The Stillwater Complex: Integrating Zircon Geochronological and Geochemical Constraints on the Age, Emplacement History and Crystallization of a Large, Open-System Layered Intrusion

The Stillwater Complex: Integrating Zircon Geochronological and Geochemical Constraints on the... Abstract The Neoarchean Stillwater Complex, one of the world’s largest known layered intrusions and host to a rich platinum-group element deposit known as the J-M Reef, represents one of the cornerstones for the study of magmatic processes in the Earth’s crust. A complete framework for crystallization of the Stillwater Complex is presented based on the trace element geochemistry of zircon and comprehensive U–Pb zircon–baddeleyite–titanite–rutile geochronology of 22 samples through the magmatic stratigraphy. Trace element concentrations and ratios in zircon are highly variable and support crystallization of zircon from fractionated interstitial melt at near-solidus temperatures in the ultramafic and mafic cumulates (Ti-in-zircon thermometry = 980–720°C). U–Pb geochronological results indicate that the Stillwater Complex crystallized over a ∼3 million-year interval from 2712 Ma (Basal series) to 2709 Ma (Banded series); late-stage granophyres and at least one phase of post-emplacement mafic dikes also crystallized at 2709 Ma. The dates reveal that the intrusion was not constructed in a strictly sequential stratigraphic order from the base (oldest) to the top (youngest) such that the cumulate succession in the complex does not follow the stratigraphic law of superposition. Two distinct age groups are recognized in the Ultramafic series. The lowermost Peridotite zone, up to and including the G chromitite, crystallized at 2710 Ma from magmas emplaced below the overlying uppermost Peridotite and Bronzitite zones that crystallized earlier at 2711 Ma. Based on the age and locally discordant nature of the J-M Reef, the base of this sequence likely represents an intrusion-wide magmatic unconformity that formed during the onset of renewed and voluminous magmatism at 2709 Ma. The thick anorthosite units in the Middle Banded series are older (2710 Ma) than the rest of the Banded series, a feature consistent with a flotation cumulate or ‘rockberg’ model. The anorthosites are related to crystallization of mafic and ultramafic rocks now preserved in the Ultramafic series and in the lower part of the Lower Banded series below the J-M Reef. The Stillwater Complex was constructed by repeated injections of magma that crystallized to produce a stack of amalgamated sills, some out-of-sequence, consequently it does not constitute the crystallized products of a progressively filled and cooled magma chamber. This calls into question current concepts regarding the intrusive and crystallization histories of major open-system layered intrusions and challenges us to rethink our understanding of the timescales of magma processes and emplacement in these large and petrologically significant and remarkable complexes. INTRODUCTION Layered intrusions play a key role in illustrating compositional diversity in magmas in the Earth’s crust (Wager & Brown, 1967; Parsons, 1987; Cawthorn, 1996; Charlier et al., 2015). They represent a critical link in the differentiation pathway of basaltic magma from partial melting in the uppermost mantle, through storage in crustal reservoirs to eruption, in many cases as voluminous and extensive flood basalts (e.g. Duluth Complex, Paces & Miller, 1993; Stillwater Complex, Helz, 1995; Muskox intrusion, Mackie et al., 2009). These intrusions also host world-class ore bodies of chromium, platinum group elements (PGE), and vanadium (e.g. Jackson, 1961; Naldrett et al., 1987; Cawthorn et al., 2005). From a temporal perspective, layered intrusions constitute parts of Earth’s earliest greenstone belts (e.g. layered sills associated with the Ujaraaluk unit – O’Neil et al., 2007, 2012; Stella intrusion; Maier et al., 2003) and are present throughout geologic time to the Cenozoic intrusions associated with continental and oceanic flood basalts (e.g. Eocene Skaergaard intrusion, East Greenland, Wotzlaw et al., 2012; Oligocene Val intrusion, Kerguelen Archipelago, Scoates et al., 2007). Studies of the Neoarchean Stillwater Complex, a large mafic–ultramafic layered intrusion in the Beartooth Mountains of southern Montana (USA), have profoundly influenced our understanding of the origin and evolution of igneous processes and mineralization in crustal magma chambers (e.g. Howland et al., 1936; Peoples & Howland, 1940; Hess, 1960; Jackson, 1961, 1969; Conn, 1979; Page, 1979; McCallum et al., 1980; Todd et al., 1982; Campbell et al., 1983; Irvine et al., 1983; Raedeke & McCallum, 1984; Boudreau, 1988, 2016; Zientek & Ripley, 1990; McCallum, 1996; Meurer & Boudreau, 1996; Godel & Barnes, 2008; Selkin et al., 2008; Keays et al., 2012; Aird & Boudreau, 2013; Barnes et al., 2015). In contrast, a systematic geochronological framework from the base to the top of the intrusion for assessing the age and duration of magmatism of the Stillwater Complex has not been established, despite the intrusion being the focus of numerous geochronological investigations since the late 1960 s (e.g. Fenton & Faure, 1969; Nunes & Tilton, 1971; DePaolo & Wasserburg, 1979; Premo et al., 1990). Although historically considered as relatively poor candidates for dating studies, mafic and ultramafic rocks of layered intrusions may locally contain small pockets of late-crystallized interstitial minerals where U–Th–Pb-bearing phases (e.g. zircon, baddeleyite, apatite, rutile) occur with quartz, Na-rich plagioclase, K-feldspar, or biotite (e.g. Scoates & Chamberlain, 1995; Schwartz et al., 2005; Scoates & Friedman, 2008; Grimes et al., 2009; Morisset et al., 2009; Zeh et al., 2015; Scoates & Wall, 2015; Mungall et al., 2016). During the past decade, major advances in sample pretreatment, instrument sensitivities, and data reduction protocols for U–Th–Pb geochronology (e.g. chemical abrasion-ID-TIMS or CA-ID-TIMS, Mattinson, 2005; EARTHTIME tracers and synthetic standards, Condon, 2005; Condon et al., 2015; McLean et al., 2015) have led to significantly improved precision and accuracy of dates (Schmitz & Kuiper, 2013; Schoene, 2014). Combined with the ability to efficiently extract zircon from layered intrusions based on selecting appropriate samples (e.g. Scoates & Wall, 2015; Zeh et al., 2015; Mungall et al., 2016), these advances open up the possibility of determining ages of crystallization for multiple samples throughout a stratigraphic sequence of cumulates, specifically for those intrusions that show mineralogical and geochemical evidence for open-system behaviour and magma replenishment events like the Stillwater Complex (e.g. Jackson, 1961; McCallum et al., 1980; Irvine et al., 1983; Wooden et al., 1991; Lipin, 1993; McCallum, 1996). Together with its use as a mineral chronometer, the trace elements of zircon allow for evaluating the geochemical evolution of crystallizing melt as a function of temperature and time (e.g. Finch & Hanchar, 2003; Whitehouse & Platt, 2003; Ferry & Watson, 2007; Harley & Kelly, 2007; Schoene et al., 2012; Barboni & Schoene, 2014; DesOrmeau et al., 2015; Samperton et al., 2015; Deering et al., 2016). In this study, a high-precision geochronologic framework for the crystallization of the Stillwater Complex is provided based on U–Pb dating results for 22 samples, including mafic and ultramafic cumulates representing the entire ∼6000 m thick stratigraphic sequence, mineralized units, and cross-cutting mafic dikes and granophyres. Samples collected from many of the major petrologic units in the intrusion allow for the production of an unprecedented record of age variations for a major layered intrusion. The high-precision U–Pb dating includes ages from both of the enigmatic thick anorthosite horizons in the uppermost part of the exposed intrusion (e.g. Anorthosite-II or AN2; Wall et al., 2016) and from the PGE-rich J-M Reef (Wall & Scoates, 2016), which was sampled along strike to test for lateral variations in age. These dates, combined with trace element compositions of zircon, are used to determine the age and duration of magmatism in the Stillwater Complex and to assess the composition and evolution of late-stage interstitial melts. The results of this study contribute to understanding whether there is evidence for variations in the timescales of magma addition and magma flux (i.e. periods of enhanced or reduced magmatic activity) and whether the intrusion was constructed in a sequential stratigraphic order from oldest at the bottom to youngest at the top. GEOLOGIC SETTING AND STRATIGRAPHY OF THE STILLWATER COMPLEX The c.2·7 Ga Stillwater Complex is exposed within the Beartooth Mountains of southern Montana, one of the major exposed blocks of the Archean Wyoming Province. It crops out over an area of approximately 200 km2 and consists of a ∼45 km-long, 6·5 km-thick, relatively steeply dipping sheet of layered mafic–ultramafic rocks and an associated marginal suite of sills and dikes (Hess, 1960; Jackson, 1961; McCallum, 1996; Boudreau, 2016) (Fig. 1). Early regional gravity and magnetic surveys indicated that the Stillwater Complex extends at depth approximately 25 km to the northeast of the Beartooth Mountains front (Blakely & Simpson, 1984; Kleinkopf, 1985) and more recent analysis from a 3 D gravity model indicates that the buried extent of the Stillwater Complex extends 30 km to the north and 40 km to the east (Finn et al., 2013, 2016). Fig. 1. View largeDownload slide Geology of the Stillwater Complex. (a) Simplified stratigraphic section showing the major subdivisions of the layered mafic–ultramafic rocks and the position of the Ni–Cu sulphide deposits (Ni) in the Basal series, the chromitite seams (A–K) in the Peridotite zone, the PGE-rich J-M Reef near the top of Olivine-bearing zone I, and the PGE-rich Picket Pin mineralized zone (PPZ) at the top of Anorthosite zone II (stratigraphic section after McCallum, 1996). Abbreviations on the section: N1, Norite zone I; GN1, Gabbronorite zone I; OB1, Olivine-bearing zone I; N2, Norite zone II; GN2, Gabbronorite zone II; OB2, Olivine-bearing zone II; AN1, Anorthosite zone I; OB3, Olivine-bearing zone III; OB4, Olivine-bearing zone IV; AN2, Anorthosite zone II; OB5, Olivine-bearing zone V; GN3, Gabbronorite zone III. (b) Generalized geologic map of the Stillwater Complex showing the major series, the position of the J-M Reef (black solid line), and the sample locations of mafic–ultramafic rocks (yellow stars) and granophyres (red stars); map modified from Zientek et al. (2005). Also shown are the locations of the two PGE mines, Stillwater and East Boulder. Abbreviations on the map: PP, Picket Pin; IM, Iron Mountain; LM, Lost Mountain; CRM, Chrome Mountain; CTM, Contact Mountain; CP, Castle Point; WFC, West Fork Creek; BB, Benbow. Inset in the lower right shows the location of the Stillwater Complex in the southwestern part of the state of Montana (USA). Fig. 1. View largeDownload slide Geology of the Stillwater Complex. (a) Simplified stratigraphic section showing the major subdivisions of the layered mafic–ultramafic rocks and the position of the Ni–Cu sulphide deposits (Ni) in the Basal series, the chromitite seams (A–K) in the Peridotite zone, the PGE-rich J-M Reef near the top of Olivine-bearing zone I, and the PGE-rich Picket Pin mineralized zone (PPZ) at the top of Anorthosite zone II (stratigraphic section after McCallum, 1996). Abbreviations on the section: N1, Norite zone I; GN1, Gabbronorite zone I; OB1, Olivine-bearing zone I; N2, Norite zone II; GN2, Gabbronorite zone II; OB2, Olivine-bearing zone II; AN1, Anorthosite zone I; OB3, Olivine-bearing zone III; OB4, Olivine-bearing zone IV; AN2, Anorthosite zone II; OB5, Olivine-bearing zone V; GN3, Gabbronorite zone III. (b) Generalized geologic map of the Stillwater Complex showing the major series, the position of the J-M Reef (black solid line), and the sample locations of mafic–ultramafic rocks (yellow stars) and granophyres (red stars); map modified from Zientek et al. (2005). Also shown are the locations of the two PGE mines, Stillwater and East Boulder. Abbreviations on the map: PP, Picket Pin; IM, Iron Mountain; LM, Lost Mountain; CRM, Chrome Mountain; CTM, Contact Mountain; CP, Castle Point; WFC, West Fork Creek; BB, Benbow. Inset in the lower right shows the location of the Stillwater Complex in the southwestern part of the state of Montana (USA). The Stillwater Complex was emplaced at shallow depths in the crust (6–7 km), possibly as a sub-volcanic intrusion, into a c.3·3 Ga metasedimentary sequence consisting of pelitic schists, iron formations and quartzites (Page, 1977; Mogk & Mueller, 1990; Helz, 1995; Labotka & Kath, 2001). Contact metamorphism produced a large, high-temperature contact aureole along the base of the intrusion, including an inner 500–1000 m wide hypersthene hornfels zone at the contact (650–800°C), and an outer 500–1500 m wide cordierite + cummingtonite hornfels zone (Labotka & Kath, 2001; Thomson, 2008). The Mouat quartz monzonite, a coarse-grained quartz monzonite, occurs along the southeastern basal part of the Stillwater Complex and based on contact relations appears to be younger than the mafic–ultramafic rocks (Nunes & Tilton, 1971; Page & Nokleberg, 1972). The complex was intruded by many generations of mafic dikes and was subjected to regional metamorphism at c.1·7 Ga, an event that locally produced greenschist facies mineral assemblages (Nunes & Tilton, 1971; Page, 1977; McCallum et al., 1999). The Stillwater Complex was uplifted, tilted, and eroded prior to the Middle Cambrian. The uppermost part of the intrusion and its upper intrusive contact are missing and the complex is overlain by Paleozoic and Mesozoic sedimentary rocks. The Stillwater Complex has been divided into the Basal, Ultramafic, Lower Banded, Middle Banded, and Upper Banded series (McCallum et al., 1980; Raedeke, 1982; McCallum, 1996) (Fig. 1). The Basal series (60–400 m thick) is an irregular sheet-like body dominated by orthopyroxenite with subordinate norite and sulphide-bearing assemblages, and it contains xenoliths of cordierite–pyroxene hornfels (Page, 1979; McCallum, 1996). A sill/dike suite includes discontinuous sills and dikes of diabase, mafic norite, and massive sulphide that intrude metasedimentary rocks beneath the complex (Zientek, 1983); five chemical groups are distinguished in the sill/dike suite (Helz, 1985). The Ultramafic series (840–2000 m thick) is subdivided into the lower Peridotite zone and upper Bronzitite zone (Jackson, 1961; Raedeke & McCallum, 1984; McCallum, 1996) (Fig. 1). The Peridotite zone contains approximately 20 cyclic units, where a complete unit consists of peridotite–harzburgite–bronzitite (orthopyroxenite), with or without chromitite seams, that are attributed to multiple magma injections during open-system magmatism (Raedeke & McCallum, 1984; Cooper, 1997). The Banded series, ranging in thickness from 4468 m in the Contact Mountain area to 1935 m in the Picket Pin area, is dominated by plagioclase-rich rocks (norite, gabbronorite, troctolite, anorthosite). The 1800 m thick Middle Banded series contains two thick anorthosite zones, Anorthosite-I (AN1) and Anorthosite-II (AN2) (Fig. 1) (Haskin & Salpas, 1992; McCallum, 1996). The Stillwater Complex hosts a variety of mineral deposits, including Ni–Cu sulphide deposits associated with the Basal series (Howland et al., 1936; Page et al., 1985), chromitite seams (A–K) in the Peridotite zone (Jackson, 1961; Raedeke & McCallum, 1984; Lipin, 1993; Cooper, 1997; Horan et al., 2001; Lenaz et al., 2012), and the world-class platinum group element (PGE) deposit known as the J-M Reef in the Lower Banded series (Todd et al., 1982; Irvine et al., 1983; Barnes & Naldrett, 1985, 1986; Mann et al., 1985; Page et al., 1985; Lambert & Simmons, 1988; Boudreau, 1988; Godel & Barnes, 2008) (Fig. 1). Although volumetrically minor, small bodies of granophyre occur throughout the Banded series (Czamanske et al., 1991). They are present as white to pink veins, 1–12 cm thick and up to 100 m long, composed nearly exclusively of quartz and sodic plagioclase with some veins containing large splays (30 cm length) of tremolitic to actinolitic amphibole. A large ‘alaskite’ (coarse-grained alkali granite) body with quartz-rich segregations and veins occurs over an area of 130 x 210 m within AN1 of the Middle Banded series about 2 km south of Picket Pin Mountain (Fig. 1). The granophyres, typically oriented at high angles to layering, have sharp linear contacts and narrow tapering terminations indicating that they were emplaced after consolidation of the host mafic rocks. The granophyres have been interpreted as crystallization differentiates that evolved in equilibrium with a high-temperature aqueous chloride solution during the final stages of crystallization of the mafic cumulates of the Banded series (Czamanske et al. 1991). PREVIOUS GEOCHRONOLOGY OF THE STILLWATER COMPLEX The Stillwater Complex has been a focus of geochronology studies since the late 1960s (e.g. Fenton & Faure, 1969; Kistler et al., 1969; Nunes & Tilton, 1971). A variety of dating methods and isotopic systems (e.g. K–Ar, Rb–Sr, U–Pb, Sm–Nd, Pb–Pb, Re–Os, 40Ar/39Ar) have been applied and a wide range of ages have been reported, mostly between 2600 and 2750 Ma (Table 1). Nunes & Tilton (1971) separated zircon from a 100 pound (45 kilograms) sample of norite from the Basal series and published the first U–Pb zircon date from the Stillwater Complex based on the discordant U–Pb results from two multi-grain fractions that yielded 207Pb/206Pb dates of 2745 Ma and 2750 Ma (no uncertainty reported). Nunes (1981) subsequently re-analyzed three multi-grain zircon fractions (up to 1 mg) from the same sample and provided a revised age of 2713 ± 3 Ma, an upper intercept 207Pb/206Pb date from strongly discordant analyses. DePaolo & Wasserburg (1979) produced a relatively precise whole rock–mineral Sm–Nd