Abstract
Samples from the J-M Reef and the G-chromitite horizons of the Stillwater Complex were analyzed for their Re and Os concentrations and Os isotopic compositions. There is large variability in the 187Os/ 186Os ratio both within the G-chromitite and between the horizons, that is unsupported by in-situ Re decay since the time at which the Stillwater Complex formed. The lowest initial 187Os/ 186Os of 0.895 ± 0.014, determined on a sample from the G-chromitite, indicates a normal mantle source region for the Ultramafic Series parent magma. Other samples from the G-chromitite are intermediate in 187Os/ 186Os between this value and the J-M Reef average initial 187Os/ 186Os of 1.32 ± 0.10. The Os concentration of the G-chromitite samples is also quite variable and is probably caused by heterogeneous distribution of early crystallizing platinum group element-bearing phases, predominantly laurite (Ru, Os, Ir)S 2). Oxygen isotopic analyses of the J-M Reef indicate that the variability of initial Os isotopic composition is the result of magmatic processes. A model is presented in which a normal mantle melt was contaminated with mafic to intermediate crust, at the time of the Ultramafic-Banded Series transition, to produce the J-M Reef Os isotopic enrichment. The G-chromitite horizon recorded two events with Os isotopes: first, the magmas that mixed to become saturated in chromite also precipitated laurite, which carried the mantle Os isotopic signature, and second, intercumulus liquid that migrated to the G-chromitite level by convection in the cumulus pile precipitated late-stage intercumulus sulfides, which carried radiogenic Os.
Published Version
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