Geochemistry, Geophysics, Geosystems

Sleep, Norman H.

doi: 10.1029/2003GC000511pmid: N/A

Studies of xenolith suites yield depth‐temperature arrays for the lithosphere at the time of eruption. These studies constrain the conductive heat flow through the lithosphere and limit the temperature range across the rheological boundary layer to <300 K. Numerical models of convection illustrate features that may be found within suites of samples. An individual dike may sample the geotherm along a finite length of its strike having variable geotherms, causing scatter in the depth‐temperature array. In a suite from the Slave Province of Canada, local heating within a deep stockwork is a more likely cause of scatter. Plume material ponded beneath a craton may yield a linear array extrapolating to the shallow geotherm without any noticeable kink or curvature. This situation existed beneath Lesotho in South Africa at ∼90 Ma. Convection at the base of the lithosphere may produce stress concentrations that trigger dike formation. Dikes may stagnate and form stockworks in low‐stress regions above the thermal boundary layer.

Gnos, E.; Peters, T.

doi: 10.1029/2001GC000229pmid: N/A

Mantle xenolith‐bearing alkali basalts, basanites and tephrites with 1.5–2 K2O and 4–5 wt% Na2O occur as small (<100 m) plugs and dikes in the Batain and Haushi‐Huqf areas, and WNW of Muscat. Their black color and the common presence of peridotite xenoliths allow a separation from older alkaline rocks and from the ophiolitic extrusive rocks. Two main dike directions, approximately E‐W and N‐S oriented, are observed. Intrusions seem to occur where these two fault systems intersect. The basalts crosscut the Upper Maastrichtian siliciclastic rocks of the Fayah Formation and the nappe stack of the Eastern Ophiolite Belt of the Batain area. They have not been observed intruding the Tertiary shallow marine carbonates although K‐Ar whole rock dating on these lavas yielded Late Eocene 37 ± 1 to 44 ± 1 Ma ages. The rocks are aphyric and fine‐grained with microgranular, and less common microtrachytic texture. They contain magmatic olivine (Fo80–82), nepheline (Ne82–86Ks14–18), clinopyroxene (Di46En25Wo28) with 2.5 wt% Al2O3, accessory phlogopite (XMg0.6, with 5.5 wt% TiO2), in a microcrystalline or glassy matrix. Plagioclase is only observed in few samples. Locally occurring mm to cm‐sized immiscible leucocratic melt droplets consist of potassium feldspar (An1–4Ab37–39Or60–70), plagioclase (An20–22Ab65Or13–15), nepheline (Ne85Ks15), phlogopite (XMg0.8, with ∼6–7 wt% TiO2), and titanomagnetite with ∼8 wt% TiO2. More than 95% of the xenoliths are <5cm‐sized weakly to clearly foliated spinel peridotites of mantle origin. Sedimentary xenoliths (hornfelses) are rare and lower crustal xenoliths were not found. The peridotite xenoliths consist of olivine (Fo90–92), enstatite (En89Fs09Wo02) with 3.1–3.3 wt% Al2O3, diopside (Di45En26Wo29) with 3–6 wt% Al2O3, and Cr‐spinel with ∼60 mol% spinel, 20% hercynite, and 15–16% magnesiochromite. The age, chemical composition, and structural position suggest a relation with local tectonic movements associated with plate reorganization in the Owens Basin region prior to the Red Sea opening.

Yamazaki, Toshitsugu; Seama, Nobukazu; Okino, Kyoko; Kitada, Kazuya; Joshima, Masato; Oda, Hirokuni; Naka, Jiro

