ratios requires small-scale heterogeneity in the plume or control of Pb isotopic ratios by entrained mantle or lithosphere components. The role of a continental component in the source of archipelago flood basalts Watkins et al., (1974) discussed the issue of a continental component in archipelago lavas. They and subsequent researchers (e.g. Dosso & Murthy, 1980; Weis et al., 1993) concluded that there is no compelling evidence for a continental component in archipelago lavas. However, the presence of a continental component in some lavas recovered from the Kerguelen Plateau ( Mahoney et al., 1995) and in a few mantle xenoliths in Kerguelen Archipelago lavas ( Hassler & Shimizu, 1995; Mattielli et al., 1996) has led to reconsideration of this issue. The isotopic characteristics of the flood basalts provide no compelling evidence for a continental component. None of the archipelago basalts have the extreme Sr, Nd and Pb isotopic ratios that require a continental component in the sources of Cretaceous basalts from Western Australia ( Frey et al., 1996) and ODP Site 738 at the southern extremity of the Kerguelen Plateau ( Mahoney et al., 1995). Also, none of the six archipelago samples analyzed have the low Os isotopic ratios that indicate a continental lithosphere origin for some of the harzburgite xenoliths ( Hassler & Shimizu, 1995). Another manifestation of continental crust is relative depletion in Nb and Ta, and this signature is present in several lavas recovered from submarine plateaux in the eastern Indian Ocean [Fig. 17 and Mahoney et al., (1995)]. Earlier studies showed that lavas from the Ninetyeast Ridge ( Frey et al., 1991; Saunders et al., 1991) and Kerguelen Archipelago ( Frey & Weis, 1996) are not depleted in Nb and Ta. This conclusion is substantiated by (Th/Ta)N, (La/Ta)N and (La/Nb)N near unity in these sections of archipelago flood basalts (Fig. 17). In addition, most of the flood basalts have Ce/Pb and Nb/U ratios typical of OIB; for example, 19 of 30 lavas have Nb/U of 40–71 and Ce/Pb of 27–39, compared with the averages for oceanic basalts of 47 ± 10 and 25 ± 5, respectively ( Hofmann et al., 1986). The deviations from these ranges do not vary systematically with MgO or isotopic ratios. We attribute these discrepant ratios to the effects of acid leaching on U and Pb abundances. In summary, there is evidence for a continental component in some refractory xenoliths in archipelago lavas, but there is no evidence for such a component in the source of the archipelago flood basalts. Is there isotopic evidence in the archipelago flood basalts for a component derived from the submarine Kerguelen Plateau? Coffin & Eldholm, (1994) used ages of Kerguelen Plateau lavas, plate reconstructions and changes in bathymetry to conclude that the Kerguelen Archipelago was not constructed on the Cretaceous Kerguelen Plateau. This interpretation is also consistent with the satellite-derived free-air gravity field [see fig. 4 of Coffin & Gahagan, (1995)]. Nevertheless, Class et al., (1996) argued for a Cretaceous lithosphere component in archipelago lavas. Frey & Weis, (1996) provided several arguments against this interpretation. Additional evidence is that the 207Pb/204Pb vs 206Pb/204Pb plot shows that none of the archipelago flood basalts have the relatively high 207Pb/204Pb at a given 206Pb/204Pb that is characteristic of lavas recovered from the Kerguelen Plateau (Fig. 11b). Furthermore, archipelago lavas lack the Nb–Ta depletion that is characteristic of some Kerguelen Plateau lavas (Fig. 17). We conclude that there is no evidence in the archipelago lavas for a component derived from the Cretaceous Kerguelen Plateau. Fig. 17. Open in new tabDownload slide La/Ta, Th/Ta and La/Nb normalized to primitive mantle ratios ( Sun & McDonough, 1989) showing that the archipelago flood basalts do not have the relative depletion in Nb and Ta that is found in some of the basalts from the Kerguelen Plateau and Broken Ridge ( Mahoney et al., 1995). Symbols as in legend for Fig. 15. Is there evidence in the archipelago flood basalts for a MORB source component? The linearity of the 87Sr/86Sr vs 143Nd/144Nd data for the flood basalts which trend toward the field for Indian Ocean MORB (Fig. 10) is consistent with mixing between plume and MORB components with similar Sr/Nd (e.g. Gautier et al., 1990). The role of two-component mixing, e.g. SEIR MORB and plume, is readily evaluated on Pb–Pb isotope ratio plots because mixing trends are linear. In general in Pb isotopic space (Fig. 11b and c), group P lavas from Rabouillère overlap with the younger lavas erupted in the Southeast Province which have been interpreted as representative of the plume ( Weis et al., 1993, 1998). All Mont Bureau lavas and group D Rabouillère