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

Abstract

Sulfide segregation from sulfur saturated basic magmas affects the compositions of chalcophile elements such as the highly siderophile elements (HSE) and the chalcogens S, Se, Te to variable extent. Whether this process predominantly occurs in the lower crust or in the mantle and how segregation of liquid sulfide and accumulation affects concentrations and ratios of these elements at different mantle depths and in presumed primitive basic magmas remains uncertain. Abundances of the HSE, S, Se and Te and Os isotopes in websterites and spinel clinopyroxenites of the Balmuccia peridotite massif (Ivrea-Verbano Zone, Italian Alps) provide new insight on sulfide segregation and the compositional change of melt and peridotite during magma transport in the mantle.Balmuccia websterites and clinopyroxenites formed from late Paleozoic and Mesozoic melt influx into stretched continental lithospheric mantle of the Ivrea-Verbano Zone, respectively. The HSE and chalcogen element compositions of websterites and clinopyroxenites reflect the segregation and accumulation of sulfide melt from S saturated silicate melts with different abundances and ratios of chalcogens and the HSE. The pyroxenites display large variations in abundances of the platinum group elements (PGE) and Te whereas abundances of less chalcophile elements S, Se and Re are much less variable. The fractionation between the PGE and fractionation of Re/Os, S/Se and Se/Te in the mantle pyroxenites are consistent with sulfide melt–silicate melt partitioning with a sequence of apparent coefficients of DPGE>DAu⩾DTe>DSe⩾DS≈DRe. Concentrations in ocean ridge basalts and in gabbros of the lower oceanic crust are also consistent with such fractionation.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.