doi: 10.1029/2002GC000492pmid: N/A

We have conducted a geophysical survey of the northern Mariana Trough from 19°N to 24°N. The trough evolves southward from incipient rifting to seafloor spreading within this region. This study aims to clarify the location and time of the rifting‐to‐spreading transition, which was controversial previously, and processes of seafloor spreading after the transition. The new data set includes swath bathymetry with side‐scan images and magnetic vector anomaly. The mantle Bouguer gravity anomaly (MBA) was calculated using the free‐air gravity anomaly from satellite altimetry. The rifting‐to‐spreading transition occurs at about 22°N, which is proved by seafloor‐spreading fabric in the bathymetry, clear magnetic lineations, and the bull's‐eye pattern in MBA. Four ridge segments separated by three nontransform discontinuities are recognized between 19°N and 22°N. The northernmost segment has relatively abundant magma supply compared with the other segments, which is estimated from a larger segment length, shallower axial depths with no rift valley, and lower MBA. The next segment to the south is, on the other hand, a magma‐starved segment with a prominent rift valley. Two anomalously deep grabens (called the Central Grabens) formed by amagmatic extension occur near the segment ends. The succession of magma‐rich, magma‐starved, and normal segments with increasing distance from the volcanic arc is the same as the observation in the Lau Basin reported by Martinez and Taylor (2002). The magnetic anomaly revealed the detailed history of the spreading. The seafloor spreading between 19°N and 20°N began prior to 5 Ma, and that between 20°N and 21°30′N began at about 4 Ma. Spreading half‐rates in the western side of the spreading center were 2 to 3 cm/year before 2.58 Ma south of 21°30′N and during the Matuyama Chron north of 21°30′N, but an average during the Brunhes Chron is 1 cm/year or less. Orientations of the ridge axes, which range from −20° to 0° at present, have rotated about 20° clockwise since the start of the spreading. These changes in rate and direction might be associated with changes in the motion of the Philippine Sea plate. Spreading has been asymmetric in the northern Mariana Trough. The spreading rates of the western side of the spreading center have been significantly larger than the eastern counterpart in general. The asymmetry may have been caused by an interaction of mantle upwelling systems under the volcanic front and the backarc spreading center and would be a characteristic of backarc spreading.

Lundstrom, C. C.

doi: 10.1029/2001GC000224pmid: N/A

When a silica‐undersaturated melt is juxtaposed with partially molten peridotite, alkali elements rapidly diffuse into the peridotite in a process referred to as the diffusive infiltration of alkalis (DIA). Four types of piston cylinder experiments were performed providing constraints on the DIA process: (1) simple phase relation experiments, (2) melt‐melt diffusion couples, (3) experiments examining the reaction between powders of basanite and spinel lherzolite (direct mixture experiments), and (4) diffusive infiltration‐reaction couples between basanite and partially molten peridotite. Melt‐melt diffusion couples constrain effective binary diffusion coefficients (EBDC) for all major elements except Na to be in the 10−6 to 10−7 cm2/s range at 1450°C and 0.9 GPa. The Na concentration profile is not binary being coupled to gradients in SiO2. EBDCs for Cl, Li, Rb, Sr, Ba, and La are in the 10−6 cm2/s range while Nb and Zr are in the 10−7 cm2/s range. A time series of direct mixture experiments shows that basanite reacts with lherzolite to reduce the orthopyroxene mode to a constant value within 15 min at 1300°C and 0.9 GPa. Quench modified melts in the direct mixture experiments have 50–51 wt.%. SiO2, independent of the basanite/lherzolite ratio. In contrast, infiltration‐reaction experiments show that sodium diffuses from basanite into partially molten peridotite in 10–30 min, resulting in quench modified melt pools with up to 64 wt.% SiO2 within the peridotite. Modal analysis shows that addition of alkalis causes orthopyroxene to incongruently break down to olivine plus silica rich melt. In experiments using a basanite with enriched Cl concentrations, a distinct boundary zone appears between the basalt‐peridotite interface and the peak in SiO2 concentration. Melts within this boundary zone have elevated CaO and Cl concentrations relative to both the basanite and melts within the peridotite beyond the boundary zone. The repeated formation of this boundary zone may indicate an important aspect of the DIA process, possibly responsible for the formation of anorthitic plagioclase and CaO rich melts.

Engels, Jennifer L.; Edwards, Margo H.; Fornari, Daniel J.; Perfit, Michael R.; Cann, Johnson R.