lavas lie between this proposed field for the plume and the field of SEIR MORB (Fig. 11b and c). However, no simple linear mixing trends are defined by the data, and there is no evidence for the low 206Pb/204Pb (<18.0) which is found in many Indian Ocean MORB. Moreover, the offset of the Lower Miocene flood basalts of the Southeast Province to relatively high 206Pb/204Pb is not explained by mixing of SEIR MORB with the field proposed for the plume (Fig. 11b and c). A component with high 206Pb/204Pb is also required by lavas from ODP Site 756 on the Ninetyeast Ridge ( Weis & Frey, 1991). In conclusion, the isotopic data are inconsistent with simple mixing of isotopically homogeneous plume and MORB-related components with the proportion of MORB-related component systematically decreasing with time. Mixing between the plume and an isotopically heterogeneous MORB source could explain many of the data, but some archipelago flood basalts, especially those from the southeast, have higher 206Pb/204Pb than proposed for the plume or found in SEIR MORB erupted distant from islands (Fig. 11b and c). Does the isotopic diversity of the flood basalts reflect plume heterogeneity? At Mont Bureau and Mont Rabouillère the high 87Sr/86Sr, low 143Nd/144Nd and low 206Pb/204Pb endmember occurs only in low (<6%) MgO lavas (Fig. 14). The simplest interpretation is that these low-MgO lavas were derived from a mafic source rather than from MgO-rich peridotite. Evidence against this interpretation is that: (1) in the Lower and Upper Miocene suites from the Southeast Province, some lavas with group P isotopic characteristics have high MgO (10–11.6%); however, these high MgO contents may reflect accumulative clinopyroxene and olivine [see tables 1 and 2 of Weis et al., (1993)]; (2) MgO-rich harzburgite xenoliths with the isotopic characteristics of group P lavas may be residual rocks related to group P lavas; however, these xenoliths were metasomatized so the source of their isotopic signature is uncertain ( Mattielli et al., 1996); (3) the initial Os isotopic ratios, 187Os/186Os ∼1.24–1.30, are inconsistent with derivation from an aged mafic source. These observations are suggestive of a MgO-rich parental magma for group P lavas. A more complex petrogenetic interpretation for group P lavas is that they evolved from group D lavas by combined fractionation and assimilation processes. This interpretation is unlikely because: (1) during the evolution of alkalic basalt to trachyte and basanite to phonolite suites erupted in the southeast part of the archipelago, there are no isotopic changes that can be attributed to assimilation of isotopically distinctive components ( Weis et al., 1993, 1998); (2) in addition, the absence in group P lavas of the distinctive geochemical characteristics of group D lavas, such as positive Eu anomalies and relative deficiency in Th (Fig. 8), is inconsistent with derivation of group P lavas from group D lavas. We conclude that the limited isotopic range of group P lavas may be intrinsic to the plume, but that the isotopic diversity of the group D lavas reflects a non-plume source. Constraints on the origin of group D flood basalts Weaver, (1991) considered the Kerguelen Archipelago as an EM-1 type of OIB and suggested that this OIB group has relatively high Ba/Th, Ba/La and Ba/Nb which may reflect relatively high Ba contents in recycled pelagic sediments. In the Mont Bureau and Mont Rabouillère lavas these ratios are highly correlated (Fig. 18), with the highest Ba/X ratios occurring in the lavas with relatively high MgO (Ba/Th vs MgO panel in Fig. 15) and low Th content (Fig. 19), i.e. the group D lavas which may have experienced less crustal processing than group P lavas. Abundance ratios among these elements are also correlated with isotopic ratios (Fig. 18). Therefore, relative to group P samples, lavas in group D had a source component with relatively lower 87Sr/86Sr, higher 143Nd/144Nd and higher Ba/X. What is this source component? The trend to low 87Sr/86Sr precludes pelagic sediment. Previously we showed that lavas in group D are also characterized by high Sr/Nd and positive Eu anomalies ( Figs 15 and 8b, respectively). Although these geochemical features are diagnostic indicators of plagioclase, among these 10 group D lavas only four (GM92–48, -50, -52 and -146) contain abundant, 10–15%, plagioclase phenocrysts or microphenocrysts and four others contain no or only sparse phenocrysts of plagioclase (Table 2). Therefore, as a group these are not plagioclase-rich cumulates, but they may contain a melt component derived from plagioclase-rich cumulate gabbros. An enigmatic geochemical characteristic of group D lavas is their relative deficiency in Th (Fig. 8a). Also, the lower 208Pb/204Pb