Extreme Variations in ɣOs, on the Thin Section Scale in Laurites from Chromite Layers from Stillwater Complex: Implications for the Origin of Laurite and the Representativeness of Whole Rock ɣOs.

English Abstract:

The origin of massive chromite layers in large layered intrusions and their role in collecting Platinium-Group Element (PGE) is poorly understood. There have been a number of studies based on whole rock geochemistry and whole rock isotopic systems, including Re-Os, which conclude that contamination of a primary magmas with crustal component played a key role in the formation of the chromite layers.

The Re-Os isotopic system is particularly appropriate for examining the origin of PGE enrichment because Os is a PGE and is enriched in chromite layers. In the case of Stillwater Complex (Montana, U.S.A) chromite layers, the host of the Os is laurite which is generally enclosed in chromite. In order to eliminate contributions of alteration and metamorphism from interstitial phases, we applied a new approach based on Os isotopes in-situ analyses on laurite by LA-MC-ICP-MS. The ɣOs obtained are variable, from sub-chondritic to radiogenic, and are surprisingly variable at the thin section scale (ɣOs = -2 to 7) and raise question about the representativeness of ɣOs from whole rock studies. These results imply mixing between at least two components. The first is chondritic to subchondritic and indicates a mantle derived source. The second is supra chondritic and implies a crustal component.

Three models may explain these results: the laurites represent i) transported residual mantle xenocrysts, ii) phenocrysts that have crystallized from magma variably contaminated “en route” to emplacement, iii) the products of Os and Ru diffusion from chromite into base metal sulfides. In this latter case unradiogenic composition may reflect chromite-derived signature whereas radiogenic composition could represent a sulfide-dominated component.