isochron of 2701 ± 8 Ma for gabbronorite from the Gabbronorite zone II of the Lower Banded series that was generally accepted as the age of crystallization of the Stillwater Complex. Table 1: Summary of published geochronology dates for rocks related to the Stillwater Complex Unit Isotope/Method Sample Material Age Method Age (Ma, ±2σ) References Middle Banded series K–Ar Phlogopite, plagioclase Plateau 2350 ± 350 Kistler et al., 1969 Middle Banded series Rb–Sr Whole rock Isochron 2900 ± 300 Fenton & Faure, 1969 Metasedimentary rock Rb–Sr Whole rock Isochron 2672 ± 150 Powell et al., 1969 Chill zone U–Pb ID-TIMS Zircon Upper intercept* 2725 ± 25 Nunes & Tilton, 1971 Metasedimentary rock Rb–Sr Whole rock Isochron 2692 ± 45 Mueller & Wooden, 1976 Lower Banded series Sm–Nd Whole rock, plagioclase, orthopyroxene, augite Isochron 2701 ± 8 DePaolo & Wasserburg, 1979 Banded series Sm–Nd Whole rock Isochron 2896 ± 34 Coffrant et al., 1980 Lower Banded series Sm–Nd Whole rock Isochron 2742 ± 34 Coffrant et al., 1980 Middle Banded series Sm–Nd Whole rock Isochron 2793 ± 21 Coffrant et al., 1980 Ultramafic series Pb–Pb Whole rock Isochron 2662 ± 12 Manhes et al., 1980 Chill zone U–Pb ID-TIMS Zircon Upper intercept* 2713 ± 3 Nunes, 1981 Lower Banded series U–Pb ID-TIMS Zircon Upper intercept* 2683 ± 87 Lambert et al., 1985 Lower Banded series U–Pb ID-TIMS Zircon, baddeleyite Upper intercept* 2705 ± 4 Premo et al., 1990 Group 1 dike U–Pb ID-TIMS Zircon Upper intercept* 2711 ± 1 Premo et al., 1990 Group 2 dike Pb–Pb Whole rock, plagioclase, orthopyroxene, augite Isochron 2704 ± 25 Premo et al., 1990 Group 2 dike Sm–Nd Whole rock, plagioclase, orthopyroxene, augite Isochron 2731 ± 92 Premo et al., 1990 Group 3 dike U–Pb ID-TIMS Zircon Upper intercept* 2703 ± 11 Premo et al., 1990 Group 4 dike U–Pb ID-TIMS Zircon Upper intercept* 2712 ± 3 Premo et al., 1990 Group 6 dike Sm–Nd Whole rock, plagioclase, orthopyroxene, augite Isochron 2706 ± 64 Premo et al., 1990 Peridotite zone Re–Os Chromitite Isochron 2740 ± 80 Marcantonio et al., 1993 Middle Banded series 40Ar/39Ar Amphibole Plateau 2744 ± 11 Selkin et al., 2008 J-M Reef U–Pb CA-TIMS Zircon Weighted Mean† 2709·11 ± 0·56 Wall & Scoates, 2016 J-M Reef U–Pb CA-TIMS Zircon Weighted Mean† 2709·28 ± 0·32 Wall & Scoates, 2016 J-M Reef U–Pb CA-TIMS Zircon Weighted Mean† 2709·00 ± 0·45 Wall & Scoates, 2016 J-M Reef U–Pb ID-TIMS Baddeleyite Weighted Mean† 2708·85 ± 0·46 Wall & Scoates, 2016 AN2 U–Pb CA-TIMS Zircon Weighted Mean† 2710·44 ± 0·32 Wall et al., 2016 AN2 U–Pb ID-TIMS Baddeleyite Weighted Mean† 2709·73 ± 0·48 Wall et al., 2016 Unit Isotope/Method Sample Material Age Method Age (Ma, ±2σ) References Middle Banded series K–Ar Phlogopite, plagioclase Plateau 2350 ± 350 Kistler et al., 1969 Middle Banded series Rb–Sr Whole rock Isochron 2900 ± 300 Fenton & Faure, 1969 Metasedimentary rock Rb–Sr Whole rock Isochron 2672 ± 150 Powell et al., 1969 Chill zone U–Pb ID-TIMS Zircon Upper intercept* 2725 ± 25 Nunes & Tilton, 1971 Metasedimentary rock Rb–Sr Whole rock Isochron 2692 ± 45 Mueller & Wooden, 1976 Lower Banded series Sm–Nd Whole rock, plagioclase, orthopyroxene, augite Isochron 2701 ± 8 DePaolo & Wasserburg, 1979 Banded series Sm–Nd Whole rock Isochron 2896 ± 34 Coffrant et al., 1980 Lower Banded series Sm–Nd Whole rock Isochron 2742 ± 34 Coffrant et al., 1980 Middle Banded series Sm–Nd Whole rock Isochron 2793 ± 21 Coffrant et al., 1980 Ultramafic series Pb–Pb Whole rock Isochron 2662 ± 12 Manhes et al., 1980 Chill zone U–Pb ID-TIMS Zircon Upper intercept* 2713 ± 3 Nunes, 1981 Lower Banded series U–Pb ID-TIMS Zircon Upper intercept* 2683 ± 87 Lambert et al., 1985 Lower Banded series U–Pb ID-TIMS Zircon, baddeleyite Upper intercept* 2705 ± 4 Premo et al., 1990 Group 1 dike U–Pb ID-TIMS Zircon Upper intercept* 2711 ± 1 Premo et al., 1990 Group 2 dike Pb–Pb Whole rock, plagioclase, orthopyroxene, augite Isochron 2704 ± 25 Premo et al., 1990 Group 2 dike Sm–Nd Whole rock, plagioclase, orthopyroxene, augite Isochron 2731 ± 92 Premo et al., 1990 Group 3 dike U–Pb ID-TIMS Zircon Upper intercept* 2703 ± 11 Premo et al., 1990 Group 4 dike U–Pb ID-TIMS Zircon Upper intercept* 2712 ± 3 Premo et al., 1990 Group 6 dike Sm–Nd Whole rock, plagioclase, orthopyroxene, augite Isochron 2706 ± 64 Premo et al., 1990 Peridotite zone Re–Os Chromitite Isochron 2740 ± 80 Marcantonio et al., 1993 Middle Banded series 40Ar/39Ar Amphibole Plateau 2744 ± 11 Selkin et al., 2008 J-M Reef U–Pb CA-TIMS Zircon Weighted Mean† 2709·11 ± 0·56 Wall & Scoates, 2016 J-M Reef U–Pb CA-TIMS Zircon Weighted Mean† 2709·28 ± 0·32 Wall & Scoates, 2016 J-M Reef U–Pb CA-TIMS Zircon Weighted Mean† 2709·00 ± 0·45 Wall & Scoates, 2016 J-M Reef U–Pb ID-TIMS Baddeleyite Weighted Mean† 2708·85 ± 0·46 Wall & Scoates, 2016 AN2 U–Pb CA-TIMS Zircon Weighted Mean† 2710·44 ± 0·32 Wall et al., 2016 AN2 U–Pb ID-TIMS Baddeleyite Weighted Mean† 2709·73 ± 0·48 Wall et al., 2016 * upper intercept 207Pb/206Pb date. † weighted mean 207Pb/206Pb date. Table 1: Summary of published geochronology dates for rocks related to the Stillwater Complex Unit Isotope/Method Sample Material Age Method Age (Ma, ±2σ) References Middle Banded series K–Ar Phlogopite, plagioclase Plateau 2350 ± 350 Kistler et al., 1969 Middle Banded series Rb–Sr Whole rock Isochron 2900 ± 300 Fenton & Faure, 1969 Metasedimentary rock Rb–Sr Whole rock Isochron 2672 ± 150 Powell et al., 1969 Chill zone U–Pb ID-TIMS Zircon Upper intercept* 2725 ± 25 Nunes & Tilton, 1971 Metasedimentary rock Rb–Sr Whole rock Isochron 2692 ± 45 Mueller & Wooden, 1976 Lower Banded series Sm–Nd Whole rock, plagioclase, orthopyroxene, augite Isochron 2701 ± 8 DePaolo & Wasserburg, 1979 Banded series Sm–Nd Whole rock Isochron 2896 ± 34 Coffrant et al., 1980 Lower Banded series Sm–Nd Whole rock Isochron 2742 ± 34 Coffrant et al., 1980 Middle Banded series Sm–Nd Whole rock Isochron 2793 ± 21 Coffrant et al., 1980 Ultramafic series Pb–Pb Whole rock Isochron 2662 ± 12 Manhes et al., 1980 Chill zone U–Pb ID-TIMS Zircon Upper intercept* 2713 ± 3 Nunes, 1981 Lower Banded series U–Pb ID-TIMS Zircon Upper intercept* 2683 ± 87 Lambert et al., 1985 Lower Banded series U–Pb ID-TIMS Zircon, baddeleyite Upper intercept* 2705 ± 4 Premo et al., 1990 Group 1 dike U–Pb ID-TIMS Zircon Upper intercept* 2711 ± 1 Premo et al., 1990 Group 2 dike Pb–Pb Whole rock, plagioclase, orthopyroxene, augite Isochron 2704 ± 25 Premo et al., 1990 Group 2 dike Sm–Nd Whole rock, plagioclase, orthopyroxene, augite Isochron 2731 ± 92 Premo et al., 1990 Group 3 dike U–Pb ID-TIMS Zircon Upper intercept* 2703 ± 11 Premo et al., 1990 Group 4 dike U–Pb ID-TIMS Zircon Upper intercept* 2712 ± 3 Premo et al., 1990 Group 6 dike Sm–Nd Whole rock, plagioclase, orthopyroxene, augite Isochron 2706 ± 64 Premo et al., 1990 Peridotite zone Re–Os Chromitite Isochron 2740 ± 80 Marcantonio et al., 1993 Middle Banded series 40Ar/39Ar Amphibole Plateau 2744 ± 11 Selkin et al., 2008 J-M Reef U–Pb CA-TIMS Zircon Weighted Mean† 2709·11 ± 0·56 Wall & Scoates, 2016 J-M Reef U–Pb CA-TIMS Zircon Weighted Mean† 2709·28 ± 0·32 Wall & Scoates, 2016 J-M Reef U–Pb CA-TIMS Zircon Weighted Mean† 2709·00 ± 0·45 Wall & Scoates, 2016 J-M Reef U–Pb ID-TIMS Baddeleyite Weighted Mean† 2708·85 ± 0·46 Wall & Scoates, 2016 AN2 U–Pb CA-TIMS Zircon Weighted Mean† 2710·44 ± 0·32 Wall et al., 2016 AN2 U–Pb ID-TIMS Baddeleyite Weighted Mean† 2709·73 ± 0·48 Wall et al., 2016 Unit Isotope/Method Sample Material Age Method Age (Ma, ±2σ) References Middle Banded series K–Ar Phlogopite, plagioclase Plateau 2350 ± 350 Kistler et al., 1969 Middle Banded series Rb–Sr Whole rock Isochron 2900 ± 300 Fenton & Faure, 1969 Metasedimentary rock Rb–Sr Whole rock Isochron 2672 ± 150 Powell et al., 1969 Chill zone U–Pb ID-TIMS Zircon Upper intercept* 2725 ± 25 Nunes & Tilton, 1971 Metasedimentary rock Rb–Sr Whole rock Isochron 2692 ± 45 Mueller & Wooden, 1976 Lower Banded series Sm–Nd Whole rock, plagioclase, orthopyroxene, augite Isochron 2701 ± 8 DePaolo & Wasserburg, 1979 Banded series Sm–Nd Whole rock Isochron 2896 ± 34 Coffrant et al., 1980 Lower Banded series Sm–Nd Whole rock Isochron 2742 ± 34 Coffrant et al., 1980 Middle Banded series Sm–Nd Whole rock Isochron 2793 ± 21 Coffrant et al., 1980 Ultramafic series Pb–Pb Whole rock Isochron 2662 ± 12 Manhes et al., 1980 Chill zone U–Pb ID-TIMS Zircon Upper intercept* 2713 ± 3 Nunes, 1981 Lower Banded series U–Pb ID-TIMS Zircon Upper intercept* 2683 ± 87 Lambert et al., 1985 Lower Banded series U–Pb ID-TIMS Zircon, baddeleyite Upper intercept* 2705 ± 4 Premo et al., 1990 Group 1 dike U–Pb ID-TIMS Zircon Upper intercept* 2711 ± 1 Premo et al., 1990 Group 2 dike Pb–Pb Whole rock, plagioclase, orthopyroxene, augite Isochron 2704 ± 25 Premo et al., 1990 Group 2 dike Sm–Nd Whole rock, plagioclase, orthopyroxene, augite Isochron 2731 ± 92 Premo et al., 1990 Group 3 dike U–Pb ID-TIMS Zircon Upper intercept* 2703 ± 11 Premo et al., 1990 Group 4 dike U–Pb ID-TIMS Zircon Upper intercept* 2712 ± 3 Premo et al., 1990 Group 6 dike Sm–Nd Whole rock, plagioclase, orthopyroxene, augite Isochron 2706 ± 64 Premo et al., 1990 Peridotite zone Re–Os Chromitite Isochron 2740 ± 80 Marcantonio et al., 1993 Middle Banded series 40Ar/39Ar Amphibole Plateau 2744 ± 11 Selkin et al., 2008 J-M Reef U–Pb CA-TIMS Zircon Weighted Mean† 2709·11 ± 0·56 Wall & Scoates, 2016 J-M Reef U–Pb CA-TIMS Zircon Weighted Mean† 2709·28 ± 0·32 Wall & Scoates, 2016 J-M Reef U–Pb CA-TIMS Zircon Weighted Mean† 2709·00 ± 0·45 Wall & Scoates, 2016 J-M Reef U–Pb ID-TIMS Baddeleyite Weighted Mean† 2708·85 ± 0·46 Wall & Scoates, 2016 AN2 U–Pb CA-TIMS Zircon Weighted Mean† 2710·44 ± 0·32 Wall et al., 2016 AN2 U–Pb ID-TIMS Baddeleyite Weighted Mean† 2709·73 ± 0·48 Wall et al., 2016 * upper intercept 207Pb/206Pb date. † weighted mean 207Pb/206Pb date. In the early 1990s, several studies provided new U–Pb zircon geochronological results for the Stillwater Complex (Table 1). Premo et al. (1990) successfully extracted zircon and baddeleyite from a range of rock types in the Stillwater Complex, including from the mafic sill/dike suite and from the Lower Banded series. They reported U–Pb dates (as upper intercepts with concordia) from the analysis of multi-grain zircon fractions, associated with variable amounts of Pb loss, that were separated from footwall mafic sills and dikes, including 2711 ± 1 Ma from a Group 1 gabbronorite, 2712 ± 3 Ma from a Group 4 high-Ti norite, and 2703 ± 10 Ma for a Group 3 mafic norite where the results were highly discordant (D = 7–32%). They combined the results from two samples from the Lower Banded series, a pegmatoidal anorthosite from the J-M Reef and a mafic pegmatoid from a thin anorthosite layer near the base of the Gabbronorite zone II, to produce a composite zircon–baddeleyite age of 2705 ± 4 Ma (upper intercept based on results from 7 multi-grain fractions), which was reduced to 2704 ± 1 Ma, with a lower intercept date of 351 Ma, when the results of two fractions from the Gabbronorite-II sample were omitted. Czamanske et al. (1991) attempted to date the age of crystallization of the large alaskite body in the Middle Banded series south of Picket Pin Mountain to confirm its timing as a late-stage differentiate. Five multi-grain zircon fractions (10–19 grains) from this sample were characterized by very high U concentrations (1444–6845 ppm), strong discordance (D < 78%), and yielded an upper intercept with concordia of 2132 ± 98 Ma and a lower intercept of 162 ± 45 Ma. They attributed the U–Pb results for zircon from the alaskite to a c.2·1–2·2 Ga thermal event that reset the U–Pb systematics of these radiation-damaged grains and to extensive Pb loss. In the context of the present study on high-precision dating of the Stillwater Complex, several samples from the Banded series have been dated using the CA-TIMS U–Pb zircon technique of Mattinson (2005) and are reported in Wall & Scoates (2016) and Wall et al. (2016) (Table 1). Zircon from a sample of AN2 in the Middle Banded series was evaluated as a potential natural reference material for U–Pb geochronology in Wall et al. (2016). The U–Pb results for nine single grains are concordant and yield a weighted mean 207Pb/206Pb date of 2710·44 ± 0·32 Ma, which is interpreted as the crystallization age of this leucogabbro. In Wall & Scoates (2016), the age of the PGE-rich J-M Reef was investigated as a time marker in the crystallization of the Stillwater Complex. The analyses yield statistically indistinguishable ages, including 2709·11 ± 0·56 Ma from the Frog Pond adit, 2709·28 ± 0·32 Ma from the East Boulder mine, and 2709·00 ± 0·45 Ma from the West Fork area, and reveal that the J-M Reef is younger than the dated sample of AN2 at higher stratigraphic levels of the intrusion. These CA-TIMS results for the Banded series indicate that the Premo et al. (1990) date of 2704 Ma is likely too young to be considered as a crystallization age and that there are age differences preserved within the stratigraphic sequence of cumulates that comprise the Stillwater Complex. SAMPLING, PROCESSING, and IMAGING Sampling and U–Th–Pb-Bearing Accessory Mineral Distribution Sampling was conducted over the course of two field seasons (2005, ST05-series; 2011, ST11-series; Table 2) and the sampling strategy has been outlined in Scoates & Wall (2015). A total of 29 samples for dating were collected, representing the exposed stratigraphic sequence of the Stillwater Complex. Material was sampled from most of the major zones of the intrusion and included nine ultramafic rocks, 15 mafic rocks (including anorthosites), four granophyres, and one monzonitic rock (Table 2, Fig. 2). The samples cover ∼3/4 of the strike-length of the exposed Stillwater Complex from the Benbow area in the east to the Chrome Mountain area of the East Boulder Plateau in the west (Fig. 1). To test synchroneity along 15 km of strike length, three samples of the PGE-rich J-M Reef were collected (ST05–08, Frog Pond adit; ST11–16, West Fork adit; ST11–37, East Boulder mine; Table 2); the results on these samples are reported in Wall & Scoates (2016) as noted above. A sample (ST11–34) of Archean quartz monzonite (Mouat quartz monzonite of Butler, 1966) from near the Stillwater mine was collected to verify that this felsic intrusion is indeed younger than the Stillwater Complex (Page & Nokleberg, 1972). Finally, a mafic dike (ST12–01) that cuts the J-M Reef was also sampled to establish the timing of post-crystallization mafic magmatism in the Stillwater Complex. Table 2: Locations, descriptions, and accessory mineral content of samples analyzed for U–Pb geochronology from the Stillwater Complex Sample Latitude/location1 Longitude/level Area Series/ Intrusion2 Zone2 Rock Type Weight (kg) Zircon Baddeleyite Rutile Titanite Apatite ST05-01 45°27'34·86”N 110°3'4·94”W Picket Pin UBS GN3 granophyre 15·2 X – X X – ST05-033 45°26'54·34”N 110°3'3·37”W Picket Pin MBS AN2 leucogabbro 50·0 X X X – X ST05-04 45°26'37·93”N 110°3'13·91”W Picket Pin MBS OB3 olivine-bearing gabbronorite 15·8 X X – – X ST05-06 45°26'16·87”N 110°3'2·02”W Alaskite MBS AN1 alaskite 10·2 X – X X – ST05-07 45°26'13·67”N 110°2'58·89”W Alaskite MBS AN1 granophyre 11·3 X – X X – ST05-084 45°27'8·51”N 110°7'24·08”W Frog Pond LBS OB1, J-M troctolite 14·3 X X – – X ST05-13 45°27'2·96”N 110°8'14·22”W Chrome/Contact LBS N1 granite pegmatite 8·6 X – X X X ST05-14 45°27'1·13”N 110°8'15·67”W Chrome/Contact US BZ orthopyroxenite 12·9 X X X – X ST11-02 45°21'23·74”N 109°48'19·16”W Benbow BS sulphide-bearing orthopyroxenite 19·3 X – – – X ST11-04 45°21'23·81”N 109°48'16·53”W Benbow US PZ, BC harzburgite 9·5 X X – – X ST11-05 45°21'41·13”N 109°48'12·49”W Benbow US PZ, JC pyroxenite 23·8 X X – – X ST11-08 45°22'42·83”N 109°47'1·53”W Benbow MBS AN1 anorthosite 20·0 X – X – X ST11-164 45°23'55·20”N 109°58'8·03”W West Fork LBS OB1, J-M anorthosite 15·8 X X – – – ST11-19 45°24'43·34”N 110°3'32·80”W Iron Mountain US BZ orthopyroxenite 17·1 X – – – X ST11-20 45°28'3·86”N 110°3'5·33”W Mish Mash Ridge UBS GN3 gabbronorite 15·5 X X – – X ST11-22 45°27'46·26”N 110°2'44·77”W Castle Creek Cirque UBS GN3 magnetite-bearing anorthosite 14·6 X X – – X ST11-24 45°23'20·85”N 109°54'18·53”W Mountain View US PZ, GC harzburgite 14·5 X X – – X ST11-33 45°23'31·89”N 109°54'3·55”W Mountain View LBS N1 norite 16·8 X – – – X ST11-34 45°22'47·89”N 109°52'27·57”W Stillwater Mine Mouat quartz monzonite 12·4 X – – X – ST11-374 East Boulder Mine 7500-foot level (75-685 + 175W) East Boulder Mine LBS OB1, J-M troctolite 18·4 X X – – X ST12-01 East Boulder Mine 7900-foot level East Boulder Mine LBS mafic dike 38·7 X X – – X Sample Latitude/location1 Longitude/level Area Series/ Intrusion2 Zone2 Rock Type Weight (kg) Zircon Baddeleyite Rutile Titanite Apatite ST05-01 45°27'34·86”N 110°3'4·94”W Picket Pin UBS GN3 granophyre 15·2 X – X X – ST05-033 45°26'54·34”N 110°3'3·37”W Picket Pin MBS AN2 leucogabbro 50·0 X X X – X ST05-04 45°26'37·93”N 110°3'13·91”W Picket Pin MBS OB3 olivine-bearing gabbronorite 15·8 X X – – X ST05-06 45°26'16·87”N 110°3'2·02”W Alaskite MBS AN1 alaskite 10·2 X – X X – ST05-07 45°26'13·67”N 110°2'58·89”W Alaskite MBS AN1 granophyre 11·3 X – X X – ST05-084 45°27'8·51”N 110°7'24·08”W Frog Pond LBS OB1, J-M troctolite 14·3 X X – – X ST05-13 45°27'2·96”N 110°8'14·22”W Chrome/Contact LBS N1 granite pegmatite 8·6 X – X X X ST05-14 45°27'1·13”N 110°8'15·67”W Chrome/Contact US BZ orthopyroxenite 12·9 X X X – X ST11-02 45°21'23·74”N 109°48'19·16”W Benbow BS sulphide-bearing orthopyroxenite 19·3 X – – – X ST11-04 45°21'23·81”N 109°48'16·53”W Benbow US PZ, BC harzburgite 9·5 X X – – X ST11-05 45°21'41·13”N 109°48'12·49”W Benbow US PZ, JC pyroxenite 23·8 X X – – X ST11-08 45°22'42·83”N 109°47'1·53”W Benbow MBS AN1 anorthosite 20·0 X – X – X ST11-164 45°23'55·20”N 109°58'8·03”W West Fork LBS OB1, J-M anorthosite 15·8 X X – – – ST11-19 45°24'43·34”N 110°3'32·80”W Iron Mountain US BZ orthopyroxenite 17·1 X – – – X ST11-20 45°28'3·86”N 110°3'5·33”W Mish Mash Ridge UBS GN3 gabbronorite 15·5 X X – – X ST11-22 45°27'46·26”N 110°2'44·77”W Castle Creek Cirque UBS GN3 magnetite-bearing anorthosite 14·6 X X – – X ST11-24 45°23'20·85”N 109°54'18·53”W Mountain View US PZ, GC harzburgite 14·5 X X – – X ST11-33 45°23'31·89”N 109°54'3·55”W Mountain View LBS N1 norite 16·8 X – – – X ST11-34 45°22'47·89”N 109°52'27·57”W Stillwater Mine Mouat quartz monzonite 12·4 X – – X – ST11-374 East Boulder Mine 7500-foot level (75-685 + 175W) East Boulder Mine LBS OB1, J-M troctolite 18·4 X X – – X ST12-01 East Boulder Mine 7900-foot level East Boulder Mine LBS mafic dike 38·7 X X – – X 1NAD83 Datum. 