at a given 206Pb/204Pb of group D lavas relative to group P lavas (Fig. 11a) indicates evolution in a relatively low Th/U environment; hence, the Th deficiency may reflect a source characteristic. Hofmann & Jochum, (1996) noted that Hawaiian shield lavas have much lower Th/Ba than primitive mantle and average depleted MORB; group D archipelago lavas also have anomalously low Th/Ba ratios (Fig. 19). Hofmann & Jochum, (1996) proposed that low Th/Ba is a source characteristic of Hawaiian lavas and speculated that this is evidence for recycled plagioclase-rich oceanic gabbros as a magma source. The association of low Th/Ba with high Sr/Nd and positive Eu anomalies in group D lavas is consistent with the speculation of Hofmann & Jochum, (1996). A recent geochemical study provides further support for cumulate rocks as an important source for group D lavas. Cumulate mafic rocks associated with harzburgite in the Oman Ophiolite contain abundant plagioclase and very little trapped melt ( Benoit et al., 1996). Consequently, these cumulates have positive Eu anomalies and range to very high Sr/Nd and Ba/Th, >1000 in several samples; they also range to 0.7044 in 87Sr/86Sr ( Benoit et al., 1996). Therefore these Oman cumulates have the geochemical characteristics required for a source component of the archipelago group D lavas. In fact, the Ba/Th vs 1/Th trend defined by the flood basalts can be explained by mixing of partial melts derived from cumulates, such as the Oman troctolites, with a typical group P lava (Fig. 19). However, in detail, isotopic heterogeneity is required in at least one of the components to explain the scatter of group D lavas in the Ba/Th vs 87Sr/86Sr plot (Fig. 18) and the lack of simple mixing trends in isotopic plots ( Figs 10, 11 and 12); such heterogeneity occurs in the Oman troctolites ( Benoit et al., 1996). If group P is attributed to the Kerguelen Plume, what are possible explanations for the origin of group D? Based on incompatible element abundance ratios, the group D lavas can be explained as mixtures involving melts with group P characteristics and melts derived from plagioclase-rich cumulates (Fig. 19). The characteristic deficiency of Th in group D lavas is not present in lavas from the Ninetyeast Ridge; not even in the plagioclase-rich accumulates from ODP drill Site 757 ( Frey et al., 1991). Moreover, the isotopic characteristics of group D do not occur in younger, Upper Miocene to Pleistocene, archipelago lavas ( Weis et al., 1993, 1998) or in flood basalts from the Southeast Province ( Weis et al., 1993) or in a 1 km section from Mont Crozier in the northeast part of the archipelago ( Damasceno et al., 1997). Therefore lavas with group D isotopic ratios are not abundant, and they are confined to the time interval from ∼40 Ma (the gabbroic Anse du Jardin pluton in Fig. 16) to ∼24 Ma ( Nicolaysen et al., 1996). Therefore a component with group D isotopic characteristics is not a long-term characteristic of the plume. It is more likely that group D lavas are related to the thick lithosphere below the archipelago. Consistent with this inference, relatively young alkaline lavas in the archipelago contain xenoliths with relict cumulate textures, positive Eu anomalies and isotopic characteristics similar to group D lavas [see insert of Fig. 10 and Mattielli et al., (1996)]. In fact, a cumulate troctolite protolith was inferred for the granulite xenoliths ( Grégoire et al., 1994). Moreover, some of these xenoliths contain garnet ( Grégoire et al., 1995) and residual garnet could explain the crossing REE patterns of group D lavas in the Mont Bureau section (Fig. 8b). Fig. 18. Open in new tabDownload slide Ba/La and Ba/Th vs Ba/Nb (subscript N indicates normalized to primitive mantle ratios) showing that these abundance ratios are positively correlated with the highest ratios in group D lavas (symbols as in Fig. 15). Ratios such as Ba/Th and Nb/Th are also correlated with 87Sr/86Sr and 143Nd/144Nd, respectively, but two group D samples (GM92–46 and GM92–55) from Mont Bureau deviate from the general trends. Fig. 19. Open in new tabDownload slide (a) Th/La vs Th/Ba normalized to primitive mantle showing that group D (filled symbols) basalts have relative depletions in Th; this is also a characteristic of Hawaiian lavas ( Hofmann & Jochum, 1996). (b) In the Ba/Th vs 1/Th panel the three mixing lines are defined by a typical group P sample with 1/Th = 0.33 and Ba/Th = 85 mixing with three melts derived by 10% batch melting (DTh = 0 and DBa = 0.1) of Oman gabbros with Ba/Th ratios of 795, 3380, and 6522 ( Benoit et al., 1996). Ticks on lines are increments of 10% mixing; symbols as in Fig. 15. Where did these cumulate rocks form? When and how were they introduced into the lithosphere