Fractionation of highly siderophile and chalcogen elements during magma transport in the mantle: Constraints from pyroxenites of the Balmuccia peridotite massif

Websterites which have formed during refertilization of depleted peridotites display ratios of the HSE and moderately suprachondritic initial 187Os/188Os similar to interstitial sulfides of refertilized peridotites. These compositions are different from 'residual' compositions of sulfide inclusions in such peridotites. These observations suggest that preferential dissolution of interstitial sulfides from peridotites into migrating silicate melts, and thus chemical and isotopic disequilibrium, may be common during open-system melt extraction and melt-peridotite reaction in the mantle. Precipitation of sulfides and pyroxenes from such migrating melts may lead to refertilization of peridotites and formation of pyroxenites elsewhere in the mantle. The enrichment of the PGE and Te in early precipitated sulfides in these cumulates explains the depletion of the PGE and Te in many basalts and lower crustal gabbros relative to peridotites. Similar ranges of Re/Os and Pd/Ir and suprachondritic initial 187Os/188Os in pyroxenites and in oceanic crust indicate that most of the compositional variation of these elements in basic magmas may reflect repeated cycles of assimilation of mainly interstitial, peridotite-hosted sulfide liquid and subsequent liquid sulfide segregation during magma transport in the mantle.