2Abbreviations: UBS, Upper Banded series; MBS, Middle Banded series; LBS, Lower Banded series; US, Ultramafic series, PZ, Peridotite zone; BZ, Bronzitite zone; BC, B chromitite; GC, G chromitite; JC, J chromitite; OB1, Olivine-bearing zone I; N1, Norite zone I; GN1, Gabbronorite zone I; AN1, Anorthosite zone I; AN2, Anorthosite zone II; GN3, Gabbronorite zone III; OB3, Olivine-bearing zone III; OB4, Olivine-bearing zone IV; J-M, J-M Reef. 3Complete description and analytical results are in Wall et al. (2016). 4Complete descriptions and analytical results are in Wall & Scoates (2016). Table 2: Locations, descriptions, and accessory mineral content of samples analyzed for U–Pb geochronology from the Stillwater Complex Sample Latitude/location1 Longitude/level Area Series/ Intrusion2 Zone2 Rock Type Weight (kg) Zircon Baddeleyite Rutile Titanite Apatite ST05-01 45°27'34·86”N 110°3'4·94”W Picket Pin UBS GN3 granophyre 15·2 X – X X – ST05-033 45°26'54·34”N 110°3'3·37”W Picket Pin MBS AN2 leucogabbro 50·0 X X X – X ST05-04 45°26'37·93”N 110°3'13·91”W Picket Pin MBS OB3 olivine-bearing gabbronorite 15·8 X X – – X ST05-06 45°26'16·87”N 110°3'2·02”W Alaskite MBS AN1 alaskite 10·2 X – X X – ST05-07 45°26'13·67”N 110°2'58·89”W Alaskite MBS AN1 granophyre 11·3 X – X X – ST05-084 45°27'8·51”N 110°7'24·08”W Frog Pond LBS OB1, J-M troctolite 14·3 X X – – X ST05-13 45°27'2·96”N 110°8'14·22”W Chrome/Contact LBS N1 granite pegmatite 8·6 X – X X X ST05-14 45°27'1·13”N 110°8'15·67”W Chrome/Contact US BZ orthopyroxenite 12·9 X X X – X ST11-02 45°21'23·74”N 109°48'19·16”W Benbow BS sulphide-bearing orthopyroxenite 19·3 X – – – X ST11-04 45°21'23·81”N 109°48'16·53”W Benbow US PZ, BC harzburgite 9·5 X X – – X ST11-05 45°21'41·13”N 109°48'12·49”W Benbow US PZ, JC pyroxenite 23·8 X X – – X ST11-08 45°22'42·83”N 109°47'1·53”W Benbow MBS AN1 anorthosite 20·0 X – X – X ST11-164 45°23'55·20”N 109°58'8·03”W West Fork LBS OB1, J-M anorthosite 15·8 X X – – – ST11-19 45°24'43·34”N 110°3'32·80”W Iron Mountain US BZ orthopyroxenite 17·1 X – – – X ST11-20 45°28'3·86”N 110°3'5·33”W Mish Mash Ridge UBS GN3 gabbronorite 15·5 X X – – X ST11-22 45°27'46·26”N 110°2'44·77”W Castle Creek Cirque UBS GN3 magnetite-bearing anorthosite 14·6 X X – – X ST11-24 45°23'20·85”N 109°54'18·53”W Mountain View US PZ, GC harzburgite 14·5 X X – – X ST11-33 45°23'31·89”N 109°54'3·55”W Mountain View LBS N1 norite 16·8 X – – – X ST11-34 45°22'47·89”N 109°52'27·57”W Stillwater Mine Mouat quartz monzonite 12·4 X – – X – ST11-374 East Boulder Mine 7500-foot level (75-685 + 175W) East Boulder Mine LBS OB1, J-M troctolite 18·4 X X – – X ST12-01 East Boulder Mine 7900-foot level East Boulder Mine LBS mafic dike 38·7 X X – – X Sample Latitude/location1 Longitude/level Area Series/ Intrusion2 Zone2 Rock Type Weight (kg) Zircon Baddeleyite Rutile Titanite Apatite ST05-01 45°27'34·86”N 110°3'4·94”W Picket Pin UBS GN3 granophyre 15·2 X – X X – ST05-033 45°26'54·34”N 110°3'3·37”W Picket Pin MBS AN2 leucogabbro 50·0 X X X – X ST05-04 45°26'37·93”N 110°3'13·91”W Picket Pin MBS OB3 olivine-bearing gabbronorite 15·8 X X – – X ST05-06 45°26'16·87”N 110°3'2·02”W Alaskite MBS AN1 alaskite 10·2 X – X X – ST05-07 45°26'13·67”N 110°2'58·89”W Alaskite MBS AN1 granophyre 11·3 X – X X – ST05-084 45°27'8·51”N 110°7'24·08”W Frog Pond LBS OB1, J-M troctolite 14·3 X X – – X ST05-13 45°27'2·96”N 110°8'14·22”W Chrome/Contact LBS N1 granite pegmatite 8·6 X – X X X ST05-14 45°27'1·13”N 110°8'15·67”W Chrome/Contact US BZ orthopyroxenite 12·9 X X X – X ST11-02 45°21'23·74”N 109°48'19·16”W Benbow BS sulphide-bearing orthopyroxenite 19·3 X – – – X ST11-04 45°21'23·81”N 109°48'16·53”W Benbow US PZ, BC harzburgite 9·5 X X – – X ST11-05 45°21'41·13”N 109°48'12·49”W Benbow US PZ, JC pyroxenite 23·8 X X – – X ST11-08 45°22'42·83”N 109°47'1·53”W Benbow MBS AN1 anorthosite 20·0 X – X – X ST11-164 45°23'55·20”N 109°58'8·03”W West Fork LBS OB1, J-M anorthosite 15·8 X X – – – ST11-19 45°24'43·34”N 110°3'32·80”W Iron Mountain US BZ orthopyroxenite 17·1 X – – – X ST11-20 45°28'3·86”N 110°3'5·33”W Mish Mash Ridge UBS GN3 gabbronorite 15·5 X X – – X ST11-22 45°27'46·26”N 110°2'44·77”W Castle Creek Cirque UBS GN3 magnetite-bearing anorthosite 14·6 X X – – X ST11-24 45°23'20·85”N 109°54'18·53”W Mountain View US PZ, GC harzburgite 14·5 X X – – X ST11-33 45°23'31·89”N 109°54'3·55”W Mountain View LBS N1 norite 16·8 X – – – X ST11-34 45°22'47·89”N 109°52'27·57”W Stillwater Mine Mouat quartz monzonite 12·4 X – – X – ST11-374 East Boulder Mine 7500-foot level (75-685 + 175W) East Boulder Mine LBS OB1, J-M troctolite 18·4 X X – – X ST12-01 East Boulder Mine 7900-foot level East Boulder Mine LBS mafic dike 38·7 X X – – X 1NAD83 Datum. 2Abbreviations: UBS, Upper Banded series; MBS, Middle Banded series; LBS, Lower Banded series; US, Ultramafic series, PZ, Peridotite zone; BZ, Bronzitite zone; BC, B chromitite; GC, G chromitite; JC, J chromitite; OB1, Olivine-bearing zone I; N1, Norite zone I; GN1, Gabbronorite zone I; AN1, Anorthosite zone I; AN2, Anorthosite zone II; GN3, Gabbronorite zone III; OB3, Olivine-bearing zone III; OB4, Olivine-bearing zone IV; J-M, J-M Reef. 3Complete description and analytical results are in Wall et al. (2016). 4Complete descriptions and analytical results are in Wall & Scoates (2016). Fig. 2. View largeDownload slide Photographs of mafic-ultramafic rocks from the Stillwater Complex showing the macroscopic textures of samples and outcrops that were targeted in this U–Pb dating study; coin is 28 mm in diameter. All samples from these outcrops yielded zircon or baddeleyite. (a) Sulphide-bearing orthopyroxenite from the Basal series (ST11–02, Benbow). (b) Rubbly weathering poikilitic harzburgite from the Peridotite zone in the vicinity of the B chromitite (ST11–04, Benbow); GPS unit is 10 cm long. (c) Chromite-bearing feldspathic harzburgite from the Peridotite zone associated with the G chromitite (ST11–24, Mountain View). (d) Chromite-bearing feldspathic pyroxenite (websterite) from the Peridotite zone associated with the J chromitite (ST11–05, Benbow). (e) Feldspathic orthopyroxenite from the base of the Bronzitite zone, just above the contact with the Peridotite zone (ST11–19, Iron Mountain). (f) Feldspathic orthopyroxenite from the Bronzitite zone (ST05–14, Iron Mountain). (g) Pegmatitic norite from Norite zone I of the Lower Banded series just above the contact between the Ultramafic series and Banded series (ST11–33, Mountain View); pen is 15 cm long. (h) Mineralized portion of the J-M Reef package from Olivine-bearing zone I showing coarse-grained character, heterogeneous textures, and sulphide clots that are typical of the reef (ST11–37, East Boulder mine); glove for scale. (i) Mottled anorthosite of the J-M Reef with rusty weathering oxidized sulphide clots (ST11–16, West Fork Stillwater River). (j) Coarse-grained anorthosite from Anorthosite zone I (ST11–08, Benbow); hammer handle is 60 cm long. (k) Olivine-bearing gabbronorite of Olivine-bearing zone III with polycrystalline aggregates of plagioclase (ST05–04, Picket Pin Mountain area). (l) Coarse-grained leucogabbro with poikilitic inverted pigeonite of Anorthosite zone II (ST05–03, south face of Picket Pin Mountain). (m) Magnetite-bearing anorthosite of Gabbronorite zone III (ST11–22, Castle Creek cirque). (n) Amphibole-bearing quartz- and K-feldspar-rich pegmatite near the base of Norite zone I from the Lower Banded series (ST05–13, Contact Mountain area). (o) Coarse-grained equigranular Mouat quartz monzonite to the south of the Stillwater mine (ST11–34, Mountain View). Abbreviations: BS, Basal series; US, Ultramafic series; PZ, Peridotite zone; BZ, Bronzitite zone; LBS, Lower Banded series; MBS, Middle Banded series; UBS, Upper Banded series. BC, B chromitite; GC, G chromitite; JC, J chromitite. Fig. 2. View largeDownload slide Photographs of mafic-ultramafic rocks from the Stillwater Complex showing the macroscopic textures of samples and outcrops that were targeted in this U–Pb dating study; coin is 28 mm in diameter. All samples from these outcrops yielded zircon or baddeleyite. (a) Sulphide-bearing orthopyroxenite from the Basal series (ST11–02, Benbow). (b) Rubbly weathering poikilitic harzburgite from the Peridotite zone in the vicinity of the B chromitite (ST11–04, Benbow); GPS unit is 10 cm long. (c) Chromite-bearing feldspathic harzburgite from the Peridotite zone associated with the G chromitite (ST11–24, Mountain View). (d) Chromite-bearing feldspathic pyroxenite (websterite) from the Peridotite zone associated with the J chromitite (ST11–05, Benbow). (e) Feldspathic orthopyroxenite from the base of the Bronzitite zone, just above the contact with the Peridotite zone (ST11–19, Iron Mountain). (f) Feldspathic orthopyroxenite from the Bronzitite zone (ST05–14, Iron Mountain). (g) Pegmatitic norite from Norite zone I of the Lower Banded series just above the contact between the Ultramafic series and Banded series (ST11–33, Mountain View); pen is 15 cm long. (h) Mineralized portion of the J-M Reef package from Olivine-bearing zone I showing coarse-grained character, heterogeneous textures, and sulphide clots that are typical of the reef (ST11–37, East Boulder mine); glove for scale. (i) Mottled anorthosite of the J-M Reef with rusty weathering oxidized sulphide clots (ST11–16, West Fork Stillwater River). (j) Coarse-grained anorthosite from Anorthosite zone I (ST11–08, Benbow); hammer handle is 60 cm long. (k) Olivine-bearing gabbronorite of Olivine-bearing zone III with polycrystalline aggregates of plagioclase (ST05–04, Picket Pin Mountain area). (l) Coarse-grained leucogabbro with poikilitic inverted pigeonite of Anorthosite zone II (ST05–03, south face of Picket Pin Mountain). (m) Magnetite-bearing anorthosite of Gabbronorite zone III (ST11–22, Castle Creek cirque). (n) Amphibole-bearing quartz- and K-feldspar-rich pegmatite near the base of Norite zone I from the Lower Banded series (ST05–13, Contact Mountain area). (o) Coarse-grained equigranular Mouat quartz monzonite to the south of the Stillwater mine (ST11–34, Mountain View). Abbreviations: BS, Basal series; US, Ultramafic series; PZ, Peridotite zone; BZ, Bronzitite zone; LBS, Lower Banded series; MBS, Middle Banded series; UBS, Upper Banded series. BC, B chromitite; GC, G chromitite; JC, J chromitite. Yields for U–Th–Pb-accessory minerals in the Stillwater Complex were excellent with zircon ± baddeleyite present in 17 of the 24 mafic–ultramafic samples, zircon + titanite + rutile present in all four granophyres, and abundant zircon recovered from the Mouat quartz monzonite (Table 2). Sample selection in the field in summer 2005 was based in part on accessibility and guided by using whole-rock trace element concentrations (e.g. Zr, U, REE; W. P. Meurer, unpublished data) as indices of higher abundances of minerals that may have crystallized from fractionated interstitial melt, thus reflecting a higher probability for the recovery of U–Th–Pb-bearing minerals. Based on these initial sampling criteria, the success rate was 50% with four of the eight mafic–ultramafic samples containing zircon ± baddeleyite. In summer 2011, sampling specifically targeted rocks of the Ultramafic series and the strategy for selecting samples focused primarily on textural features using knowledge gained from sampling mafic–ultramafic rocks in the intervening years (e.g. Bushveld Complex, Scoates & Friedman, 2008, Scoates & Wall, 2015; Grenville anorthosites, Morisset et al., 2009; Muskox intrusion, Mackie et al., 2009; Bird River sill, Scoates & Scoates, 2013; Thompson Nickel Belt, Scoates et al., 2017). Outcrops that contained coarse-grained (pegmatitic) patches, especially with coarse interstitial plagioclase (e.g. feldspathic pyroxenites), or plagioclase-rich rocks with relatively abundant or irregular distributed poikilitic to sub-poikilitic pyroxene were preferentially sampled (Fig. 2) (Scoates & Wall, 2015). As a result, zircon (± baddeleyite) was recovered from more than 90% of the samples (13 out of 14 samples processed), including nearly all of the rocks collected from the Ultramafic series. In the mafic–ultramafic rocks (Fig. 2), zircon occurs predominantly as subequant grains, 25 to 150 μm in diameter, in interstitial pockets associated with quartz, biotite, plagioclase, and alkali feldspar (completely replaced by secondary sericite) (Fig. 3a–h). Zircon is also found locally along chromite grain boundaries in ultramafic rocks (Fig. 3c) and included within plagioclase in the Banded series (Fig. 3h). Despite evidence for strong alteration in the silicates around some zircon (Fig. 3a, b, e, f), the crystals are transparent and show no visual signs of alteration. Baddeleyite grains are scarce in thin section in the mafic–ultramafic rocks, reflecting its relatively low recovery in mineral separates. Where present, baddeleyite occurs as small (<100 μm in length), euhedral grains commonly associated with the ‘felsic’ pockets of interstitial quartz and alkali feldspar (Fig. 3i); zircon and baddeleyite have not been observed together in the same interstitial pocket. Small amounts of rutile are present in the AN2 sample ST05–03 (Table 2) where it occurs as small (30–60 μm diameter) anhedral crystals in clusters that formed from the breakdown of magmatic titanite. Based on its low U concentrations (<0·1 ppm) and high common Pb contents (Wall et al., 2016), this rutile is likely secondary and formed during a post-crystallization hydrothermal alteration event. Fig. 3. View largeDownload slide Photomicrographs showing the textural setting of representative zircon and baddeleyite grains in mafic-ultramafic rocks (panels (a)–(j)) from the Stillwater Complex and associated granitoids (panels (k)–(l)); scale bar is 200 microns in each panel, except for panel (i) where the scale bar is 50 microns. (a) Anhedral zircon along the grain boundary between serpentinized olivine grains (PPL: ST11–04). (b) Anhedral zircon in an interstitial pocket associated with plagioclase and secondary chlorite (PPL: ST11–24). (c) Zircon crystal along the grain boundary between orthopyroxene and chromite (PPL: ST11–05). (d) Anhedral zircon crystals associated with interstitial plagioclase, sericite, and rutile needles (PPL: ST05–14). (e) Anhedral zircon associated with interstitial K-feldspar that has been replaced by sericite (PPL: ST11–33). (f) Several zircon crystals in an interstitial pocket containing quartz and minor biotite (PPL: ST11–33). (g) Zircon grain along the boundary of a plagioclase primocryst and interstitial sulphide (PPL: ST11–08). (h) Zircon crystal in plagioclase with minor secondary sericite (PPL: ST11–22). (i) Elongate baddeleyite grain in interstitial plagioclase that has been replaced by secondary sericite (PPL: ST05–03). (j) Metamict euhedral zircon in plagioclase with minor sericite alteration (XPL: ST05–01, granophyre). (k) Large, metamict zircon crystal along the margin of quartz and K-feldspar (XPL: ST05–06, alaskite); note the sharp contact between zircon, quartz, and K-feldspar and the irregular contact between zircon and highly altered K-feldspar (replaced by secondary sericite). (l) Euhedral zircon crystal at the contact between quartz and altered hornblende (PPL: ST11–34, Mouat quartz monzonite). Mineral abbreviations: ol, olivine; serp, serpentine; plag, plagioclase; chl, chlorite; chr, chromite; rut, rutile; ser, sericite; qtz, quartz; po, pyrrhotite; badd, baddeleyite; hbl, hornblende. Fig. 3. View largeDownload slide Photomicrographs showing the textural setting of representative zircon and baddeleyite grains in mafic-ultramafic rocks (panels (a)–(j)) from the Stillwater Complex and associated granitoids (panels (k)–(l)); scale bar is 200 microns in each panel, except for panel (i) where the scale bar is 50 microns. (a) Anhedral zircon along the grain boundary between serpentinized olivine grains (PPL: ST11–04). (b) Anhedral zircon in an interstitial pocket associated with plagioclase and secondary chlorite (PPL: ST11–24). (c) Zircon crystal along the grain boundary between orthopyroxene and chromite (PPL: ST11–05). (d) Anhedral zircon crystals associated with interstitial plagioclase, sericite, and rutile needles (PPL: ST05–14). (e) Anhedral zircon associated with interstitial K-feldspar that has been replaced by sericite (PPL: ST11–33). (f) Several zircon crystals in an interstitial pocket containing quartz and minor biotite (PPL: ST11–33). (g) Zircon grain along the boundary of a plagioclase primocryst and interstitial sulphide (PPL: ST11–08). (h) Zircon crystal in plagioclase with minor secondary sericite (PPL: ST11–22). (i) Elongate baddeleyite grain in interstitial plagioclase that has been replaced by secondary sericite (PPL: ST05–03). (j) Metamict euhedral zircon in plagioclase with minor sericite alteration (XPL: ST05–01, granophyre). (k) Large, metamict zircon crystal along the margin of quartz and K-feldspar (XPL: ST05–06, alaskite); note the sharp contact between zircon, quartz, and K-feldspar and the irregular contact between zircon and highly altered K-feldspar (replaced by secondary sericite). (l) Euhedral zircon crystal at the contact between quartz and altered hornblende (PPL: ST11–34, Mouat quartz monzonite). Mineral abbreviations: ol, olivine; serp, serpentine; plag, plagioclase; chl, chlorite; chr, chromite; rut, rutile; ser, sericite; qtz, quartz; po, pyrrhotite; badd, baddeleyite; hbl, hornblende. All four granophyres sampled from cross-cutting pegmatites, and from granophyric veins and intrusions in the Banded series (Fig. 2n), contain