Platinum-group elements and the multistage metasomatic history of Kerguelen lithospheric mantle (South Indian Ocean)

Abstract

Chalcophile and highly siderophile trace elements in Kerguelen mantle xenoliths monitor a multistage melt-depletion/metasomatic history in the lithospheric mantle. A high-degree partial melting event in a fast-spreading oceanic ridge setting generated protogranular harzburgites strongly depleted in S (<5 ppm), Se (<7 ppb), Cu (<5.1 ppm) and Pd (Pd/Ir N<0.2; N=CI-chondrite normalised). This refractory protolith was subsequently percolated in a within-plate setting by large fractions of basaltic to highly alkaline melts, which produced poikiloblastic harzburgites. Their PGE concentrations (including the IPGE; Os, Ir, Ru) are depleted by factors of 2 to 5 (0.002–0.005×CI-chondrites) compared with those of the protogranular harzburgites. Spinel dunites sampled as wall-rocks to hornblendite dykes are similarly depleted. These strongly compatible elements form intergranular discrete phases rather than being locked in silicates and can be dissolved in the percolating melts. After precipitation of amphibole and phlogopite in some harzburgites and the dunites, the highly alkaline melts evolved towards volatile-rich (carbonatite) small-melt fractions that ultimately unmixed into a highly modified, silica-rich and volatile-poor alkaline fraction and a carbonated fraction strongly enriched in C–O–H–S volatiles. The latter fluid affected the refractory protolith to various degrees by precipitating Pd–Pt-enriched sulfides of metasomatic origin. Bulk-rock S/Se, Cu/S, Os/Ir and Pd/Pt ratios combined with petrographic observations argue for two sulfide precipitation processes. A group of samples with broadly chondritic S/Se and Os/Ir ratios and no enrichment in Pd over Pt (Pd/Pt N=1) displays Cu–Fe–Ni-rich sulfide melt droplets unmixed from metasomatic carbonate melt pockets. In contrast, some dunites with abundant Fe–Ni sulfides have superchondritic S/Se ratios (up to 10,000), coupled with superchondritic Os/Ir N (2.7) and Pd/Pt N (6). Such a sympathetic behaviour of Os, Pd and S suggests transport from a S- and Cl-bearing CO 2 vapour phase.