below the Kerguelen Archipelago? Cumulate troctolites from the Oman Ophiolite have the geochemical characteristics required for the source of group D lavas [Fig. 19 and Benoit et al., (1996)]. Therefore a plausible scenario is that these cumulates formed in the oceanic lithosphere at ∼43 Ma when the SEIR was coincident with the Kerguelen Plume. Apparently, fragments of thesecumulates were incorporated into the lithosphere of the Kerguelen Plateau as the SEIR migrated to the northeast away from the Kerguelen Plume. From ∼43 to 26 Ma these cumulates contributed a melt component to the volumetrically minor group D lavas. Conclusions When combined with previous studies of lavas associated with the Kerguelen Plume, this study of flood basalts exposed in the northern part of the Kerguelen Archipelago leads to the following conclusions. From ∼40 to 0.1 Ma magmatism in the archipelago was dominated by a source with relatively high 87Sr/86Sr (>0.7050) and low 143Nd/144Nd (<0.5127). These geochemical features are attributed to the Kerguelen Plume. Flood basalts with these enriched isotopic characteristics are typically transitional basalts with only 4–6% MgO. Either the plume source was mafic rather than ultramafic, or MgO-rich parental melts were extensively modified during ascent through the thick, ∼20 km, crustal section. The latter hypothesis is preferred because: (a) a few lavas from the Southeast Province have high MgO and87Sr/86Sr, and (b) an isolated and old mafic composition in the plume would have higher Os isotopic ratios than the observed initial values of 187Os/186Os ∼1.24–1.30. Most of the oldest lavas studied (∼29–30 Ma) have geochemical features (low MgO content, major element compositions transitional between tholeiitic and alkalic, relatively high abundances of incompatible elements, and relatively high 87Sr/86Sr and low 143Nd/144Nd) that are not consistent with plume–MORB mixing at a ridge-centered plume. A small subset of the older flood basalts differ significantly in Sr and Nd isotopic ratios. These lavas trend to lower 87Sr/86Sr (to 0.70396) and higher 143Nd/144Nd (to 0.51289) but they do not represent mixtures of plume and MORB-related melts. The trace element abundance characteristics of these lavas require a role for a component derived by melting of a plagioclase-rich cumulate rock. These cumulates may have formed in ∼43 Ma oceanic crust at the Southeast Indian Ridge (SEIR); subsequently, as the SEIR migrated away from the plume these cumulate rocks were melted by the plume. This relatively low 87Sr/86Sr and high 143Nd/144Nd component with a cumulate history does not occur in <24 Ma rocks in the archipelago, but it is found as xenoliths of cumulate rocks in young alkaline dikes. The temporal history recorded in two sections of flood basalts shows abrupt and large geochemical variations, including isotopic ratios of Sr, Nd and Pb, in the oldest lavas, but within 1 my this heterogeneity subsided, and the lavas achieved a near steady-state composition with isotopic characteristics attributed to the Kerguelen Plume. There is evidence for a continental component in some ∼110 Ma basement lavas of the Kerguelen Plateau and in the isotopic ratios of a few dunite and harzburgite xenoliths found in young alkaline Kerguelen Archipelago lavas. However, this continental component has not contributed to the magmatic products of the Kerguelen Archipelago. Acknowledgements This research was supported by NSF Grants OPP-9417774 and EAR-9614532 and Belgian Grant FNRS 1.5.019.95F. The Belgian Francqui Foundation is especially thanked for its support of scientific collaboration between US and Belgian scientists. This research would not be possible without the support of the French IFRTP for field efforts in the Kerguelen Archipelago. We thank Dr P. Ila for supervision of the MIT Neutron Activation Facility, Dr J. M. Rhodes for access to the University of Massachusetts X-ray Fluorescence Facility, J. P. Mennessier for help with the chemical processing for Sr, Nd and Pb isotopic analyses at ULB, and Dr S. Hart and Dr G. Ravizza for instruction in Os chemistry and mass spectrometry. We thank Dimitri Damasceno de Oliveira for help with petrographic descriptions. F. Albarède, D. Geist, J. Mahoney and A. Saunders are thanked for their constructive reviews. References Albarède F. . 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TI - Petrogenesis of the Flood Basalts Forming the Northern Kerguelen Archipelago: Implications for the Kerguelen Plume
JO - Journal of Petrology
DO - 10.1093/petroj/39.4.711
DA - 1998-04-01
UR - https://www.deepdyve.com/lp/oxford-university-press/petrogenesis-of-the-flood-basalts-forming-the-northern-kerguelen-fNPIIgCaNN
SP - 711
EP - 748
VL - 39
IS - 4
DP - DeepDyve
ER -