zircon, titanite, and rutile (Table 2). In contrast to zircon from the mafic–ultramafic rocks of the Stillwater Complex, zircon in the granophyres is large (50–300 μm in diameter) and typically euhedral (Fig. 3j–k). Titanite occurs as large (100–250 μm in diameter), euhedral crystals associated with quartz and plagioclase-rich zones in the granophyres and primary rutile (20–60 microns in diameter) is found within altered biotite or hornblende crystals (Wall, 2009). The Mouat quartz monzonite (Fig. 2o), contains abundant euhedral zircon with well-developed prismatic terminations. Sample Processing and Imaging Mineral treatment protocols were carried out at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia, Vancouver (Canada). Zircon, baddeleyite, titanite, and rutile grains were separated from samples using conventional crushing, grinding, and Wilfley table techniques, followed by final concentration using heavy liquids and magnetic separation. All mineral fractions for analysis were hand-picked in methanol under magnification using a binocular microscope and selected based on grain morphology, quality, and size. Backscattered electron (BSE), secondary electron (SE), and cathodoluminescence (CL) imaging of zircon and baddeleyite grains was carried out on a Philips XL-30 scanning electron microscope (SEM) at a voltage of 15 kV and equipped with a Bruker Quanta 200 energy-dispersion X-ray microanalysis system at the Electron Microbeam/X-Ray Diffraction Facility (EMXDF) at the University of British Columbia. Cathodoluminescence images highlight the character of zoning present in zircon from mafic–ultramafic rocks of the Stillwater Complex (Fig. 4a–t). The total CL emission intensity is relatively low. The images demonstrate a lack of resorbed cores and there is no evidence for inheritance or xenocrysts in any of the samples throughout the entire stratigraphic sequence. Overgrowths and inclusions are rare in the zircon grains from the least magnetic fractions (N2) selected for U–Pb dating and trace element analysis. The external morphology of zircon ranges from irregular grains (Fig. 4b–h, o–p) to grains with well-developed crystal faces (Fig. 4i–n); based on zoning patterns, many of the imaged (and dated) grains appear to be fragments of larger crystals. Sector zoning is the most predominant feature in zircon grains from the Ultramafic and Lower Banded series (Fig. 4c–l) with oscillatory zoning more common in rocks from the Middle and Upper Banded series (Fig. 4n, q–t). Sector zoning is characteristic of zircon that crystallized from fractionated interstitial melt late in the consolidation history of mafic rocks (Scoates & Wall, 2015) and may reflect relatively rapid crystallization and the formation of minor defects related to local differences in the primary crystallinity of the grains (e.g. Corfu et al., 2003; Nasdala et al., 2003). Zircon from a coarse-grained gabbronorite at the top of Gabbronorite zone III in the Upper Banded series is distinguished by its euhedral morphology and well-developed oscillatory zoning (Fig. 4q,r). Cathodoluminescence images of zircon from most of the granophyres (ST05–01, ST05–06, ST05–07) reveal complex internal features, including partially to completely metamict zones and abundant micrometer-scale pores resulting from extensive radiation damage and hydrothermal alteration (Ver Hoeve, 2013); zircon from ST05–13 is pristine with simple oscillatory zoning and no metamictization. Fig. 4. View largeDownload slide Scanning electron microscope images of zircon and baddeleyite from mafic–ultramafic rocks of the Stillwater Complex; scale bars as noted on each panel. Panels (a)–(t) show cathodoluminescence (CL) images of zircon and panels (u)–(x) show backscattered electron (BSE) and secondary electron (SE) images of baddeleyite. The CL images show the typical irregular and anhedral morphology of interstitial zircon, characteristic sector zoning, and the of inherited cores. The CL images are arranged in stratigraphic order from the base (a) to the top (t) of the intrusion: (a) & (b) ST11–02 (BS); (c) & (d) ST11–04 (PZ, US); (e) & (f) ST11–05 (PZ, US); (g) & (h) ST11–19 (BZ, US); (i) &) (j) ST11–33 (N1, LBS); (k) & (l) ST05–08 (OB1, LBS); (m) & (n) ST11–08 (AN1, MBS); (o) & (p) ST05–03; (q) & (r) ST11–20 (GN3, UBS) —note the euhedral morphology and oscillatory zoning in these zircon grains; (s) & (t) ST11–22 (GN3, UBS). (u) BSE image of a baddeleyite crystal without secondary zircon overgrowths (ST05–08, OB1, LBS). (v) BSE image of a baddeleyite grain with secondary zircon overgrowths (ST05–03, AN2, MBS). (w) SE image of a single baddeleyite grain showing the surface appearance of secondary zircon overgrowths (ST05–03, AN2, MBS). (x) BSE image of a baddeleyite grain containing metamict and altered secondary zircon (ST05–03, AN2, MBS). Refer to the caption of Figure 2 for letter codes. Fig. 4. View largeDownload slide Scanning electron microscope images of zircon and baddeleyite from mafic–ultramafic rocks of the Stillwater Complex; scale bars as noted on each panel. Panels (a)–(t) show cathodoluminescence (CL) images of zircon and panels (u)–(x) show backscattered electron (BSE) and secondary electron (SE) images of baddeleyite. The CL images show the typical irregular and anhedral morphology of interstitial zircon, characteristic sector zoning, and the of inherited cores. The CL images are arranged in stratigraphic order from the base (a) to the top (t) of the intrusion: (a) & (b) ST11–02 (BS); (c) & (d) ST11–04 (PZ, US); (e) & (f) ST11–05 (PZ, US); (g) & (h) ST11–19 (BZ, US); (i) &) (j) ST11–33 (N1, LBS); (k) & (l) ST05–08 (OB1, LBS); (m) & (n) ST11–08 (AN1, MBS); (o) & (p) ST05–03; (q) & (r) ST11–20 (GN3, UBS) —note the euhedral morphology and oscillatory zoning in these zircon grains; (s) & (t) ST11–22 (GN3, UBS). (u) BSE image of a baddeleyite crystal without secondary zircon overgrowths (ST05–08, OB1, LBS). (v) BSE image of a baddeleyite grain with secondary zircon overgrowths (ST05–03, AN2, MBS). (w) SE image of a single baddeleyite grain showing the surface appearance of secondary zircon overgrowths (ST05–03, AN2, MBS). (x) BSE image of a baddeleyite grain containing metamict and altered secondary zircon (ST05–03, AN2, MBS). Refer to the caption of Figure 2 for letter codes. The backscattered and secondary electron images of baddeleyite grains from the Stillwater Complex reveal their tabular morphology, typically 100–150 microns in length (Fig. 4u–x). There is no internal zoning present in the baddeleyite grains. Baddeleyite ranges from pristine grains with no secondary zircon overgrowths or replacement patches (Fig. 4u) through grains with rough external surfaces that consist of 1–3 micron-thick zircon overgrowths (Fig. 4v,w) to grains that contain secondary zircon in irregular patches in the interior (Fig. 4v). The secondary zircon formed during post-crystallization reaction and replacement and is a common feature of baddeleyite (e.g. Heaman & LeCheminant, 1993; Rioux et al., 2010). ANALYTICAL METHODS All analyses, LA-ICP-MS and U–Pb TIMS geochronology, were done at PCIGR (Vancouver, Canada). A brief overview of the methodology is provided below and the complete details of the different analytical methods may be found in the Supplementary Data; supplementary data are available for downloading at http://www.petrology.oxfordjournals.org. LA-ICP-MS Analytical Procedures for Trace Elements in Zircon Following cathodoluminescence imaging of zircon, in situ trace element analyses were done using a Resonetics RESOlution M-50-LR laser attached to an Agilent 7700x quadrupole ICP-MS. Raw data were reduced using the Iolite 2·5 extension (Paton et al., 2011) for Igor ProTM, yielding concentration values and respective propagated uncertainties. Multiple spots per grain were used to test both grain-to-grain and intragrain variability. Standards were analyzed throughout the sequence to allow for drift correction. NIST 612 glass was used for drift correction with sample spacing between every five unknowns and 90Zr was used as the internal standard assuming stoichiometric values for zircon. NIST 610 glass was analyzed after each NIST 612 analysis and used as a monitor standard for trace elements to assess the accuracy and precision of the runs (Supplementary DataFig. A1). Results for NIST-610 (n = 129) over the course of the laser ablation sessions overlap within uncertainty with published values for all elements determined for this study (Jochum et al., 2011). Analytical sessions were run within a 5-day interval to ensure maximum instrument stability and similar mass bias between runs. Representative ranges of trace element concentrations in zircon are reported in Table 3 and the complete trace element concentration results can be found in Supplementary DataTable A1. Table 3: Trace element concentrations (ppm) by LA-ICP-MS in zircon from the Stillwater Complex Quartile Li Sc Ti Rb Sr Y Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U Th/U Tzrc (°C) ST11-02 (Basal series, n = 20)  Q1 21·29 265 20 0·04 0·17 319 2·7 0·01 4·2 0·02 0·45 1·11 0·23 6·72 2·15 26·6 9·7 46 9·36 80 16 8587 1·59 312 103 1·7 856  Median 23·65 308 23 0·06 0·20 409 3·7 0·01 4·6 0·03 0·63 1·28 0·37 8·42 2·88 36·6 12·6 57 11·0 95 19 8796 2·11 439 175 2·7 871  Q3 25·81 338 25 0·09 0·24 597 4·3 0·51 5·9 0·06 1·57 2·18 0·76 10·8 3·56 47·9 17·8 82 16·1 134 27 9144 2·41 781 259 3·2 883 ST11-04 (Peridotite zone, n = 20)  Q1 51·23 183 17 0·09 0·24 422 1·4 0·017 4·3 0·04 0·68 1·36 0·36 9·80 2·83 37·0 12·2 61 11·2 88 15 8220 0·45 164 205 0·7 836  Median 81·80 250 20 0·14 0·28 707 1·5 0·032 6·9 0·11 1·83 2·85 0·62 15·2 5·32 65·7 21·5 92 16·6 132 23 8500 0·62 266 272 0·9 856  Q3 148·7 283 22 0·19 0·31 1029 1·8 0·042 7·8 0·27 3·73 5·95 1·06 28·6 8·90 100 32·8 133 23·0 170 28 8720 0·93 296 288 1·1 865 ST11-05 (Peridotite zone, n = 17)  Q1 20·18 293 26 0·09 0·20 328 2·33 0·007 3·0 0·03 0·68 1·64 0·38 8·12 2·37 30·9 10·6 44 8·47 68 13 9061 0·69 139 40 2·7 886  Median 23·10 393 33 0·14 0·26 495 2·73 0·017 3·3 0·11 2·21 3·15 0·73 12·7 4·38 52·5 16·2 66 11·6 91 17 9620 1·03 208 49 4·0 914  Q3 24·91 592 38 0·28 0·44 560 3·23 0·063 4·3 0·13 2·75 3·60 1·11 15·3 4·63 55·0 17·8 72 12·2 96 18 10 040 1·29 269 64 7·4 932 ST11-19 (Bronzitite zone, n = 25)  Q1 19·74 403 26 0·04 0·16 298 0·84 0·002 7·8 0·03 0·62 1·34 0·36 6·36 2·29 28·0 9·4 40 7·37 63 12 8407 0·41 107 24 3·7 886  Median 21·69 502 30 0·08 0·19 384 0·93 0·001 9·6 0·06 1·09 2·36 0·65 9·22 3·01 37·9 12·4 53 9·95 81 15 9390 0·47 134 28 4·2 902  Q3 23·40 593 35 0·13 0·21 631 1·01 0·003 10·3 0·09 1·62 4·00 1·17 13·7 4·71 64·1 20·9 83 14·4 114 20 9497 0·54 184 30 5·5 925 ST05-14 (Bronzitite zone, n = 19)  Q1 20·03 428 15 0·06 0·21 345 1·13 0·005 2·4 0·01 0·37 0·86 0·22 6·53 2·19 27·7 10·4 52 10·4 90 19 9885 0·09 164 9 10 820  Median 22·74 535 17 0·10 0·25 469 1·22 0·013 2·9 0·02 0·92 1·35 0·52 9·98 3·24 40·4 14·4 68 12·6 101 20 10 340 0·13 181 13 17 836  Q3 23·86 766 22 0·17 0·29 597 1·35 0·026 3·1 0·07 1·33 2·69 0·65 13·5 4·19 53·8 18·8 82 14·9 120 23 12 080 0·17 240 16 20 868 ST11-33 (Lower Banded series, N1, n = 20)  Q1 37·50 260 15 0·12 0·28 544 1·78 0·002 4·7 0·04 0·68 1·84 0·24 10·6 4·03 48·3 17·3 80 15·3 127 25 8226 0·45 102 105 0·8 821  Median 57·65 278 16 0·29 0·41 1247 1·89 0·005 7·6 0·12 1·86 3·59 0·51 23·2 7·84 105 39·3 186 38·5 333 65 8701 0·57 158 150 1·0 828  Q3 65·13 298 21 0·46 0·48 1538 2·26 0·018 11·6 0·17 2·67 5·88 0·83 32·8 10·7 136 49·8 225 44·6 390 75 9173 0·66 181 174 1·1 858 ST12-01 (OB1, mafic dike, n = 20)  Q1 20·50 302 20 0·23 0·25 1185 1·39 0·002 6·4 0·01 0·42 1·62 0·24 15·4 6·23 92·8 37·0 175 34·7 296 56 7788 0·20 97 97 0·8 853  Median 22·85 303 22 0·30 0·31 1353 1·43 0·003 7·1 0·02 0·98 3·62 0·53 27·5 9·04 119 44·0 206 41·6 355 66 8368 0·22 109 102 1·0 864  Q3 25·48 308 23 0·35 0·36 1482 1·54 0·004 7·5 0·11 2·48 5·64 0·77 31·8 10·4 132 48·2 219 42·7 364 69 8940 0·25 110 107 1·1 874 ST11-37 (Lower Banded series, OB1, J-M Reef, n = 20)  Q1 68·93 336 22 0·19 0·34 804 1·7 0·007 4·6 0·08 0·89 1·76 0·34 13·1 5·25 72·6 26·6 115 20·8 170 31 8168 0·40 127 142 0·8 865  Median 87·45 342 30 0·22 0·36 825 2·11 0·019 5·1 0·11 2·01 4·07 0·69 22·2 6·82 80·8 28·0 119 22·0 178 34 9082 0·57 167 218 0·9 904  Q3 116·4 355 37 0·25 0·43 891 2·39 0·075 5·6 0·17 2·43 4·40 0·72 22·6 7·03 84·4 29·1 133 25·8 214 41 10 228 0·68 278 309 1·0 931 ST11-16 (Lower Banded series, OB1, J-M Reef, n = 17)  Q1 58·60 320 23 0·08 0·28 427 3·12 0·007 1·1 0·00 0·09 0·61 0·04 5·56 2·44 31·8 12·5 60 13·6 127 29 9040 1·07 116 138 0·8 871  Median 83·00 470 29 0·16 0·31 521 4·34 0·007 1·6 0·03 0·54 1·82 0·19 13·2 3·89 36·5 15·6 76 17·6 164 34 9390 1·75 211 268 0·8 899  Q3 97·10 561 36 0·26 0·63 870 6·49 3·804 19·4 2·17 17·9 12·8 0·87 24·5 6·40 80·0 27·7 126 26·9 246 45 9890 2·35 590 360 1·1 926 ST05-08 (Lower Banded series, OB1, J-M Reef, n = 20)  Q1 20·50 349 23 0·09 0·21 453 1·41 0·006 4·8 0·02 0·80 1·69 0·34 9·87 3·51 43·5 14·8 62 11·2 89 17 8016 0·29 75 81 0·9 870  Median 22·90 480 30 0·18 0·24 634 1·47 0·010 6·2 0·07 1·11 2·60 0·50 14·1 3·86 51·5 21·0 106 19·0 167 32 9065 0·33 115 108 1·0 904  Q3 24·86 656 41 0·27 0·28 1311 1·68 0·019 7·7 0·10 1·83 4·04 0·92 23·8 8·23 116 42·3 190 36·2 301 56 9335 0·39 129 115 1·1 944 ST11-08 (Middle Banded series, AN1, n = 20)  Q1 13·64 294 8 0·44 0·49 1502 2·16 0·033 2·9 0·02 0·73 2·46 0·10 20·8 8·26 117 47·6 249 54·7 499 10 7255 0·94 109 202 0·557 753  Median 23·43 302 10 0·48 0·56 1716 2·57 0·052 3·1 0·04 0·90 3·03 0·14 24·4 9·88 137 55·8 280 60·8 551 11 8083 1·05 146 238 0·617 778  Q3 53·00 307 11 0·62 0·61 1938 2·69 0·062 3·6 0·07 1·35 3·54 0·25 27·3 10·6 153 63·2 329 72·0 666 13 8829 1·24 193 294 0·672 791 ST05-04 (Middle Banded series, OB3, n = 14)  Q1 57·23 419 27 0·11 0·28 357 3·66 0·007 2·4 0·02 0·30 0·66 0·15 5·13 1·85 25·2 10·4 56 13·0 127 27 7434 1·32 244 310 0·729 889  Median 68·05 480 28 0·13 0·32 398 3·98 0·081 2·9 0·08 0·81 0·80 0·18 5·48 2·03 30·5 12·1 63 14·5 138 29 8215 1·44 294 393 0·768 896  Q3 75·85 538 34 0·17 0·76 415 4·13 0·174 4·0 0·21 1·07 1·23 0·33 6·09 2·22 31·8 12·6 64 15·3 144 31 8608 1·52 330 432 0·793 918 Quartile Li Sc Ti Rb Sr Y Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U Th/U Tzrc (°C) ST11-02 (Basal series, n = 20)  Q1 21·29 265 20 0·04 0·17 319 2·7 0·01 4·2 0·02 0·45 1·11 0·23 6·72 2·15 26·6 9·7 46 9·36 80 16 8587 1·59 312 103 1·7 856  Median 23·65 308 23 0·06 0·20 409 3·7 0·01 4·6 0·03 0·63 1·28 0·37 8·42 2·88 36·6 12·6 57 11·0 95 19 8796 2·11 439 175 2·7 871  Q3 25·81 338 25 0·09 0·24 597 4·3 0·51 5·9 0·06 1·57 2·18 0·76 10·8 3·56 47·9 17·8 82 16·1 134 27 9144 2·41 781 259 3·2 883 ST11-04 (Peridotite zone, n = 20)  Q1 51·23 183 17 0·09 0·24 422 1·4 0·017 4·3 0·04 0·68 1·36 0·36 9·80 2·83 37·0 12·2 61 11·2 88 15 8220 0·45 164 205 0·7 836  Median 81·80 250 20 0·14 0·28 707 1·5 0·032 6·9 0·11 1·83 2·85 0·62 15·2 5·32 65·7 21·5 92 16·6 132 23 8500 0·62 266 272 0·9 856  Q3 148·7 283 22 0·19 0·31 1029 1·8 0·042 7·8 0·27 3·73 5·95 1·06 28·6 8·90 100 32·8 133 23·0 170 28 8720 0·93 296 288 1·1 865 ST11-05 (Peridotite zone, n = 17)  Q1 20·18 293 26 0·09 0·20 328 2·33 0·007 3·0 0·03 0·68 1·64 0·38 8·12 2·37 30·9 10·6 44 8·47 68 13 9061 0·69 139 40 2·7 886  Median 23·10 393 33 0·14 0·26 495 2·73 0·017 3·3 0·11 2·21 3·15 0·73 12·7 4·38 52·5 16·2 66 11·6 91 17 9620 1·03 208 49 4·0 914  Q3 24·91 592 38 0·28 0·44 560 3·23 0·063 4·3 0·13 2·75 3·60 1·11 15·3 4·63 55·0 17·8 72 12·2 96 18 10 040 1·29 269 64 7·4 932 ST11-19 (Bronzitite zone, n = 25)  Q1 19·74 403 26 0·04 0·16 298 0·84 0·002 7·8 0·03 0·62 1·34 0·36 6·36 2·29 28·0 9·4 40 7·37 63 12 8407 0·41 107 24 3·7 886  Median 21·69 502 30 0·08 0·19 384 0·93 0·001 9·6 0·06 1·09 2·36 0·65 9·22 3·01 37·9 12·4 53 9·95 81 15 9390 0·47 134 28 4·2 902  Q3 23·40 593 35 0·13 0·21 631 1·01 0·003 10·3 0·09 1·62 4·00 1·17 13·7 4·71 64·1 20·9 83 14·4 114 20 9497 0·54 184 30 5·5 925 ST05-14 (Bronzitite zone, n = 19)  Q1 20·03 428 15 0·06 0·21 345 1·13 0·005 2·4 0·01 0·37 0·86 0·22 6·53 2·19 27·7 10·4 52 10·4 90 19 9885 0·09 164 9 10 820  Median 22·74 535 17 0·10 0·25 469 1·22 0·013 2·9 0·02 0·92 1·35 0·52 9·98 3·24 40·4 14·4 68 12·6 101 20 10 340 0·13 181 13 17 836  Q3 23·86 766 22 0·17 0·29 597 1·35 0·026 3·1 0·07 1·33 2·69 0·65 13·5 4·19 53·8 18·8 82 14·9 120 23 12 080 0·17 240 16 20 868 ST11-33 (Lower Banded series, N1, n = 20)  Q1 37·50 260 15 0·12 0·28 544 1·78 0·002 4·7 0·04 0·68 1·84 0·24 10·6 4·03 48·3 17·3 80 15·3 127 25 8226 0·45 102 105 0·8 821  Median 57·65 278 16 0·29 0·41 1247 1·89 0·005 7·6 0·12 1·86 3·59 0·51 23·2 7·84 105 39·3 186 38·5 333 65 8701 0·57 158 150 1·0 828  Q3 65·13 298 21 0·46 0·48 1538 2·26 0·018 11·6 0·17 2·67 5·88 0·83 32·8 10·7 136 49·8 225 44·6 390 75 9173 0·66 181 174 1·1 858 ST12-01 (OB1, mafic dike, n = 20)  Q1 20·50 302 20 0·23 0·25 1185 1·39 0·002 6·4 0·01 0·42 1·62 0·24 15·4 6·23 92·8 37·0 175 34·7 296 56 7788 0·20 97 97 0·8 853  Median 22·85 303 22 0·30 0·31 1353 1·43 0·003 7·1 0·02 0·98 3·62 0·53 27·5 9·04 119 44·0 206 41·6 355 66 8368 0·22 109 102 1·0 864  Q3 25·48 308 23 0·35 0·36 1482 1·54 0·004 7·5 0·11 2·48 5·64 0·77 31·8 10·4 132 48·2 219 42·7 364 69 8940 0·25 110 107 1·1 874 ST11-37 (Lower Banded series, OB1, J-M Reef, n = 20)  Q1 68·93 336 22 0·19 0·34 804 1·7 0·007 4·6 0·08 0·89 1·76 0·34 13·1 5·25 72·6 26·6 115 20·8 170 31 8168 0·40 127 142 0·8 865  Median 87·45 342 30 0·22 0·36 825 2·11 0·019 5·1 0·11 2·01 4·07 0·69 22·2 6·82 80·8 28·0 119 22·0 178 34 9082 0·57 167 218 0·9 904  Q3 116·4 355 37 0·25 0·43 891 2·39 0·075 5·6 0·17 2·43 4·40 0·72 22·6 7·03 84·4 29·1 133 25·8 214 41 10 228 0·68 278 309 1·0 931 ST11-16 (Lower Banded series, OB1, J-M Reef, n = 17)  Q1 58·60 320 23 0·08 0·28 427 3·12 0·007 1·1 0·00 0·09 0·61 0·04 5·56 2·44 31·8 12·5 60 13·6 127 29 9040 1·07 116 138 0·8 871  Median 83·00 470 29 0·16 0·31 521 4·34 0·007 1·6 0·03 0·54 1·82 0·19 13·2 3·89 36·5 15·6 76 17·6 164 34 9390 1·75 211 268 0·8 899  Q3 97·10 561 36 0·26 0·63 870 6·49 3·804 19·4 2·17 17·9 12·8 0·87 24·5 6·40 80·0 27·7 126 26·9 246 45 9890 2·35 590 360 1·1 926 ST05-08 (Lower Banded series, OB1, J-M Reef, n = 20)  Q1 20·50 349 23 0·09 0·21 453 1·41 0·006 4·8 