doi: 10.1029/2002GC000483pmid: N/A

Collapse pits and an associated suite of collapse‐related features that form in submarine lava flows are ubiquitous on the global mid‐ocean ridge crest. Collapse pits, the lava tube systems they expose, and lenses of talus created by the collapse process combine to produce a permeable region in the shallow ocean crust and are thought to contribute significantly to the 100–300 m thick low velocity zone observed at intermediate to fast‐spreading mid‐ocean ridges. This horizon of low‐density, high‐porosity material is likely to be an important aquifer for the transfer of hydrothermal fluids in the upper ocean crust. In a recent survey of the East Pacific Rise at 9°37′N, we used photographs, video and observations from the submersible Alvin, and DSL‐120A side scan data to determine that 13% of the 720,000 m2 of seafloor imaged had foundered to form collapse pits. In 98% of the images collapse pits occurred in lobate flows, and the rest in sheet flows. On the basis of our observations and analyses of collapse features, and incorporating data from previous models for collapse formation plus laboratory and theoretical models of basalt lava behavior in the deep ocean, we develop a detailed multistage physical model for collapse formation in the deep ocean. In our model, lava extruded on the seafloor traps pockets of seawater beneath the flow that are instantly vaporized to a briny steam. The seawater is transformed to vapor at temperatures above 480°C with a 20 times expansion in volume. Bubbles of vapor rise through the lava and concentrate below the chilled upper crust of the lava flow, creating gas‐filled cavities at magmatic temperatures. Fluid lava from the cavity roofs drips into the vapor pockets to create delicate drip and septa structures, a process that may be enhanced by water vapor diffusing into the magma and reducing its melting point. As the vapor pocket cools, the pressure within it drops, causing a pressure gradient to develop across the upper crust. The pressure gradient often causes the roof crust to collapse during cooling, though vapor pocket geometry may be such that the roof remains intact during subsidence of the underlying lava. Alternatively, drainaway of the molten lava may cause collapse in locations where inflated lava roof crusts are not supported from below by bounding walls or lava pillars. Post‐eruption seismicity, lava movement, or hydrovolcanic explosions may cause continued collapse of the lava carapace after the eruption.

Hall, Paul S.; Kincaid, Chris

doi: 10.1029/2003GC000567pmid: N/A

We present results from a series of two‐dimensional numerical experiments in which a thermally buoyant, off‐axis mantle plume interacts with a nearby ridge axis. These experiments incorporate melting and a number of related dynamical feedbacks, including energy loss to latent heating and Fe depletion buoyancy as well as viscosity increases due to dehydration during melting. Results indicate that dehydration, which acts to increase the viscosity of the residual mantle during melting, profoundly impacts the morphology of flow between plume and ridge axis. As upwelling plume material begins to melt, it becomes significantly more viscous and readily accretes to the overlying lithosphere. This creates a viscous plug that grows downward from the base of the lithosphere directly above the plume conduit. Plume material traveling to the ridge axis is then deflected horizontally at subsolidus depths by this plug, allowing pristine, unmelted plume material to reach the ridge axis. Experiments without dehydration show that the loss of thermal energy to latent heating during melting marginally decreases the flow of plume material along the base of the lithosphere to the ridge axis. Alternatively, extraction of Fe from the solid during melting slightly increases plume flow to the ridge. These results also demonstrate that the flow of plume material to the ridge axis is sensitive to the slope of the base of the rheological lithosphere, consistent with previous studies of off‐axis plume ridge interaction.

Umino, Susumu; Miyashita, Sumio; Hotta, Fumiko; Adachi, Yoshiko

doi: 10.1029/2001GC000233pmid: N/A

Along‐strike variations of the 60‐km‐long sheeted dike complex in the northern Oman Ophiolite were studied in order to understand the shallow magma plumbing system beneath the fossil fast spreading ridge. The presence of numerous dikes intruding into the layered gabbro defines the northern end of the paleoridge segment at Wadi Fizh. The postulated segment center is located at Wadi Thuqbah, which has a thicker Mono transition zone than elsewhere along the fossil segment. Aphyric dikes predominate in the sheeted dikes, of which 99% are simple, while multiple and composite dikes are few. The thickness of 1511 dikes ranges from <1 cm to >13 m, with an average thickness of 71.3 cm. Restored dike trends in the 30‐km‐long northern half of the dike complex display the NS trending north domain and NNE‐NS trending south domain bordered at the south of Wadi Bani Umar al Gharbi. In the domain boundary, dikes gradually change strikes or are mutually intrusive. Dike thickens northward with the largest peak along Wadi Fizh at the northern end of the paleoridge segment and a small peak at Wadi Bani Umar al Gharbi. Most dikes have a bulk Mg# of 55–66, which overlaps the majority of MORB. Less common, highly evolved dikes with Mg# 34–40 characterize the north domain. Thicker dikes (>3 m) tend to have high Mg#, while thinner dikes (<2 m) are highly variable in Mg#. The regional variations of the dike trends and the whole rock compositions can be explained by the secular variation in the structures of the paleoridge segment comparable to a third‐order segment of the present mid‐ocean ridge system spanning a few tens of thousands of years. Initially, the segment‐long melt lens developed along the paleoridge axis, which fed long and thick dikes with high Mg#. With decreasing supply of magma, the melt lens split up into the north and south smaller lenses bordered by a DEVAL that fed thinner dikes intruding into the former thick dikes. Cut‐off of the northern melt lens from the magma source changed the melt composition to highly evolved, low‐Mg# magmas, which subsequently intruded as short evolved dikes. Meanwhile, the main melt lens in the segment center continued feeding the high‐Mg# dikes which were maintained by the larger melt lens size and intermittent magma supply from deep magma chambers.

