Abstract

The sulfur isotopic compositions, trace and rare earth element (REE), and platinum group element (PGE) contents were determined for a suite of anhydrous spinel lherzolite xenoliths and their metasomatized equivalents from Dish Hill, California, USA, to evaluate the mobility of these elements in metasomatic fluids. Anhydrous xenoliths have REE patterns that vary from being slightly LREE-depleted to flat, as well as variable enrichments of LILE (Ba and Sr) and HFSE (Th and Nb) that indicate they were affected by cryptic metasomatism. The formation of amphibole in xenoliths (modal metasomatism) is accompanied by pronounced enrichments in HFSEs and REES, particularly LREEs. With the exception of a single xenolith with slightly elevated PGE contents, the metasomatic fluids did not produce any appreciable changes in the platinum group element (PGE) contents, suggesting these elements are relatively immobile under these mantle conditions. Typical concentrations of PGEs in xenoliths from Dish Hill are 0.8 ppb Os, 1 ppb Ir, 0.6 ppb Rh, 2.5 ppb Pt, 1.8 ppb Pd, and 1.7 ppb Au. Modal metasomatism has resulted in a variable increase in sulfur content of the xenoliths, from a value of <10 ppm for anhydrous xenoliths, to between 25 and 120 ppm for amphibole-rich xenoliths. The δ 34S values of sulfides in anhydrous xenoliths from Dish Hill are near +7, which are unusually high for the mantle and indicate that the lithospheric mantle under California was most likely affected by subduction of crustal sulfur. Sulfides in amphibole-bearing xenoliths have variable but generally lower δ 34S values of 3 to 8.7, suggesting that some portion of the metasomatic sulfur was derived from an asthenospheric mantle source with a more primordial sulfur isotopic composition.

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