0·02 0·80 1·69 0·34 9·87 3·51 43·5 14·8 62 11·2 89 17 8016 0·29 75 81 0·9 870  Median 22·90 480 30 0·18 0·24 634 1·47 0·010 6·2 0·07 1·11 2·60 0·50 14·1 3·86 51·5 21·0 106 19·0 167 32 9065 0·33 115 108 1·0 904  Q3 24·86 656 41 0·27 0·28 1311 1·68 0·019 7·7 0·10 1·83 4·04 0·92 23·8 8·23 116 42·3 190 36·2 301 56 9335 0·39 129 115 1·1 944 ST11-08 (Middle Banded series, AN1, n = 20)  Q1 13·64 294 8 0·44 0·49 1502 2·16 0·033 2·9 0·02 0·73 2·46 0·10 20·8 8·26 117 47·6 249 54·7 499 10 7255 0·94 109 202 0·557 753  Median 23·43 302 10 0·48 0·56 1716 2·57 0·052 3·1 0·04 0·90 3·03 0·14 24·4 9·88 137 55·8 280 60·8 551 11 8083 1·05 146 238 0·617 778  Q3 53·00 307 11 0·62 0·61 1938 2·69 0·062 3·6 0·07 1·35 3·54 0·25 27·3 10·6 153 63·2 329 72·0 666 13 8829 1·24 193 294 0·672 791 ST05-04 (Middle Banded series, OB3, n = 14)  Q1 57·23 419 27 0·11 0·28 357 3·66 0·007 2·4 0·02 0·30 0·66 0·15 5·13 1·85 25·2 10·4 56 13·0 127 27 7434 1·32 244 310 0·729 889  Median 68·05 480 28 0·13 0·32 398 3·98 0·081 2·9 0·08 0·81 0·80 0·18 5·48 2·03 30·5 12·1 63 14·5 138 29 8215 1·44 294 393 0·768 896  Q3 75·85 538 34 0·17 0·76 415 4·13 0·174 4·0 0·21 1·07 1·23 0·33 6·09 2·22 31·8 12·6 64 15·3 144 31 8608 1·52 330 432 0·793 918 Quartile Li Sc Ti Rb Sr Y Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U Th/U Tzrc (°C) ST05-03 (Middle Banded series, AN2, n = 65)  Q1 19·58 170 12 0·26 0·32 1004 1·37 0·001 2·5 0·02 0·49 1·67 0·08 14·7 5·56 78·6 32·2 169 37·3 338 72 7684 0·51 103 170 0·531 799  Median 21·61 209 12 0·36 0·39 1375 1·74 0·002 2·9 0·04 1·25 3·31 0·17 23·0 7·27 108 44·1 216 48·1 445 93 8550 0·98 127 206 0·573 810  Q3 23·38 253 14 0·44 0·47 1704 2·24 0·010 3·7 0·10 1·90 4·15 0·20 36·8 9·02 135 54·8 280 59·4 555 115 8988 1·25 161 291 0·633 823 ST11-20 (Upper Banded series, GN3, n = 20)  Q1 50·28 495 18 0·17 0·37 630 1·49 0·131 6·6 0·18 2·19 1·79 0·38 9·86 3·65 47·6 18·8 96 20·2 187 39 7918 0·52 231 291 0·805 840  Median 90·80 646 20 0·19 0·50 992 1·66 0·275 7·9 0·34 3·71 4·21 0·59 21·3 6·77 85·1 30·9 145 29·1 250 49 8685 0·56 268 345 0·828 857  Q3 129·2 808 24 0·27 0·68 1065 1·89 1·395 10·0 0·79 6·08 5·37 1·78 23·8 7·17 88·8 32·8 155 30·8 264 53 8940 0·62 561 451 0·909 877 ST11-22 (Upper Banded series, GN3, n = 19)  Q1 97·30 281 7 0·31 0·61 860 4·30 0·011 6·5 0·08 1·67 3·38 0·25 16·4 4·88 63·0 27·2 139 33·6 315 71 7995 1·92 167 381 0·423 750  Median 103·1 291 9 0·47 0·79 1780 4·99 0·082 7·7 0·19 2·77 6·30 0·59 32·6 12·1 161 54·7 285 64·2 575 121 8580 2·41 293 421 0·676 770  Q3 113·9 298 11 0·64 1·43 2626 6·95 0·212 14·2 0·57 6·81 7·80 2·22 41·9 14·9 209 84·1 420 88·2 759 156 9520 5·11 323 459 0·721 792 ST11-34 (Mouat quartz monzonite, n = 20)  Q1 77·20 321 9 0·23 0·61 747 2·74 5·038 44·3 2·19 14·7 8·11 1·29 24·7 5·81 64·6 24·1 118 25·8 239 51 7729 0·64 196 254 0·623 767  Median 105·7 344 11 0·35 1·12 967 3·27 8·400 53·7 3·95 24·9 11·2 1·73 29·5 7·04 84·2 31·9 157 32·8 302 61 8701 0·95 252 342 0·713 791  Q3 109·9 361 18 0·48 2·00 1176 3·62 9·775 66·6 4·88 32·2 16·7 2·52 33·9 9·35 112 38·8 183 38·7 346 68 8808 1·14 335 429 0·828 840 Quartile Li Sc Ti Rb Sr Y Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U Th/U Tzrc (°C) ST05-03 (Middle Banded series, AN2, n = 65)  Q1 19·58 170 12 0·26 0·32 1004 1·37 0·001 2·5 0·02 0·49 1·67 0·08 14·7 5·56 78·6 32·2 169 37·3 338 72 7684 0·51 103 170 0·531 799  Median 21·61 209 12 0·36 0·39 1375 1·74 0·002 2·9 0·04 1·25 3·31 0·17 23·0 7·27 108 44·1 216 48·1 445 93 8550 0·98 127 206 0·573 810  Q3 23·38 253 14 0·44 0·47 1704 2·24 0·010 3·7 0·10 1·90 4·15 0·20 36·8 9·02 135 54·8 280 59·4 555 115 8988 1·25 161 291 0·633 823 ST11-20 (Upper Banded series, GN3, n = 20)  Q1 50·28 495 18 0·17 0·37 630 1·49 0·131 6·6 0·18 2·19 1·79 0·38 9·86 3·65 47·6 18·8 96 20·2 187 39 7918 0·52 231 291 0·805 840  Median 90·80 646 20 0·19 0·50 992 1·66 0·275 7·9 0·34 3·71 4·21 0·59 21·3 6·77 85·1 30·9 145 29·1 250 49 8685 0·56 268 345 0·828 857  Q3 129·2 808 24 0·27 0·68 1065 1·89 1·395 10·0 0·79 6·08 5·37 1·78 23·8 7·17 88·8 32·8 155 30·8 264 53 8940 0·62 561 451 0·909 877 ST11-22 (Upper Banded series, GN3, n = 19)  Q1 97·30 281 7 0·31 0·61 860 4·30 0·011 6·5 0·08 1·67 3·38 0·25 16·4 4·88 63·0 27·2 139 33·6 315 71 7995 1·92 167 381 0·423 750  Median 103·1 291 9 0·47 0·79 1780 4·99 0·082 7·7 0·19 2·77 6·30 0·59 32·6 12·1 161 54·7 285 64·2 575 121 8580 2·41 293 421 0·676 770  Q3 113·9 298 11 0·64 1·43 2626 6·95 0·212 14·2 0·57 6·81 7·80 2·22 41·9 14·9 209 84·1 420 88·2 759 156 9520 5·11 323 459 0·721 792 ST11-34 (Mouat quartz monzonite, n = 20)  Q1 77·20 321 9 0·23 0·61 747 2·74 5·038 44·3 2·19 14·7 8·11 1·29 24·7 5·81 64·6 24·1 118 25·8 239 51 7729 0·64 196 254 0·623 767  Median 105·7 344 11 0·35 1·12 967 3·27 8·400 53·7 3·95 24·9 11·2 1·73 29·5 7·04 84·2 31·9 157 32·8 302 61 8701 0·95 252 342 0·713 791  Q3 109·9 361 18 0·48 2·00 1176 3·62 9·775 66·6 4·88 32·2 16·7 2·52 33·9 9·35 112 38·8 183 38·7 346 68 8808 1·14 335 429 0·828 840 All concentrations reported in parts per million (ppm). Raw LA-ICP-MS data reduced using Iolite 2·5 trace elements reduction scheme with Zr concentration obtained by EPMA as internal standard. Q1, quartile 1; Q3, quartile 3. Q1–Q3 represents the range of the middle 75% of analyses for each sample. Tzrc (°C) refers to the Ti-in-zircon temperature calculated following the method of Ferry & Watson (2007) – see Discussion for additional calculation details. Table 3: Trace element concentrations (ppm) by LA-ICP-MS in zircon from the Stillwater Complex Quartile Li Sc Ti Rb Sr Y Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U Th/U Tzrc (°C) ST11-02 (Basal series, n = 20)  Q1 21·29 265 20 0·04 0·17 319 2·7 0·01 4·2 0·02 0·45 1·11 0·23 6·72 2·15 26·6 9·7 46 9·36 80 16 8587 1·59 312 103 1·7 856  Median 23·65 308 23 0·06 0·20 409 3·7 0·01 4·6 0·03 0·63 1·28 0·37 8·42 2·88 36·6 12·6 57 11·0 95 19 8796 2·11 439 175 2·7 871  Q3 25·81 338 25 0·09 0·24 597 4·3 0·51 5·9 0·06 1·57 2·18 0·76 10·8 3·56 47·9 17·8 82 16·1 134 27 9144 2·41 781 259 3·2 883 ST11-04 (Peridotite zone, n = 20)  Q1 51·23 183 17 0·09 0·24 422 1·4 0·017 4·3 0·04 0·68 1·36 0·36 9·80 2·83 37·0 12·2 61 11·2 88 15 8220 0·45 164 205 0·7 836  Median 81·80 250 20 0·14 0·28 707 1·5 0·032 6·9 0·11 1·83 2·85 0·62 15·2 5·32 65·7 21·5 92 16·6 132 23 8500 0·62 266 272 0·9 856  Q3 148·7 283 22 0·19 0·31 1029 1·8 0·042 7·8 0·27 3·73 5·95 1·06 28·6 8·90 100 32·8 133 23·0 170 28 8720 0·93 296 288 1·1 865 ST11-05 (Peridotite zone, n = 17)  Q1 20·18 293 26 0·09 0·20 328 2·33 0·007 3·0 0·03 0·68 1·64 0·38 8·12 2·37 30·9 10·6 44 8·47 68 13 9061 0·69 139 40 2·7 886  Median 23·10 393 33 0·14 0·26 495 2·73 0·017 3·3 0·11 2·21 3·15 0·73 12·7 4·38 52·5 16·2 66 11·6 91 17 9620 1·03 208 49 4·0 914  Q3 24·91 592 38 0·28 0·44 560 3·23 0·063 4·3 0·13 2·75 3·60 1·11 15·3 4·63 55·0 17·8 72 12·2 96 18 10 040 1·29 269 64 7·4 932 ST11-19 (Bronzitite zone, n = 25)  Q1 19·74 403 26 0·04 0·16 298 0·84 0·002 7·8 0·03 0·62 1·34 0·36 6·36 2·29 28·0 9·4 40 7·37 63 12 8407 0·41 107 24 3·7 886  Median 21·69 502 30 0·08 0·19 384 0·93 0·001 9·6 0·06 1·09 2·36 0·65 9·22 3·01 37·9 12·4 53 9·95 81 15 9390 0·47 134 28 4·2 902  Q3 23·40 593 35 0·13 0·21 631 1·01 0·003 10·3 0·09 1·62 4·00 1·17 13·7 4·71 64·1 20·9 83 14·4 114 20 9497 0·54 184 30 5·5 925 ST05-14 (Bronzitite zone, n = 19)  Q1 20·03 428 15 0·06 0·21 345 1·13 0·005 2·4 0·01 0·37 0·86 0·22 6·53 2·19 27·7 10·4 52 10·4 90 19 9885 0·09 164 9 10 820  Median 22·74 535 17 0·10 0·25 469 1·22 0·013 2·9 0·02 0·92 1·35 0·52 9·98 3·24 40·4 14·4 68 12·6 101 20 10 340 0·13 181 13 17 836  Q3 23·86 766 22 0·17 0·29 597 1·35 0·026 3·1 0·07 1·33 2·69 0·65 13·5 4·19 53·8 18·8 82 14·9 120 23 12 080 0·17 240 16 20 868 ST11-33 (Lower Banded series, N1, n = 20)  Q1 37·50 260 15 0·12 0·28 544 1·78 0·002 4·7 0·04 0·68 1·84 0·24 10·6 4·03 48·3 17·3 80 15·3 127 25 8226 0·45 102 105 0·8 821  Median 57·65 278 16 0·29 0·41 1247 1·89 0·005 7·6 0·12 1·86 3·59 0·51 23·2 7·84 105 39·3 186 38·5 333 65 8701 0·57 158 150 1·0 828  Q3 65·13 298 21 0·46 0·48 1538 2·26 0·018 11·6 0·17 2·67 5·88 0·83 32·8 10·7 136 49·8 225 44·6 390 75 9173 0·66 181 174 1·1 858 ST12-01 (OB1, mafic dike, n = 20)  Q1 20·50 302 20 0·23 0·25 1185 1·39 0·002 6·4 0·01 0·42 1·62 0·24 15·4 6·23 92·8 37·0 175 34·7 296 56 7788 0·20 97 97 0·8 853  Median 22·85 303 22 0·30 0·31 1353 1·43 0·003 7·1 0·02 0·98 3·62 0·53 27·5 9·04 119 44·0 206 41·6 355 66 8368 0·22 109 102 1·0 864  Q3 25·48 308 23 0·35 0·36 1482 1·54 0·004 7·5 0·11 2·48 5·64 0·77 31·8 10·4 132 48·2 219 42·7 364 69 8940 0·25 110 107 1·1 874 ST11-37 (Lower Banded series, OB1, J-M Reef, n = 20)  Q1 68·93 336 22 0·19 0·34 804 1·7 0·007 4·6 0·08 0·89 1·76 0·34 13·1 5·25 72·6 26·6 115 20·8 170 31 8168 0·40 127 142 0·8 865  Median 87·45 342 30 0·22 0·36 825 2·11 0·019 5·1 0·11 2·01 4·07 0·69 22·2 6·82 80·8 28·0 119 22·0 178 34 9082 0·57 167 218 0·9 904  Q3 116·4 355 37 0·25 0·43 891 2·39 0·075 5·6 0·17 2·43 4·40 0·72 22·6 7·03 84·4 29·1 133 25·8 214 41 10 228 0·68 278 309 1·0 931 ST11-16 (Lower Banded series, OB1, J-M Reef, n = 17)  Q1 58·60 320 23 0·08 0·28 427 3·12 0·007 1·1 0·00 0·09 0·61 0·04 5·56 2·44 31·8 12·5 60 13·6 127 29 9040 1·07 116 138 0·8 871  Median 83·00 470 29 0·16 0·31 521 4·34 0·007 1·6 0·03 0·54 1·82 0·19 13·2 3·89 36·5 15·6 76 17·6 164 34 9390 1·75 211 268 0·8 899  Q3 97·10 561 36 0·26 0·63 870 6·49 3·804 19·4 2·17 17·9 12·8 0·87 24·5 6·40 80·0 27·7 126 26·9 246 45 9890 2·35 590 360 1·1 926 ST05-08 (Lower Banded series, OB1, J-M Reef, n = 20)  Q1 20·50 349 23 0·09 0·21 453 1·41 0·006 4·8 0·02 0·80 1·69 0·34 9·87 3·51 43·5 14·8 62 11·2 89 17 8016 0·29 75 81 0·9 870  Median 22·90 480 30 0·18 0·24 634 1·47 0·010 6·2 0·07 1·11 2·60 0·50 14·1 3·86 51·5 21·0 106 19·0 167 32 9065 0·33 115 108 1·0 904  Q3 24·86 656 41 0·27 0·28 1311 1·68 0·019 7·7 0·10 1·83 4·04 0·92 23·8 8·23 116 42·3 190 36·2 301 56 9335 0·39 129 115 1·1 944 ST11-08 (Middle Banded series, AN1, n = 20)  Q1 13·64 294 8 0·44 0·49 1502 2·16 0·033 2·9 0·02 0·73 2·46 0·10 20·8 8·26 117 47·6 249 54·7 499 10 7255 0·94 109 202 0·557 753  Median 23·43 302 10 0·48 0·56 1716 2·57 0·052 3·1 0·04 0·90 3·03 0·14 24·4 9·88 137 55·8 280 60·8 551 11 8083 1·05 146 238 0·617 778  Q3 53·00 307 11 0·62 0·61 1938 2·69 0·062 3·6 0·07 1·35 3·54 0·25 27·3 10·6 153 63·2 329 72·0 666 13 8829 1·24 193 294 0·672 791 ST05-04 (Middle Banded series, OB3, n = 14)  Q1 57·23 419 27 0·11 0·28 357 3·66 0·007 2·4 0·02 0·30 0·66 0·15 5·13 1·85 25·2 10·4 56 13·0 127 27 7434 1·32 244 310 0·729 889  Median 68·05 480 28 0·13 0·32 398 3·98 0·081 2·9 0·08 0·81 0·80 0·18 5·48 2·03 30·5 12·1 63 14·5 138 29 8215 1·44 294 393 0·768 896  Q3 75·85 538 34 0·17 0·76 415 4·13 0·174 4·0 0·21 1·07 1·23 0·33 6·09 2·22 31·8 12·6 64 15·3 144 31 8608 1·52 330 432 0·793 918 Quartile Li Sc Ti Rb Sr Y Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U Th/U Tzrc (°C) ST11-02 (Basal series, n = 20)  Q1 21·29 265 20 0·04 0·17 319 2·7 0·01 4·2 0·02 0·45 1·11 0·23 6·72 2·15 26·6 9·7 46 9·36 80 16 8587 1·59 312 103 1·7 856  Median 23·65 308 23 0·06 0·20 409 3·7 0·01 4·6 0·03 0·63 1·28 0·37 8·42 2·88 36·6 12·6 57 11·0 95 19 8796 2·11 439 175 2·7 871  Q3 25·81 338 25 0·09 0·24 597 4·3 0·51 5·9 0·06 1·57 2·18 0·76 10·8 3·56 47·9 17·8 82 16·1 134 27 9144 2·41 781 259 3·2 883 ST11-04 (Peridotite zone, n = 20)  Q1 51·23 183 17 0·09 0·24 422 1·4 0·017 4·3 0·04 0·68 1·36 0·36 9·80 2·83 37·0 12·2 61 11·2 88 15 8220 0·45 164 205 0·7 836  Median 81·80 250 20 0·14 0·28 707 1·5 0·032 6·9 0·11 1·83 2·85 0·62 15·2 5·32 65·7 21·5 92 16·6 132 23 8500 0·62 266 272 0·9 856  Q3 148·7 283 22 0·19 0·31 1029 1·8 0·042 7·8 0·27 3·73 5·95 1·06 28·6 8·90 100 32·8 133 23·0 170 28 8720 0·93 296 288 1·1 865 ST11-05 (Peridotite zone, n = 17)  Q1 20·18 293 26 0·09 0·20 328 2·33 0·007 3·0 0·03 0·68 1·64 0·38 8·12 2·37 30·9 10·6 44 8·47 68 13 9061 0·69 139 40 2·7 886  Median 23·10 393 33 0·14 0·26 495 2·73 0·017 3·3 0·11 2·21 3·15 0·73 12·7 4·38 52·5 16·2 66 11·6 91 17 9620 1·03 208 49 4·0 914  Q3 24·91 592 38 0·28 0·44 560 3·23 0·063 4·3 0·13 2·75 3·60 1·11 15·3 4·63 55·0 17·8 72 12·2 96 18 10 040 1·29 269 64 7·4 932 ST11-19 (Bronzitite zone, n = 25)  Q1 19·74 403 26 0·04 0·16 298 0·84 0·002 7·8 0·03 0·62 1·34 0·36 6·36 2·29 28·0 9·4 40 7·37 63 12 8407 0·41 107 24 3·7 886  Median 21·69 502 30 0·08 0·19 384 0·93 0·001 9·6 0·06 1·09 2·36 0·65 9·22 3·01 37·9 12·4 53 9·95 81 15 9390 0·47 134 28 4·2 902  Q3 23·40 593 35 0·13 0·21 631 1·01 0·003 10·3 0·09 1·62 4·00 1·17 13·7 4·71 64·1 20·9 83 14·4 114 20 9497 0·54 184 30 5·5 925 ST05-14 (Bronzitite zone, n = 19)  Q1 20·03 428 15 0·06 0·21 345 1·13 0·005 2·4 0·01 0·37 0·86 0·22 6·53 2·19 27·7 10·4 52 10·4 90 19 9885 0·09 164 9 10 820  Median 22·74 535 17 0·10 0·25 469 1·22 0·013 2·9 0·02 0·92 1·35 0·52 9·98 3·24 40·4 14·4 68 12·6 101 20 10 340 0·13 181 13 17 836  Q3 23·86 766 22 0·17 0·29 597 1·35 0·026 3·1 0·07 1·33 2·69 0·65 13·5 4·19 53·8 18·8 82 14·9 120 23 12 080 0·17 240 16 20 868 ST11-33 (Lower Banded series, N1, n = 20)  Q1 37·50 260 15 0·12 0·28 544 1·78 0·002 4·7 0·04 0·68 1·84 0·24 10·6 4·03 48·3 17·3 80 15·3 127 25 8226 0·45 102 105 0·8 821  Median 57·65 278 16 0·29 0·41 1247 1·89 0·005 7·6 0·12 1·86 3·59 0·51 23·2 7·84 105 39·3 186 38·5 333 65 8701 0·57 158 150 1·0 828  Q3 65·13 298 21 0·46 0·48 1538 2·26 0·018 11·6 0·17 2·67 5·88 0·83 32·8 10·7 136 49·8 225 44·6 390 75 9173 0·66 181 174 1·1 858 ST12-01 (OB1, mafic dike, n = 20)  Q1 20·50 302 20 0·23 0·25 1185 1·39 0·002 6·4 0·01 0·42 1·62 0·24 15·4 6·23 92·8 37·0 175 34·7 296 56 7788 0·20 97 97 0·8 853  Median 22·85 303 22 0·30 0·31 1353 1·43 0·003 7·1 0·02 0·98 3·62 0·53 27·5 9·04 119 44·0 206 41·6 355 66 8368 0·22 109 102 1·0 864  Q3 25·48 308 23 0·35 0·36 1482 1·54 0·004 7·5 0·11 2·48 5·64 0·77 31·8 10·4 132 48·2 219 42·7 364 69 8940 0·25 110 107 1·1 874 ST11-37 (Lower Banded series, OB1, J-M Reef, n = 20)  Q1 68·93 336 22 0·19 0·34 804 1·7 0·007 4·6 0·08 0·89 1·76 0·34 13·1 5·25 72·6 26·6 115 20·8 170 31 8168 0·40 127 142 0·8 865  Median 87·45 342 30 0·22 0·36 825 2·11 0·019 5·1 0·11 2·01 4·07 0·69 22·2 6·82 80·8 28·0 119 22·0 178 34 9082 0·57 167 218 0·9 904  Q3 116·4 355 37 0·25 0·43 891 2·39 0·075 5·6 0·17 2·43 4·40 0·72 22·6 7·03 84·4 29·1 133 25·8 214 41 10 228 0·68 278 309 1·0 931 ST11-16 (Lower Banded series, OB1, J-M Reef, n = 17)  Q1 58·60 320 23 0·08 0·28 427 3·12 0·007 1·1 0·00 0·09 0·61 0·04 5·56 2·44 31·8 12·5 60 13·6 127 29 9040 1·07 116 138 0·8 871  Median 83·00 470 29 0·16 0·31 521 4·34 0·007 1·6 0·03 0·54 1·82 0·19 13·2 3·89 36·5 15·6 76 17·6 164 34 9390 1·75 211 268 0·8 899  Q3 97·10 561 36 0·26 0·63 870 6·49 3·804 19·4 2·17 17·9 12·8 0·87 24·5 6·40 80·0 27·7 126 26·9 246 45 9890 2·35 590 360 1·1 926 ST05-08 (Lower Banded series, OB1, J-M Reef, n = 20)  Q1 20·50 349 23 0·09 0·21 453 1·41 0·006 4·8 0·02 0·80 1·69 0·34 9·87 3·51 43·5 14·8 62 11·2 89 17 8016 0·29 75 81 0·9 870  Median 22·90 480 30 0·18 0·24 634 1·47 0·010 6·2 0·07 1·11 2·60 0·50 14·1 3·86 51·5 21·0 106 19·0 167 32 9065 0·33 115 108 1·0 904  Q3 24·86 656 41 0·27 0·28 1311 1·68 0·019 7·7 0·10 1·83 4·04 0·92 23·8 8·23 116 42·3 190 36·2 301 56 9335 0·39 129 115 1·1 944 ST11-08 (Middle Banded series, AN1, n = 20)  Q1 13·64 294 8 0·44 0·49 1502 2·16 0·033 2·9 0·02 0·73 2·46 0·10 20·8 8·26 117 47·6 249 54·7 499 10 7255 0·94 109 202 0·557 753  Median 23·43 302 10 0·48 0·56 1716 2·57 0·052 3·1 0·04 0·90 3·03 0·14 24·4 9·88 137 55·8 280 60·8 551 11 8083 1·05 146 238 0·617 778  Q3 53·00 307 11 0·62 0·61 1938 2·69 0·062 3·6 0·07 1·35 3·54 0·25 27·3 10·6 153 63·2 329 72·0 666 13 8829 1·24 193 294 0·672 791 ST05-04 (Middle Banded series, OB3, n = 14)  Q1 57·23 419 27 0·11 0·28 357 3·66 0·007 2·4 0·02 0·30 0·66 0·15 5·13 1·85 25·2 10·4 56 13·0 127 27 7434 1·32 244 310 0·729 889  Median 68·05 480 28 0·13 0·32 398 3·98 0·081 2·9 0·08 0·81 0·80 0·18 5·48 2·03 30·5 12·1 63 14·5 138 29 8215 1·44 294 393 0·768 896  Q3 75·85 538 34 0·17 0·76 415 4·13 0·174 4·0 0·21 1·07 1·23 0·33 6·09 2·22 31·8 12·6 64 15·3 144 31 8608 1·52 330 432 0·793 918 Quartile Li Sc Ti Rb Sr Y Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U Th/U Tzrc (°C) ST05-03 (Middle Banded series, AN2, n = 65)  Q1 19·58 170 12 0·26 0·32 1004 1·37 0·001 2·5 0·02 0·49 1·67 0·08 14·7 5·56 78·6 32·2 169 37·3 338 72 7684 0·51 103 170 0·531 799  Median 21·61 209 12 0·36 0·39 1375 1·74 0·002 2·9 0·04 1·25 3·31 0·17 23·0 7·27 108 44·1 216 48·1 445 93 8550 0·98 127 206 0·573 810  Q3 23·38 253 14 0·44 0·47 1704 2·24 0·010 3·7 0·10 1·90 4·15 0·20 36·8 9·02 135 54·8 280 59·4 555 115 8988 1·25 161 291 0·633 823 ST11-20 (Upper Banded series, GN3, n = 20)  Q1 50·28 495 18 0·17 0·37 630 1·49 0·131 6·6 0·18 2·19 1·79 0·38 9·86 3·65 47·6 18·8 96 20·2 187 39 7918 0·52 231 291 0·805 840  Median 90·80 646 20 0·19 0·50 992 1·66 0·275 7·9 0·34 3·71 4·21 0·59 21·3 6·77 85·1 30·9 145 29·1 250 49 8685 0·56 268 345 0·828 857  Q3 129·2 808 24 0·27 0·68 1065 1·89 1·395 10·0 0·79 6·08 5·37 1·78 23·8 7·17 88·8 32·8 155 30·8 264 53 8940 0·62 561 451 0·909 877 ST11-22 (Upper Banded series, GN3, n = 19)  Q1 97·30 281 7 0·31 0·61 860 4·30 0·011 6·5 0·08 1·67 3·38 0·25 16·4 4·88 63·0 27·2 139 33·6 315 71 7995 1·92 167 381 0·423 750  Median 103·1 291 9 0·47 0·79 1780 4·99 0·082 7·7 0·19 2·77 6·30 0·59 32·6 12·1 161 54·7 285 64·2 575 121 8580 2·41 293 421 0·676 770  Q3 113·9 298 11 0·64 1·43 2626 6·95 0·212 14·2 0·57 6·81 7·80 2·22 41·9 14·9 209 84·1 420 88·2 759 156 9520 5·11 323 459 0·721 792 ST11-34 (Mouat quartz monzonite, n = 20)  Q1 77·20 321 9 0·23 0·61 747 2·74 5·038 44·3 2·19 14·7 8·11 1·29 24·7 5·81 64·6 24·1 118 25·8 239 51 7729 0·64 196 254 0·623 767  Median 105·7 344 11 0·35 1·12 967 3·27 8·400 53·7 3·95 24·9 11·2 1·73 29·5 7·04 84·2 31·9 157 32·8 302 61 8701 0·95 252 342 0·713 791  Q3 109·9 361 18 0·48 2·00 1176 3·62 9·775 66·6 4·88 32·2 16·7 2·52 33·9 9·35 112 38·8 183 38·7 346 68 8808 1·14 335 429 0·828 840 Quartile Li Sc Ti Rb Sr Y Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U Th/U Tzrc (°C) ST05-03 (Middle Banded series, AN2, n = 65)  Q1 19·58 170 12 0·26 0·32 1004 1·37 0·001 2·5 0·02 0·49 1·67 0·08 14·7 5·56 78·6 32·2 169 37·3 338 72 7684 0·51 103 170 0·531 799  Median 21·61 209 12 0·36 0·39 1375 1·74 0·002 2·9 0·04 1·25 3·31 0·17 23·0 7·27 108 44·1 216 48·1 445 93 8550 0·98 127 206 0·573 810  Q3 23·38 253 14 0·44 