Ridgwell, Andy J.

doi: 10.1029/2003GC000563pmid: N/A

The “iron hypothesis” posits a role for increased supply of mineral aerosol to the ocean surface during glacial periods in driving atmospheric CO2 lower; that changes in CO2 and climate strongly affect dust supply raises the possibility of feedback. Here I take a systems view in analyzing the properties and implications of such a feedback and consider three primary state variables that can be related empirically to each other: dust supply, atmospheric CO2, and “climate” (surface air temperature). The results of this analysis suggest that the dust‐CO2‐climate feedback is primarily an intraglacial phenomenon, when it can account for about a third of the temperature variability recorded in Antarctic ice cores. Since glacial‐interglacial cyclicity prior to ca. 800 kyr BP is characterized by the absence of a “full” glacial state (such as the Last Glacial Maximum), it is possible that destabilization of climate by the marine iron cycle is fundamental to the differences between “41 kyr” and “100 kyr” climatic regimes. The critical role played by the state of the land surface in this feedback also has implications for the longer‐term evolution of the Earth system during the Cenozoic.

Borissova, Irina; Coffin, Millard F.; Charvis, Philippe; Operto, Stéphane

doi: 10.1029/2003GC000535pmid: N/A

Microcontinents appear to commonly form on young continental margins close to hot spots, but difficulties in understanding their geology and evolution have inhibited assessment of their global distribution and significance. Thick volcanic accumulations in areas affected by hot spot magmatism only complicate the issue. Elan Bank, a large western salient of the Kerguelen Plateau, is a microcontinent that originally lay between India and Antarctica in Gondwana. Recent regional plate tectonic reconstructions suggest that during Gondwana breakup, Elan Bank and India initially separated from Antarctica, and Elan Bank became isolated in the Southern Ocean via a ridge jump to the north between Elan Bank and India. In Albian time (∼108 Ma), voluminous magmatism attributed to the Kerguelen hot spot overprinted and radically altered the original microcontinent and its surroundings. Recent ODP investigations, deep seismic reflection data, and a wide‐angle seismic line on Elan Bank allow us to gain the first insight into the feature's integrated crustal structure and geological evolution and the adjacent continent‐ocean transition zone. Our analysis shows that Elan Bank's crust is at least 16 km thick. The upper igneous crust consists of a 2–3 km thick layer with seismic velocities ranging from 4.4 to 5.9 km/s that can be interpreted as the result of accumulation of lava flows originating from the Kerguelen hot spot. Seismic velocities at the base of the crust are as low as 6.6 km/s, which is consistent with a fragment of thinned continental crust ∼14 km thick. A high velocity body, located at depths of 5 to 10 km, could be interpreted as plutonic rocks emplaced during the major regional magmatic episode. On the basis of deep seismic reflection data, we interpret extensional structures beneath the volcanic flows. In Albian time, when the area was affected by the Kerguelen hot spot, volcaniclastic material and lava flows accumulated in faulted grabens and basins both on the bank and within the continent‐ocean transition zone to the south, creating the appearance of flat, unstructured basement. The seismic structure and inferred composition of Elan Bank revealed by this study contribute to our understanding of microcontinent formation as well as provide a template for identifying microcontinents in accreted terranes and mountain belts.

Cherkaoui, Abdellah S. M.; Wilcock, William S. D.; Dunn, Robert A.; Toomey, Douglas R.

doi: 10.1029/2001GC000215pmid: N/A

There are two competing models for lower crustal accretion at fast spreading ridges. In the gabbro glacier model, all the lower crust solidifies in a magma lens at the dike/gabbro boundary and then flows to its final level. In the sheeted sill model, the lower crust is formed by sills emplaced at a variety of depths. We present a steady state numerical representation of the sill model that explicitly includes hydrothermal circulation. The crust is accreted uniformly at the ridge axis at all depths and hydrothermal circulation occurs in crustal rocks that have cooled below a threshold temperature. The results show that when the permeability exceeds a threshold value (∼4 × 10−14 m2 for a cracking temperature of 800°C and a uniform permeability), hydrothermal circulation can cool the entire crust near the ridge axis, producing vertical isotherms in the lower crust. At lower permeabilities, the properties of seawater lead to bimodal cooling with the lowermost crust cooling more slowly than shallower depths. These results are reasonably consistent with studies of the seismic structure of the East Pacific Rise and of plagioclase crystal size distributions in gabbros from Oman. We infer that the sheeted sill model cannot be discarded on thermal grounds.