0·47 1704 2·24 0·010 3·7 0·10 1·90 4·15 0·20 36·8 9·02 135 54·8 280 59·4 555 115 8988 1·25 161 291 0·633 823 ST11-20 (Upper Banded series, GN3, n = 20)  Q1 50·28 495 18 0·17 0·37 630 1·49 0·131 6·6 0·18 2·19 1·79 0·38 9·86 3·65 47·6 18·8 96 20·2 187 39 7918 0·52 231 291 0·805 840  Median 90·80 646 20 0·19 0·50 992 1·66 0·275 7·9 0·34 3·71 4·21 0·59 21·3 6·77 85·1 30·9 145 29·1 250 49 8685 0·56 268 345 0·828 857  Q3 129·2 808 24 0·27 0·68 1065 1·89 1·395 10·0 0·79 6·08 5·37 1·78 23·8 7·17 88·8 32·8 155 30·8 264 53 8940 0·62 561 451 0·909 877 ST11-22 (Upper Banded series, GN3, n = 19)  Q1 97·30 281 7 0·31 0·61 860 4·30 0·011 6·5 0·08 1·67 3·38 0·25 16·4 4·88 63·0 27·2 139 33·6 315 71 7995 1·92 167 381 0·423 750  Median 103·1 291 9 0·47 0·79 1780 4·99 0·082 7·7 0·19 2·77 6·30 0·59 32·6 12·1 161 54·7 285 64·2 575 121 8580 2·41 293 421 0·676 770  Q3 113·9 298 11 0·64 1·43 2626 6·95 0·212 14·2 0·57 6·81 7·80 2·22 41·9 14·9 209 84·1 420 88·2 759 156 9520 5·11 323 459 0·721 792 ST11-34 (Mouat quartz monzonite, n = 20)  Q1 77·20 321 9 0·23 0·61 747 2·74 5·038 44·3 2·19 14·7 8·11 1·29 24·7 5·81 64·6 24·1 118 25·8 239 51 7729 0·64 196 254 0·623 767  Median 105·7 344 11 0·35 1·12 967 3·27 8·400 53·7 3·95 24·9 11·2 1·73 29·5 7·04 84·2 31·9 157 32·8 302 61 8701 0·95 252 342 0·713 791  Q3 109·9 361 18 0·48 2·00 1176 3·62 9·775 66·6 4·88 32·2 16·7 2·52 33·9 9·35 112 38·8 183 38·7 346 68 8808 1·14 335 429 0·828 840 All concentrations reported in parts per million (ppm). Raw LA-ICP-MS data reduced using Iolite 2·5 trace elements reduction scheme with Zr concentration obtained by EPMA as internal standard. Q1, quartile 1; Q3, quartile 3. Q1–Q3 represents the range of the middle 75% of analyses for each sample. Tzrc (°C) refers to the Ti-in-zircon temperature calculated following the method of Ferry & Watson (2007) – see Discussion for additional calculation details. U–Pb Geochronology Analytical Procedures for Zircon, Baddeleyite, Titanite, and Rutile Zircon was analyzed by CA-ID-TIMS following the procedures modified from Mattinson (2005) and adapted at PCIGR as outlined in Scoates & Friedman (2008), Scoates & Wall (2015), and Wall et al. (2016). Baddeleyite, titanite, and rutile were analyzed by ID-TIMS – see the Supplementary Data for additional details. Isotopic ratios were measured on a modified single collector VG-354S (with Sector 54 electronics) thermal ionization mass spectrometer equipped with analogue Daly photomultipliers. Uranium fractionation was determined directly on individual runs using the EARTHTIME ET535 mixed 205Pb–233 U–235 U isotope tracer and Pb isotope ratios were corrected for fractionation of 0·25%/amu ± 0·03% (2σ) based on replicate analyses of NBS-982 reference material (Scoates & Wall, 2015). Data reduction was completed using the Excel-based program of Schmitz & Schoene (2007). Standard concordia diagrams, regression intercepts, and weighted averages were produced with Isoplot 3·09 (Ludwig, 2003). Unless otherwise noted all errors are quoted at the 95% confidence level (2σ). The complete U–Pb analytical results can be found in Supplementary DataTable A2 (zircon), Table A3 (baddeleyite), Table A4 (titanite), and Table A5 (rutile). A summary table of all zircon and baddeleyite weighted mean dates from each sample is provided in Table 4. The 207Pb/206Pb and 206Pb/238 U dates have been corrected for initial 230Th disequilibrium with a Th/Uliquid value of 3·25 ± 1·5 and a 238 U/235 U isotopic composition of 137·88; for comparison, Table 4 also contains a column with the dates calculated using 238 U/235 U = 137·818 (Hiess et al., 2012). Table 4: Summary of U-Pb zircon-baddeleyite dating results from the Stillwater Complex Sample# Series Rock Name Mineral Th/U Grains Weighted Mean 207Pb/206Pb Age1 2σ Weighted Mean 207Pb/206Pb Age2 2σ ST11-02 BS3 sulphide-bearing orthopyroxenite zircon 2·3–45 5 2712·24 0·63 2711·50 0·63 ST11-04 PZ orthopyroxenite zircon 0·91–1·3 5 2710·26 0·36 2709·52 0·36 ST11-24 PZ harzburgite zircon 0·17–0·47 4 2710·32 0·47 2709·58 0·47 ST11-24 PZ harzburgite baddeleyite 0·011–0·032 5 2709·99 0·40 2709·25 0·40 ST11-05 PZ websterite zircon 0·58–14 4 2711·37 0·36 2710·63 0·36 ST11-05 PZ websterite baddeleyite 0·028–0·074 2 2711·06 0·66 2710·31 0·66 ST11-19 BZ orthopyroxenite zircon 5·8–31 5 2711·35 0·39 2710·60 0·39 ST05-14 BZ orthopyroxenite zircon 0·90–17 9 2711·09 0·75 2710·35 0·75 ST11-33 N1 norite pegmatite zircon 0·69–1·1 8 2710·17 0·35 2709·43 0·35 ST05-13 N1 granite pegmatite zircon 0·59–0·86 5 2709·00 0·39 2708·26 0·39 ST12-01 JMD mafic dike zircon 0·66–1·1 5 2709·33 0·35 2708·59 0·35 ST12-01 JMD mafic dike baddeleyite 0·004–0·066 1 2709·40 0·90 2708·70 0·90 ST11-37 OB1 troctolite zircon 0·71–0·98 6 2709·28 0·32 2708·54 0·32 ST11-16 OB1 anorthosite zircon 0·031–1·1 7 2709·00 0·45 2708·25 0·45 ST05-08 OB1 troctolite zircon 0·84–1·1 5 2709·11 0·56 2708·37 0·56 ST05-08 OB1 troctolite baddeleyite 0·012–0·017 5 2708·85 0·46 2408·11 0·46 ST11-08 AN1 anorthosite zircon 0·58–0·70 7 2710·27 0·34 2709·52 0·34 ST05-04 OB3 olivine-bearing gabbronorite zircon 0·56–0·75 5 2709·00 0·53 2708·25 0·53 ST05-04 OB3 olivine-bearing gabbronorite baddeleyite 0·010–0·018 2 2708·45 0·78 2707·71 0·78 ST05-03 AN2 leucogabbro zircon 0·45–0·84 9 2710·44 0·32 2709·70 0·32 ST05-03 AN2 leucogabbro baddeleyite 0·008–0·018 3 2709·73 0·49 2708·99 0·49 ST11-20 GN3 gabbronorite zircon 0·39–0·90 4 2708·96 0·43 2708·22 0·43 ST11-22 GN3 magnetite-bearing anorthosite zircon 0·41–0·64 5 2709·01 0·44 2708·27 0·44 ST11-34 MQM quartz monzonite zircon 0·66–0·73 4 2803·43 0·55 2802·69 0·55 Sample# Series Rock Name Mineral Th/U Grains Weighted Mean 207Pb/206Pb Age1 2σ Weighted Mean 207Pb/206Pb Age2 2σ ST11-02 BS3 sulphide-bearing orthopyroxenite zircon 2·3–45 5 2712·24 0·63 2711·50 0·63 ST11-04 PZ orthopyroxenite zircon 0·91–1·3 5 2710·26 0·36 2709·52 0·36 ST11-24 PZ harzburgite zircon 0·17–0·47 4 2710·32 0·47 2709·58 0·47 ST11-24 PZ harzburgite baddeleyite 0·011–0·032 5 2709·99 0·40 2709·25 0·40 ST11-05 PZ websterite zircon 0·58–14 4 2711·37 0·36 2710·63 0·36 ST11-05 PZ websterite baddeleyite 0·028–0·074 2 2711·06 0·66 2710·31 0·66 ST11-19 BZ orthopyroxenite zircon 5·8–31 5 2711·35 0·39 2710·60 0·39 ST05-14 BZ orthopyroxenite zircon 0·90–17 9 2711·09 0·75 2710·35 0·75 ST11-33 N1 norite pegmatite zircon 0·69–1·1 8 2710·17 0·35 2709·43 0·35 ST05-13 N1 granite pegmatite zircon 0·59–0·86 5 2709·00 0·39 2708·26 0·39 ST12-01 JMD mafic dike zircon 0·66–1·1 5 2709·33 0·35 2708·59 0·35 ST12-01 JMD mafic dike baddeleyite 0·004–0·066 1 2709·40 0·90 2708·70 0·90 ST11-37 OB1 troctolite zircon 0·71–0·98 6 2709·28 0·32 2708·54 0·32 ST11-16 OB1 anorthosite zircon 0·031–1·1 7 2709·00 0·45 2708·25 0·45 ST05-08 OB1 troctolite zircon 0·84–1·1 5 2709·11 0·56 2708·37 0·56 ST05-08 OB1 troctolite baddeleyite 0·012–0·017 5 2708·85 0·46 2408·11 0·46 ST11-08 AN1 anorthosite zircon 0·58–0·70 7 2710·27 0·34 2709·52 0·34 ST05-04 OB3 olivine-bearing gabbronorite zircon 0·56–0·75 5 2709·00 0·53 2708·25 0·53 ST05-04 OB3 olivine-bearing gabbronorite baddeleyite 0·010–0·018 2 2708·45 0·78 2707·71 0·78 ST05-03 AN2 leucogabbro zircon 0·45–0·84 9 2710·44 0·32 2709·70 0·32 ST05-03 AN2 leucogabbro baddeleyite 0·008–0·018 3 2709·73 0·49 2708·99 0·49 ST11-20 GN3 gabbronorite zircon 0·39–0·90 4 2708·96 0·43 2708·22 0·43 ST11-22 GN3 magnetite-bearing anorthosite zircon 0·41–0·64 5 2709·01 0·44 2708·27 0·44 ST11-34 MQM quartz monzonite zircon 0·66–0·73 4 2803·43 0·55 2802·69 0·55 1Calculated using 238 U/235 U of 137·88 for comparison with existing datasets. 2Calculated using 238 U/235 U of 137·818 (Hiess et al., 2012); note that these ages are systematically younger than the ages calculated using 238U/235U of 137·88. 3Abbreviations: BS, Basal series; PZ, Peridotite zone; BZ, Bronzitite zone; N1, Norite zone I; JMD, J-M Reef mafic dike; OB1, Olivine-bearing zone I; AN1, Anorthosite zone I; OB3, Olivine-bearing zone III; AN2, Anorthosite zone II; GN3, Gabbronorite zone III; MQM, Mouat quartz monzonite; mag, magnetite. Note: results for all grains included in weighted mean calculations range from -0·05 to 0·5% discordance. Table 4: Summary of U-Pb zircon-baddeleyite dating results from the Stillwater Complex Sample# Series Rock Name Mineral Th/U Grains Weighted Mean 207Pb/206Pb Age1 2σ Weighted Mean 207Pb/206Pb Age2 2σ ST11-02 BS3 sulphide-bearing orthopyroxenite zircon 2·3–45 5 2712·24 0·63 2711·50 0·63 ST11-04 PZ orthopyroxenite zircon 0·91–1·3 5 2710·26 0·36 2709·52 0·36 ST11-24 PZ harzburgite zircon 0·17–0·47 4 2710·32 0·47 2709·58 0·47 ST11-24 PZ harzburgite baddeleyite 0·011–0·032 5 2709·99 0·40 2709·25 0·40 ST11-05 PZ websterite zircon 0·58–14 4 2711·37 0·36 2710·63 0·36 ST11-05 PZ websterite baddeleyite 0·028–0·074 2 2711·06 0·66 2710·31 0·66 ST11-19 BZ orthopyroxenite zircon 5·8–31 5 2711·35 0·39 2710·60 0·39 ST05-14 BZ orthopyroxenite zircon 0·90–17 9 2711·09 0·75 2710·35 0·75 ST11-33 N1 norite pegmatite zircon 0·69–1·1 8 2710·17 0·35 2709·43 0·35 ST05-13 N1 granite pegmatite zircon 0·59–0·86 5 2709·00 0·39 2708·26 0·39 ST12-01 JMD mafic dike zircon 0·66–1·1 5 2709·33 0·35 2708·59 0·35 ST12-01 JMD mafic dike baddeleyite 0·004–0·066 1 2709·40 0·90 2708·70 0·90 ST11-37 OB1 troctolite zircon 0·71–0·98 6 2709·28 0·32 2708·54 0·32 ST11-16 OB1 anorthosite zircon 0·031–1·1 7 2709·00 0·45 2708·25 0·45 ST05-08 OB1 troctolite zircon 0·84–1·1 5 2709·11 0·56 2708·37 0·56 ST05-08 OB1 troctolite baddeleyite 0·012–0·017 5 2708·85 0·46 2408·11 0·46 ST11-08 AN1 anorthosite zircon 0·58–0·70 7 2710·27 0·34 2709·52 0·34 ST05-04 OB3 olivine-bearing gabbronorite zircon 0·56–0·75 5 2709·00 0·53 2708·25 0·53 ST05-04 OB3 olivine-bearing gabbronorite baddeleyite 0·010–0·018 2 2708·45 0·78 2707·71 0·78 ST05-03 AN2 leucogabbro zircon 0·45–0·84 9 2710·44 0·32 2709·70 0·32 ST05-03 AN2 leucogabbro baddeleyite 0·008–0·018 3 2709·73 0·49 2708·99 0·49 ST11-20 GN3 gabbronorite zircon 0·39–0·90 4 2708·96 0·43 2708·22 0·43 ST11-22 GN3 magnetite-bearing anorthosite zircon 0·41–0·64 5 2709·01 0·44 2708·27 0·44 ST11-34 MQM quartz monzonite zircon 0·66–0·73 4 2803·43 0·55 2802·69 0·55 Sample# Series Rock Name Mineral Th/U Grains Weighted Mean 207Pb/206Pb Age1 2σ Weighted Mean 207Pb/206Pb Age2 2σ ST11-02 BS3 sulphide-bearing orthopyroxenite zircon 2·3–45 5 2712·24 0·63 2711·50 0·63 ST11-04 PZ orthopyroxenite zircon 0·91–1·3 5 2710·26 0·36 2709·52 0·36 ST11-24 PZ harzburgite zircon 0·17–0·47 4 2710·32 0·47 2709·58 0·47 ST11-24 PZ harzburgite baddeleyite 0·011–0·032 5 2709·99 0·40 2709·25 0·40 ST11-05 PZ websterite zircon 0·58–14 4 2711·37 0·36 2710·63 0·36 ST11-05 PZ websterite baddeleyite 0·028–0·074 2 2711·06 0·66 2710·31 0·66 ST11-19 BZ orthopyroxenite zircon 5·8–31 5 2711·35 0·39 2710·60 0·39 ST05-14 BZ orthopyroxenite zircon 0·90–17 9 2711·09 0·75 2710·35 0·75 ST11-33 N1 norite pegmatite zircon 0·69–1·1 8 2710·17 0·35 2709·43 0·35 ST05-13 N1 granite pegmatite zircon 0·59–0·86 5 2709·00 0·39 2708·26 0·39 ST12-01 JMD mafic dike zircon 0·66–1·1 5 2709·33 0·35 2708·59 0·35 ST12-01 JMD mafic dike baddeleyite 0·004–0·066 1 2709·40 0·90 2708·70 0·90 ST11-37 OB1 troctolite zircon 0·71–0·98 6 2709·28 0·32 2708·54 0·32 ST11-16 OB1 anorthosite zircon 0·031–1·1 7 2709·00 0·45 2708·25 0·45 ST05-08 OB1 troctolite zircon 0·84–1·1 5 2709·11 0·56 2708·37 0·56 ST05-08 OB1 troctolite baddeleyite 0·012–0·017 5 2708·85 0·46 2408·11 0·46 ST11-08 AN1 anorthosite zircon 0·58–0·70 7 2710·27 0·34 2709·52 0·34 ST05-04 OB3 olivine-bearing gabbronorite zircon 0·56–0·75 5 2709·00 0·53 2708·25 0·53 ST05-04 OB3 olivine-bearing gabbronorite baddeleyite 0·010–0·018 2 2708·45 0·78 2707·71 0·78 ST05-03 AN2 leucogabbro zircon 0·45–0·84 9 2710·44 0·32 2709·70 0·32 ST05-03 AN2 leucogabbro baddeleyite 0·008–0·018 3 2709·73 0·49 2708·99 0·49 ST11-20 GN3 gabbronorite zircon 0·39–0·90 4 2708·96 0·43 2708·22 0·43 ST11-22 GN3 magnetite-bearing anorthosite zircon 0·41–0·64 5 2709·01 0·44 2708·27 0·44 ST11-34 MQM quartz monzonite zircon 0·66–0·73 4 2803·43 0·55 2802·69 0·55 1Calculated using 238 U/235 U of 137·88 for comparison with existing datasets. 2Calculated using 238 U/235 U of 137·818 (Hiess et al., 2012); note that these ages are systematically younger than the ages calculated using 238U/235U of 137·88. 3Abbreviations: BS, Basal series; PZ, Peridotite zone; BZ, Bronzitite zone; N1, Norite zone I; JMD, J-M Reef mafic dike; OB1, Olivine-bearing zone I; AN1, Anorthosite zone I; OB3, Olivine-bearing zone III; AN2, Anorthosite zone II; GN3, Gabbronorite zone III; MQM, Mouat quartz monzonite; mag, magnetite. Note: results for all grains included in weighted mean calculations range from -0·05 to 0·5% discordance. RESULTS Trace Element Concentrations in Zircon Trace element concentrations in zircon were determined in situ by LA-ICP-MS. A total of 376 point measurements were made on 112 grains from 16 samples and they represent a range of grain sizes, morphologies and internal zoning. The LA-ICP-MS analyses span a wide range of concentrations (Table 3; Figs 5–8), both within individual samples and between them. There is significant variation in some of the important trace elements that substitute in the crystal structure of zircon, including U (7–800 ppm), Th (5–1237 ppm), Hf (4200–13 000 ppm), and Ti (5–55 ppm) (Fig. 5). Most of the analyzed zircon from the Stillwater Complex has typically low Th/U (∼0·5–1), however, zircon in pyroxenites and harzburgites from the Ultramafic series (e.g. ST11–05, ST11–19, ST05–14), and a few spot analyses of zircon from the Basal series orthopyroxenite (ST11–02), are characterized by unusually high Th/U (2–22) (Fig. 5a). Titanium contents are highest in zircon from the J-M Reef (up to 56 ppm in ST11–16) and lowest in those from the AN1 and AN2 anorthosites from the Middle Banded series (∼5 ppm) (Fig. 5b). The analyses from a number of samples show systematic trends of decreasing Ti with increasing Hf (Fig. 5b inset, Supplementary DataFig. A2). Zircon from the anorthosites defines a distinctive field of relatively low Ti compared with the majority of zircon analyzed from other stratigraphic intervals of the Stillwater Complex. For zircon grains where multiple spot analyses were done, there does not appear to be a strong correlation between elemental concentration (e.g. Hf, Ti) and spatial distribution within individual grains (core v. rim) or CL response (e.g. dark, light) (Fig. 6). Fig. 5. View largeDownload slide Variations in trace element concentrations of zircon from the Stillwater Complex determined in situ by LA-ICP-MS. Each point represents the analysis of a single spot. (a) Th v. U (ppm); dashed lines indicate reference Th/U values of 0·5, 1, 2, 5, 10, and 15. (b) Ti v. Hf (ppm); the inset shows the results for a single sample, ST11–37, from the J-M Reef. The mean 2σ analytical uncertainties are indicated in each panel. The points are colour coded and correspond to the colours of the appropriate series or zone on the stratigraphic section in Figure 1; the symbol shapes distinguish the different rock types (e.g. norite, gabbronorite, harzburgite, etc.) or specific units (J-M Reef). The field defined by LA-ICP-MS analyses of zircon from the Mouat quartz monzonite (grey field) is shown on both plots for reference. Fig. 5. View largeDownload slide Variations in trace element concentrations of zircon from the Stillwater Complex determined in situ by LA-ICP-MS. Each point represents the analysis of a single spot. (a) Th v. U (ppm); dashed lines indicate reference Th/U values of 0·5, 1, 2, 5, 10, and 15. (b) Ti v. Hf (ppm); the inset shows the results for a single sample, ST11–37, from the J-M Reef. The mean 2σ analytical uncertainties are indicated in each panel. The points are colour coded and correspond to the colours of the appropriate series or zone on the stratigraphic section in Figure 1; the symbol shapes distinguish the different rock types (e.g. norite, gabbronorite, harzburgite, etc.) or specific units (J-M Reef). The field defined by LA-ICP-MS analyses of zircon from the Mouat quartz monzonite (grey field) is shown on both plots for reference. Fig. 6. View largeDownload slide Trace element variations in representative zircon grains from mafic-ultramafic rocks in the Stillwater Complex. (a) ST12–01 (JMD, OB1, LBS). (b) ST11–02 (BS). (c) ST11–19 (BZ). (d) ST05–14 (BZ). (e) ST11–08 (AN1, MBS). (f) ST11–20 (GN3, UBS). Each pair of images in a panel contains an upper part with a SEM-CL image and a lower part with variations in Hf and Ti (both in ppm) that correspond to the LA-ICP-MS spot analyses (white spots, ∼33 μm in diameter). The spot numbers (e.g. LA1, LA2, etc.) correspond to the analysis number. Note that the spot analyses are arranged in increasing number from left to right and do not correspond to single transects through the grains. The corresponding Hf and Ti concentrations illustrate that the elemental variability in individual grains is not spatially systematic (i.e. core-to-rim) or with respect to CL brightness. The 2σ analytical uncertainties for each spot analysis are indicated in each panel. Refer to the caption of Figure 2 for letter codes. Fig. 6. View largeDownload slide Trace element variations in representative zircon grains from mafic-ultramafic rocks in the Stillwater Complex. (a) ST12–01 (JMD, OB1, LBS). (b) ST11–02 (BS). (c) ST11–19 (BZ). (d) ST05–14 (BZ). (e) ST11–08 (AN1, MBS). (f) ST11–20 (GN3, UBS). Each pair of images in a panel contains an upper part with a SEM-CL image and a lower part with variations in Hf and Ti (both in ppm) that correspond to the LA-ICP-MS spot analyses (white spots, ∼33 μm in diameter). The spot numbers (e.g. LA1, LA2, etc.) correspond to the analysis number. Note that the spot analyses are arranged in increasing number from left to right and do not correspond to single transects through the grains. The corresponding Hf and Ti concentrations illustrate that the elemental variability in individual grains is not spatially systematic (i.e. core-to-rim) or with respect to CL brightness. The 2σ analytical uncertainties for each spot analysis are indicated in each panel. Refer to the caption of Figure 2 for letter codes. Fig. 7. View largeDownload slide Trace element characteristics of zircon from the Stillwater Complex determined in situ by LA-ICP-MS. Each point represents the analysis of a single spot. (a) Stacked histogram of Li concentrations. (b) the sum of the REE v. Hf (ppm). (c) Ti v. Ce/Nd. (d) (Sm/La)N v. La, where subscript N indicates chondrite-normalized values. The mean 2σ analytical uncertainties are indicated in each panel. The points are colour coded and correspond to the colours of the appropriate series or zone on the stratigraphic section in Figure 1; the symbol shapes distinguish the different rock types (e.g. norite, gabbronorite, harzburgite, etc.) or specific units (J-M Reef). Also shown on all plots for reference is the field defined by LA-ICP-MS analyses of zircon from the Mouat quartz monzonite (grey field). Fig. 7. View largeDownload slide Trace element characteristics of zircon from the Stillwater Complex determined in situ by LA-ICP-MS. Each point represents the analysis of a single spot. (a) Stacked histogram of Li concentrations. (b) the sum of the REE v. Hf (ppm). (c) Ti v. Ce/Nd. (d) (Sm/La)N v. La, where subscript N indicates chondrite-normalized values. The mean 2σ analytical uncertainties are indicated in each panel. The points are colour coded and correspond to the colours of the appropriate series or zone on the stratigraphic section in Figure 1; the symbol shapes distinguish the different rock types (e.g. norite, gabbronorite, harzburgite, etc.) or specific units (J-M Reef). Also shown on all plots for reference is the field defined by LA-ICP-MS analyses of zircon from the Mouat quartz monzonite (grey field). Fig. 8. View largeDownload slide Chondrite-normalized rare earth element patterns for zircon from the Stillwater Complex determined in situ by LA-ICP-MS; all diagrams are plotted at the same scale. Lines represent the median values for analyses from each sample and shaded areas correspond to the range in composition; lines and shaded areas are colour coded and correspond to the colours of the appropriate series or zone on the stratigraphic section in Figure 1. Samples are grouped by similar stratigraphic location; abbreviations for zones (e.g. OB1, GN3, etc.) are found in the caption to Figure 1. Panel (h) shows the patterns for zircon from intrusions associated with Stillwater Complex, the Mouat quartz monzonite (ST11–34) and a mafic dike (ST12–01) that cuts the J-M Reef at the East Boulder mine. Concentrations are normalized to the C1 chondrite values of McDonough & Sun (1995). Fig. 8. View largeDownload slide Chondrite-normalized rare earth element patterns for zircon from the Stillwater Complex determined in situ by LA-ICP-MS; all diagrams are plotted at the same scale. Lines represent the median values for analyses from each sample and shaded areas correspond to the range in composition; lines and shaded areas are colour coded and correspond to the colours of the appropriate series or zone on the stratigraphic section in Figure 1. Samples are grouped by similar stratigraphic location; abbreviations for zones (e.g. OB1, GN3, etc.) are found in the caption to Figure 1. Panel (h) shows the patterns for zircon from intrusions associated with Stillwater Complex, the Mouat quartz monzonite (ST11–34) and a mafic dike (ST12–01) that cuts the J-M Reef at the East Boulder mine. Concentrations are normalized to the C1 chondrite values of McDonough & Sun (1995). The use of trace elements in zircon from mafic–ultramafic rocks in the Stillwater Complex as a petrologic fingerprint also includes Li and the rare earth elements (REE) (Figs 7 and 8). Lithium, an important substituting cation in zircon (e.g. Bouvier et al., 2012), varies from 1–230 ppm with most analyses in the range of 10–30 ppm, in samples from all major series of the intrusion (e.g. Basal, Ultramafic, Banded) (Fig. 7a). Li concentrations in Stillwater zircon are significantly higher than the very low contents of oceanic crust zircon (<0·01 ppm; Grimes et al., 2009) and are typical of zircon from continental crustal environments (Ushikubo et al., 2008; Bouvier et al., 2012). Stillwater zircon displays a wide range in total REE content (61–2452 ppm). Those from ultramafic rocks are characterized by low total REE (<400 ppm), whereas high total REE (typically higher than 1000 ppm) characterize zircon from the AN1 and AN2 anorthosites (Fig. 8b), which are also distinguished by their light REE, or LREE, content (e.g. low Ce/Nd and low Ti, Fig. 7c). Zircon from the Stillwater Complex has variable (Sm/La)N chondrite-normalized ratios (2–10 000), spanning the range from normal magmatic zircon to low ratios (<10) (Fig. 7d). This may either be an effect of alteration of zircon by hydrothermal fluids (i.e. gain of LREE) or analyses of sub-micron inclusions during ablation (e.g. Hoskin & Schaltegger, 2003; Grimes et al., 2009). The chondrite-normalized REE patterns are broadly sub-parallel with strong LREE-depletion, reflecting the relative incompatibility of LREE in zircon, and they have a slight concave-down shape in the HREE (Fig. 8); chondrite-normalized REE patterns are shown for each individual spot analysis in Supplementary DataFigures A3 and A4. All patterns have the characteristic positive Ce anomalies (CeN/CeN* = 0·52–288) and negative Eu anomalies (average EuN/EuN* = 0·2) of zircon. Europium anomalies are most strongly negative (EuN/EuN* = 0·03) in the AN1 and AN2 anorthosites. Zircon from ST11–16 (J-M Reef) and ST11–34 (Mouat quartz monzonite) shows anomalous behaviour with respect to the LREE and displays relatively flat chondrite-normalized patterns (e.g. (Sm/La)N =1–10; Fig. 7d, 8). In ST11–16, these anomalous analyses are from two grains (2, 11) that in CL images lack zoning and contain fractures that may host sub-micron inclusions of secondary minerals such as apatite, monazite, or allanite. Cathodoluminescence images of zircon from the Mouat quartz monzonite show strong oscillatory zoning and reveal minor inclusions; it is likely that the consistently flat LREE in the patterns for all zircon grains in this sample resulted from intersection of sub-micron inclusions during ablation. U–Pb Geochronology Results The CA-TIMS U–Pb data for zircon from the mafic-ultramafic rocks are remarkably coherent and all samples yield weighted mean 207Pb/206Pb dates in the range from 2712 Ma to 2709 Ma (Fig. 9). Many of the U–Pb results for the analyzed baddeleyite grains in the same samples also exhibit concordant results and similar dates (Fig. 10a–g), however, some analyses are slightly discordant (up to 2·9%) with slightly younger 207Pb/206Pb dates (as young as 2702 Ma) (Supplementary DataTable A3). The U–Pb results are concordant for zircon from the Mouat quartz monzonite (ST11–34, Fig. 10h) and from one of the granophyres (ST05–13, Fig. 10i); the analyses of zircon from all other granophyres yield strongly discordant U–Pb results and variable ages (Supplementary DataFig. A5). The uncertainty on the weighted mean 207Pb/206Pb dates reported in Figures 9 and 10 is in the ±X/Y/Z format (±2σ) of Schoene et al. (2006) with internal error in the absence of all systematic errors (±X), including tracer calibration error (±Y), and including uncertainty due to decay-constant errors (±Z); in the text below, all dates are reported using only the ±X format for comparison as all analyses are from the same laboratory. These dates are interpreted as the age of crystallization of zircon at near-solidus temperatures. In addition to the 12 samples reported below, U–Pb results are also available for a leucogabbro from Anorthosite-II (Wall et al., 2016) and for three samples of the J-M Reef (Wall & Scoates, 2016). Concordia plots for these additional samples are shown in Figures 9 and 10 for direct comparison with the entire U–Pb geochronological dataset from the Stillwater Complex. Fig. 9. View largeDownload slide Concordia diagrams showing U–Pb geochronological CA-ID-TIMS results for chemically abraded zircon for all dated ultramafic–mafic rocks of the Stillwater Complex, including results for one sample (ST05–03) from Anorthosite zone II (Wall et al., 2016) and three samples (ST05–08, ST11–16, ST11–37) from the J-M Reef (Wall & Scoates, 2016). The panels are arranged to show results for samples from the base of the intrusion (a) stratigraphically upward through the top of the intrusion (o). Calculated weighted mean 207Pb/206Pb ages are indicated where the uncertainty is reported in the ± X/Y/Z format (±2σ) with internal error in the absence of all systematic errors (±X), including tracer calibration error (±Y), and including uncertainty due to decay-constant errors (±Z). Concordia curve (solid black line) shows ages in millions of years (Ma). Each ellipse indicates the analysis of a single zircon grain. Dashed lines show the error bounds of the concordia curve due to uncertainty in the decay constants of uranium. MSWD refers to the mean square of the weighted deviates. Inset panels to the right in the concordia diagrams show bar diagrams where each bar represents the analysis of each single crystal; the horizontal black line indicates the weighted mean date and the light grey band reflects the external reproducibility. Colours of the ellipses and of the individual 207Pb/206Pb dates in the insets correspond to the colour of the associated zone or series based on the stratigraphic column in Figure 1. Refer to the caption of Figure 2 for letter codes. Fig. 9. View largeDownload slide Concordia diagrams showing U–Pb geochronological CA-ID-TIMS results for chemically abraded zircon for all dated ultramafic–mafic rocks of the Stillwater Complex, including results for one sample (ST05–03) from Anorthosite zone II (Wall et al., 2016) and three samples (ST05–08, ST11–16, ST11–37) from the J-M Reef (Wall & Scoates, 2016). The panels are arranged to show results for samples from the base of the intrusion (a) stratigraphically upward through the top of the intrusion (o). Calculated weighted mean 207Pb/206Pb ages are indicated where the uncertainty is reported in the ± X/Y/Z format (±2σ) with internal error in the absence of all systematic errors (±X), including tracer calibration error (±Y), and including uncertainty due to decay-constant errors (±Z). Concordia curve (solid black line) shows ages in millions of years (Ma). Each ellipse indicates the analysis of a single zircon grain. Dashed lines show the error bounds of the concordia curve due to uncertainty in the decay constants of uranium. MSWD refers to the mean square of the weighted deviates. Inset panels to the right in the concordia diagrams show bar diagrams where each bar represents the analysis of each single crystal; the horizontal black line indicates the weighted mean date and the light grey band reflects the external reproducibility. Colours of the ellipses and of the individual 207Pb/206Pb dates in the insets correspond to the colour of the associated zone or series based on the stratigraphic column in Figure 1. Refer to the caption of Figure 2 for letter codes. Fig. 10. View largeDownload slide Concordia diagrams showing additional U–Pb geochronological results for rocks from the Stillwater Complex. (a)–(e) show the results for untreated and air-abraded baddeleyite from mafic–ultramafic rocks of the Stillwater Complex. (f)–(g) include results for baddeleyite and zircon from the mafic dike that cuts the J-M Reef (JMD) at the East Boulder mine (ST12–01). (h)–(i) show CA-ID-TIMS results for chemically abraded zircon from the Mouat quartz monzonite (ST11–34, MQM) and from a granophyre in the Norite zone 1 (ST05–13). The baddeleyite results for sample ST05–03 from Anorthosite zone II were reported in Wall et al. (2016) and the results for sample ST05–08 from the J-M Reef were reported in Wall & Scoates (2016). Each ellipse indicates the analysis of a single baddeleyite or single zircon grain. All information on the format of the diagrams and reported dates is the same as that contained in the caption to Figure 9. Refer to the caption of Figure 2 for letter codes. Fig. 10. View largeDownload slide Concordia diagrams showing additional U–Pb geochronological results for rocks from the Stillwater Complex. (a)–(e) show the results for untreated and air-abraded baddeleyite from mafic–ultramafic rocks of the Stillwater Complex. (f)–(g) include results for baddeleyite and zircon from the mafic dike that cuts the J-M Reef (JMD) at the East Boulder mine (ST12–01). (h)–(i) show CA-ID-TIMS results for chemically abraded zircon from the Mouat quartz monzonite (ST11–34, MQM) and from a granophyre in the Norite zone 1 (ST05–13). The baddeleyite results for sample ST05–03 from Anorthosite zone II were reported in Wall et al. (2016) and the results for sample ST05–08 from the J-M Reef were reported in Wall & Scoates (2016). Each ellipse indicates the analysis of a single baddeleyite or single zircon grain. All information on the format of the diagrams and reported dates is the same as that contained in the caption to Figure 9. Refer to the caption of Figure 2 for letter codes. Basal & ultramafic series The oldest dated mafic–ultramafic rock in this study is a sulphide-bearing feldspathic orthopyroxenite (ST11–02) of the Basal series (Benbow area) with a weighted mean 207Pb/206Pb date of 2712·24 ± 0·61 Ma (Fig. 9a). Three samples from the Peridotite zone of the Ultramafic series yield a range of dates, including, from stratigraphically lowest to highest, a date of 2710·26 ± 0·35 Ma for a poikilitic harzburgite (ST11–04) collected in the vicinity of the B chromitite from the Benbow area (Fig. 9b), a date of 2710·32 ± 0·46 Ma for a pegmatitic feldspathic harzburgite associated with the G chromitite (ST11–24) in the Mountain View area (Fig. 9c), and a date 2711·37 ± 0·35 Ma for a pegmatitic feldspathic pyroxenite (websterite) (ST11–05) from the J chromitite seam in the Benbow area (Fig. 9d) that is intermediate in age between the Basal series sample and the underlying samples of the Peridotite zone. The weighted mean 207Pb/206Pb dates from coexisting baddeleyite from two Peridotite zone samples overlap within uncertainty with the dates established from the chemically abraded zircon from the same samples, including ST11–24 at 2709·99 ± 0·39 Ma (n = 5; Fig. 10a) and ST11–05 at 2711·05 ± 0·64 Ma (n = 2; Fig. 10b). The dates for two samples from the Bronzitite zone are statistically indistinguishable and similar to the c.2711 Ma date from the pyroxenite of the J chromitite in the upper part of the underlying Peridotite zone. A pegmatitic feldspathic orthopyroxenite (ST11–19) from the base of the Bronzitite zone in the Iron Mountain area is dated at 2711·35 ± 0·38 Ma (Fig. 9e). The U–Pb data for all nine chemically abraded zircon grains from sample ST05–14, a pegmatitic feldspathic orthopyroxenite from the top of the Bronzitite zone in the Chrome Mountain area, yield a date of 2711·09 ± 0·74 Ma (Fig. 9f); the relatively high uncertainty of this date is related to the exceptionally low U-concentrations in zircon from this sample (median = 13 ppm, Table 3). Banded series The zircon dates are out-of-sequence in the Banded series and do not decrease systematically with increasing stratigraphic height, but rather alternate up-section between crystallization ages of 2710 Ma and of 2709 Ma (Fig. 9). Zircon from the stratigraphically lowest sample (ST11–33), a pegmatitic norite from the base of Norite zone I of the Lower Banded series in the Mountain View area, gives a weighted mean 207Pb/206Pb date of 2710·17 ± 0·35 Ma (Fig. 9g), younger than the dated samples from the underlying Bronzitite zone and from the J chromitite in the Peridotite zone. The three dated samples from the J-M Reef in Olivine-bearing zone I of the Lower Banded series are all younger at 2709·11 ± 0·56 Ma (ST05–08, Frog Pond adit, Fig. 9h), 2709·00 ± 0·45 Ma (ST11–16, West Fork Stillwater River, Fig. 9i), and 2709·28 ± 0·31 Ma (ST11–37, East Boulder mine, Fig. 9j) (Wall & Scoates, 2016). The U–Pb results for zircon from sample ST11–08, a coarse-grained anorthosite from AN1 of the Middle Banded series in the Benbow area, yield an older date of 2710·27 ± 0·34 Ma (Fig. 9k). The Olivine-bearing zone III occurs between the two thick anorthosite horizons in the Middle Banded series (AN1 and AN2) and an olivine-bearing gabbronorite (ST05–04) from this zone in the Picket Pin Mountain area gives a weighted mean 207Pb/206Pb zircon date based on the most concordant results of 2709·00 ± 0·53 Ma (Fig. 9l), a date that is younger than those from both anorthosite units (Anorthosite zone II, 2710·44 ± 0·32 Ma, Fig. 9m;Wall et al., 2016). The two analyzed samples from Gabbronorite zone III of the Upper Banded series yield statistically indistinguishable dates from the J-M Reef samples in the Lower Banded series and from OB3 in the Middle Banded series. A magnetite-bearing anorthosite (ST11–22) from the Castle Creek Cirque area is dated at 2709·01 ± 0·44 Ma (Fig. 9n) and a coarse-grained gabbronorite (ST11–20) from the uppermost exposure of the Upper Banded series in the Mish Mash Ridge area to the northwest of Picket Pin Mountain gives a date of 2708·96 ± 0·43 Ma (Fig. 9o). Mafic dike (East Boulder Mine) Sample ST12–01 is a mafic dike that crosscuts the J-M Reef from Olivine-bearing zone I of the Lower Banded series at the East Boulder mine. The U–Pb data for all five chemically abraded zircon grains yield overlapping 207Pb/206Pb dates from 2709·14 to 2709·50 Ma (Table 4) with a weighted mean 207Pb/206Pb date of 2709·33 ± 0·34 Ma (Fig. 10f), which is identical within uncertainty to the c.2709 Ma dates from the J-M Reef (Wall & Scoates, 2016). The U–Pb data for all six analyzed baddeleyite grains from the dike range from slightly discordant to discordant (D = 0·06–5·73%) and yield a range in 207Pb/206Pb dates from 2695·27 to 2709·41 Ma (Supplementary DataTable A3). The most concordant baddeleyite result has a 207Pb/206Pb date of 2709·41 ± 0·87 (Fig. 10g) that overlaps within uncertainty with the weighted mean 207Pb/206Pb date from the chemically abraded zircon. Mouat quartz monzonite Sample ST11–34 is a coarse-grained quartz monzonite from the Mouat quartz monzonite in the Stillwater mine area. The U–Pb data for all four chemically abraded zircon grains are concordant and yield overlapping 207Pb/206Pb dates from 2801·85 to 2803·66 Ma (Supplementary DataTable A2). The weighted mean 207Pb/206Pb date is 2803·43 ± 0·54 Ma (Fig. 10h), which is nearly 100 million years older than the mafic–ultramafic rocks of the Stillwater Complex. Granophyric rocks Given their complexity, the detailed U–Pb geochronological results for the four granophyres from the Banded series of the Stillwater Complex are described in the Supplementary Data. The U–Pb analytical results are listed in Supplementary DataTable A2 (zircon), Table A4 (titanite), and Table A5 (rutile), and all concordia plots are presented in Supplementary DataFig. A5. The U–Pb systematics for zircon (CA-TIMS), titanite, and rutile from the granophyres are highly variable. There is evidence for closed-system behaviour in one granophyre with a weighted mean 207Pb/206Pb date that is statistically identical to those for the mafic-ultramafic rocks of the Stillwater Complex (e.g. granophyre ST05–13; 2709·00 ± 0·39 Ma, Fig. 10i). There is also evidence for extensive open-system processes and Pb loss during post-crystallization hydrothermal and metamorphic events (e.g. ST05–01, -06, -07) (Supplementary DataFig. A5). DISCUSSION Crystallization of Zircon in Mafic–Ultramafic Rocks of the Stillwater Complex The trace element systematics of zircon from the mafic-ultramafic rocks in the Stillwater Complex provide constraints on the evolution of fractionated interstitial melts within the cumulates of this large, open-system layered intrusion and provide context for interpreting the significance of the U–Pb geochronological results. The implications of the zircon geochemistry are addressed below, including application of Ti-in-zircon thermometry and definition of the solidus temperatures of the cumulates, the significance of high-Th/U zircon in pyroxenites as a monitor of late-stage fluid saturation in interstitial melt, and the use of trace element ratios in zircon to determine both the distinctive chemical fingerprints of different series and zones in the intrusion and the tectono-magmatic setting of the Stillwater Complex. Zircon tracks the near-solidus temperatures of interstitial melt in cumulates In the Stillwater Complex, zircon occurs in mafic–ultramafic rocks characterized by heterogeneous textures (e.g. coarse grain sizes, irregular distribution of minerals) with macroscopic evidence for relatively abundant interstitial minerals, typically poikilitic plagioclase, and microscopic evidence of minerals that crystallized from highly fractionated melts (e.g. quartz, K-feldspar, biotite) at temperatures likely approaching the solidus of these cumulates. As the Ti content of zircon has a strong dependence on magma temperature during crystallization (e.g. Degeling, 2003; Troitzsch & Ellis, 2005; Zack et al., 2004; Watson et al., 2006; Ferry & Watson, 2007), the minimum temperatures at which zircon crystallized (Tzrc) in the Stillwater Complex can be calculated using the method of Ferry & Watson (2007) where: log (ppm Ti-in-zircon)=(5·711±0·072)–(4800±86)/Tzrc(K)– log aSiO2+log aTiO2 Small amounts of interstitial quartz are present in most of the mafic–ultramafic rocks that contain zircon, thus it follows that aSiO2 = 1 for all rocks from the Stillwater Complex. Although secondary rutile is found in ST05–03 from the AN2 zone in the Middle Banded series (Wall et al. 2016), the general absence of rutile in these rocks indicates that aTiO2 <1. Grimes et al. (2009) assigned aTiO2 = 0·7 for gabbroic rocks from oceanic crust following estimates of TiO2 solubility in silicate magmas from Hayden & Watson (2007) and the presence of Ti-bearing phases such as titanomagnetite and ilmenite. As Ti-bearing phases are notably absent in the Stillwater Complex, with the exception of magnetite in anorthosite layers at the top of the exposed Upper Banded series, aTiO2 was likely lower than 0·7, so we assume that aTiO2 = 0·5 for all rocks from the Stillwater Complex, a value that is consistent with the TiO2 solubility model of Hayden & Watson (2007). The Ferry & Watson (2007) thermometer was calibrated for a pressure of 1 GPa (10 kbars) and they estimated a pressure dependence of -5°C/kbar for pressures below 10 kbars, which would lower the calculated temperatures for the Stillwater Complex (pressure of emplacement ∼1·5 to 3 kbars) zircon by ∼35°C for a 3 kbar pressure. This variation is less than or equal to the uncertainty associated for each spot analysis. Application of Ti-in-zircon thermometry to the Stillwater Complex yields a wide range of temperatures for most samples (Tzrc = 990–720°C) (Fig. 11). Several analyses with Ti >50 ppm result in slightly higher temperatures (Tzrc =1020–1090°C) and there is a single anomalously low temperature value (ST11–22, LA12, 651°C). The analyses from many samples define trends of decreasing Ti with increasing Hf (Fig. 5b;Supplementary DataFig. A2) consistent with continuous zircon crystallization from progressively fractionated interstitial melt down to solidus temperatures, which are approximated by the lowest Ti contents (i.e. temperature) determined from each sample. In general, the lowest Tzrc values from each sample decrease with increasing stratigraphic height from the Basal series through the Ultramafic series to the Upper Banded series (Fig. 11). Figure 11 reveals a similar range in calculated Tzrc and similar mean temperatures (Tzrc ∼890–870°C) for rocks from the Basal series, Ultramafic series, and Lower Banded series, including the J-M Reef. In contrast, the distribution for samples from the Middle and Upper Banded series, including the AN1 and AN2 anorthosites, shifts to lower temperatures (Tzrc ∼820–800°C) (Fig. 11) and the minimum temperatures extend down to 720°C. Combined, the Ti-in-zircon systematics for the Stillwater Complex indicate that zircon crystallized over a ∼50–150°C temperature range within individual samples from the temperature of initial zircon saturation when small amounts of interstitial melt were present (<10%) down to near-solidus temperatures. Fig. 11. View largeDownload slide Ti-in-zircon thermometry values for zircon from the Stillwater Complex calculated using the method of Ferry & Watson (2007); calculations were made assuming the presence of quartz (aSiO2 = 1) and aTiO2 = 0·5 (see text for explanation). (a) Box-and-whisker plots of Ti-in-zircon thermometry. The line within the box represents the median of the dataset (quartile 2), the edges of the box define the limits of 50% of the data (quartiles 1 and 3), and the whiskers define the range of 82% of the data. (b) Stacked histogram of Ti-in-zircon thermometry values. Bars are colour coded and correspond to the colour of the relevant series or zone in the stratigraphic column in Figure 1 as indicated in the legend. Fig. 11. View largeDownload slide Ti-in-zircon thermometry values for zircon from the Stillwater Complex calculated using the method of Ferry & Watson (2007); calculations were made assuming the presence of quartz (aSiO2 = 1) and aTiO2 = 0·5 (see text for explanation). (a) Box-and-whisker plots of Ti-in-zircon thermometry. The line within the box represents the median of the dataset (quartile 2), the edges of the box define the limits of 50% of the data (quartiles 1 and 3), and the whiskers define the range of 82% of the data. (b) Stacked histogram of Ti-in-zircon thermometry values. Bars are colour coded and correspond to the colour of the relevant series or zone in the stratigraphic column in Figure 1 as indicated in the legend. High Th/U in zircon from pyroxenites reflects late-stage fluid saturation In zircon from the Stillwater Complex, Th/U varies from typical magmatic values of ∼0·5 to 1 (e.g. Belousova et al., 2002; Xiang et al., 2011; Kirkland et al., 2015) for most grains to unexpectedly high values (2–20) from pyroxenites in the Ultramafic series and for a few select zircon grains from the Basal series sample (Fig. 5a). Most of the high Th/U analyses are depleted in U relative to Th as the total range in Th concentrations in these zircon grains is broadly similar to that defined by zircon from the rest of the intrusion (Fig. 5a). Three processes may be considered to account for the high Th/U in zircon from the Stillwater Complex: (1) co-crystallization of zircon with other U-rich phases; (2) a fractionation effect of zircon alone; and (3) late-stage oxidation and fluid saturation. The occurrence of co-crystallizing phases (e.g. uraninite, zirconolite, baddeleyite) that have a greater affinity for U than Th would lead to high Th/U in the fractionating interstitial melt and would be recorded by high Th/U in any zircon that crystallized from that melt. As baddeleyite is much less abundant compared to zircon when present, and not present in most of the samples, and as there is no evidence for other U-rich phases in the mineral separates, baddeleyite crystallization cannot explain the significant increase in the Th/U required in the melt. Simple Rayleigh fractionation during crystallization of zircon with decreasing temperature will result in a reduction of zircon Th/U, opposite to what is required, due to the cooler, fractionated melts being more depleted in U (relative to Th) as U is extracted from the melt into zircon (Kirkland et al., 2015). The third process involves producing high-Th/U zircon as a result of a local change in oxidation state and late-stage fluid saturation. Apatite has been used as a monitor of relative halogen variations of interstitial melts (Boudreau et al., 1986; Boudreau & McCallum, 1989) and is a common interstitial mineral in the Stillwater Complex. Cl-rich apatite, typically containing <0·4 wt % F and >6·0 wt % Cl, occurs throughout the lower third of the intrusion, including the Peridotite and Bronzitite zones, and its presence indicates equilibration with Cl-rich fluids exsolved during solidification of the cumulate sequence (Boudreau & McCallum, 1989; Boudreau, 2016). Cl-apatite also occurs in some Basal series samples. To produce the high-Th/U zircon in pyroxenites from the Ultramafic series, it is speculated that zircon crystallized and grew from fractionated interstitial melt during and following exsolution of Cl-rich fluids that involved local oxidation of U4+ (compatible in the Zr4+ site in zircon) to U6+ (e.g. Keppler & Wyllie, 1991; Bacon et al., 2007). These fluids preferentially scavenged U6+ resulting in high Th/U in the remaining melt. This process must have begun at relatively high temperatures (∼950°C) based on the Ti-in-zircon thermometry results (Fig. 11). Variable Th/U in zircon from individual samples apparently reflects the degree to which the interstitial melt pockets were interconnected and equilibrated with the Cl-rich fluids. Trace element ratios distinguish zircon from the different series and zones Variations in the trace element concentrations in zircon may also result from changes in zircon/melt partitioning, which can vary by an order of magnitude over several hundred degrees (e.g. Sano et al., 2002; Thomas et al., 2002; Hanchar & van Westrenen, 2007; Rubatto & Hermann, 2007). There is also evidence that non-equilibrium element partitioning can have an effect on the trace element patterns in zircon at the sub-micron scale (Hofmann et al., 2009), however, zircon geochemistry generally reflects equilibrium partition between zircon and melt (e.g. Belousova et al., 2006; Claiborne et al., 2010; Schoene et al., 2010, 2012; Reid et al., 2011; Samperton et al., 2015). Trace element ratios are not as affected by the temperature effect on partitioning (e.g. Grimes et al., 2015; Samperton et al., 2015) and, as a result, changes in trace element ratios in zircon will reflect differences both in magma chemistry and in the co-crystallizing minerals driving compositional changes in the interstitial melt. Trace element ratios effectively distinguish zircon in mafic–ultramafic rocks from the different major series and zones of the Stillwater Complex (Figs 12 and 13). Two distinct groupings are evident from a range of different trace element ratios (e.g. Y/Hf, Sc/Th, Ce/Nd, Lu/Hf, Yb/Dy, Th/U). Zircon from the Ultramafic series, including samples from both the Peridotite and Bronzitite zones, and from the Basal series sample have characteristically low Y/Hf, Lu/Hf, Yb/Dy, relatively high Eu/Eu*, and variable Sc/Th (Fig. 12), and high Th/U and low total REE (Fig. 13). In contrast, zircon from both anorthosite zones (AN1 + AN2) in the Middle Banded series have high Y/Hf, Lu/Hf, and Yb/Dy, low Sc/Th and Eu/Eu* (Fig. 12), coupled with low Th/U and high total REE (Fig. 13). The trends of strongly increasing Y/Hf at near-constant Sc/Th (Fig. 12a) and strongly increasing Lu/Hf with decreasing Yb/Dy (Fig. 12c) in zircon from these anorthosites are consistent with the effects of zircon fractionation, whereas the prominent negative Eu anomalies in zircon from the anorthosites reflect the preceding crystallization of plagioclase (Fig. 12d). Trace element ratios in zircon from the Lower and Upper Banded series are generally intermediate between these two main groups. The J-M Reef samples are highly variable with many analyses yielding Y/Hf, Sc/Th, Yb/Dy, and Eu/Eu* (Fig. 12) similar to the ultramafic rocks, but with distinctly lower Th/U and higher total REE (Fig. 13). Based on the distinctive zircon chemistry from the different series and zones, combined with systematic temperature differences based on Ti-in-zircon thermometry (Fig. 11), zircon in the Stillwater Complex crystallized from progressively fractionated interstitial melt that appears to have been derived locally from the hosting cumulates (i.e. from within the same zone based on sampling constraints). Fig. 12. View largeDownload slide Trace element ratio variations for zircon from the Stillwater Complex determined in situ by LA-ICP-MS. (a) Y/Hf v. Sc/Th. (b) Ce/Nd v. Lu/Hf. (c) Lu/Hf v. Yb/Dy. (d) Eu/Eu* v. Yb/Dy. Arrows in the plots indicate fractional crystallization trends of the identified phases assuming equilibrium partitioning between zircon and melt; partition coefficients used are from Adam & Green (2006) and Bédard (2006) in (a) and from Rubatto & Hermann (2007) in (c) and (d). Data points are colour coded based on the colours of the stratigraphic section in Figure 1 and symbols represent the associated rock type of the sample. The mean 2σ analytical uncertainties are indicated in each panel. Abbreviations: zirc, zircon; badd, baddeleyite; amph, amphibole; cpx, clinopyroxene; ap, apatite; titan, titanite; plag, plagioclase. Fig. 12. View largeDownload slide Trace element ratio variations for zircon from the Stillwater Complex determined in situ by LA-ICP-MS. (a) Y/Hf v. Sc/Th. (b) Ce/Nd v. Lu/Hf. (c) Lu/Hf v. Yb/Dy. (d) Eu/Eu* v. Yb/Dy. Arrows in the plots indicate fractional crystallization trends of the identified phases assuming equilibrium partitioning between zircon and melt; partition coefficients used are from Adam & Green (2006) and Bédard (2006) in (a) and from Rubatto & Hermann (2007) in (c) and (d). Data points are colour coded based on the colours of the stratigraphic section in Figure 1 and symbols represent the associated rock type of the sample. The mean 2σ analytical uncertainties are indicated in each panel. Abbreviations: zirc, zircon; badd, baddeleyite; amph, amphibole; cpx, clinopyroxene; ap, apatite; titan, titanite; plag, plagioclase. Fig. 13. View largeDownload slide Box-and-whisker plots showing the stratigraphic variations in key trace element indices and ratios in zircon from mafic–ultramafic rocks in the Stillwater Complex. (a) Simplified stratigraphic column of the Stillwater Complex; see Figure 1 for additional details on colour coding and abbreviations. (b) Th/U. (c) Sum (REE). (d) (Eu/Eu*) = Eu anomaly in chondrite-normalized patterns. (e) Yb/Dy. (f) Lu/Hf. Box-and-whisker plots highlight the mean and variation of large datasets. The line within the box represents the median of the dataset (quartile 2), the edges of the box define the limits of 50% of the data (quartiles 1 and 3), the whiskers define the range of 82% of the data, and the black circles represent outliers in the dataset (i.e. outside of the 9th and 91st percentile). Note that compositional axes are logarithmic to effectively show the large variations present; the exception is panel (c), which shows the sum of the REE in linear format. The red-dashed line indicates the approximate stratigraphic position of the J-M Reef; the magenta-filled symbol refers to the mafic dike the cuts the J-M Reef (ST12-01, JMD). Fig. 13. View largeDownload slide Box-and-whisker plots showing the stratigraphic variations in key trace element indices and ratios in zircon from mafic–ultramafic rocks in the Stillwater Complex. (a) Simplified stratigraphic column of the Stillwater Complex; see Figure 1 for additional details on colour coding and abbreviations. (b) Th/U. (c) Sum (REE). (d) (Eu/Eu*) = Eu anomaly in chondrite-normalized patterns. (e) Yb/Dy. (f) Lu/Hf. Box-and-whisker plots highlight the mean and variation of large datasets. The line within the box represents the median of the dataset (quartile 2), the edges of the box define the limits of 50% of the data (quartiles 1 and 3), the whiskers define the range of 82% of the data, and the black circles represent outliers in the dataset (i.e. outside of the 9th and 91st percentile). Note that compositional axes are logarithmic to effectively show the large variations present; the exception is panel (c), which shows the sum of the REE in linear format. The red-dashed line indicates the approximate stratigraphic position of the J-M Reef; the magenta-filled symbol refers to the mafic dike the cuts the J-M Reef (ST12-01, JMD). Tectono-magmatic setting of the Stillwater Complex from trace elements in zircon In addition to using trace elements in zircon as tracers of magmatic processes in fractionat