Fractionation of Highly Siderophile Elements in Refertilized Mantle: Implications for the Os Isotope Composition of Basalts

Abstract:

Highly fertile lherzolite and spinel websterite in the Ronda peridotite massif are enriched in Pt and Pd compared to Os, Ir, Ru and Re. The fractionation of the highly siderophile elements (HSE) in these rocks was produced by reaction of melt with their peridotite and pyroxenite precursors. Modelling indicates that upon reaction primary HSE hosts were dissolved and magmatic sulphides precipitated, largely erasing the original HSE signature of reacting protoliths. The budget of HSE in the melt was controlled by sulphide-silicate partitioning or entrainment of molten sulphide in silicate melt. Rhenium was likely removed from precursor peridotite and pyroxenite by previous melt extraction. Refertilization of peridotite by melts increased Al₂O₃, CaO, Pt, Pd contents and ¹⁸⁷Os/¹⁸⁸Os beyond typical values for mantle peridotite and the primitive upper mantle. The interaction of melts with residual peridotite and pyroxenite, as illustrated in the Ronda massif, can generate radiogenic-Os-enriched domains in the mantle with high melt productivity, higher Pt/Os and Pt/Re than common peridotite, and lower Re/Os than typical pyroxenite. As also previously proposed for pyroxenites, these “hybrid” domains can account for the difference in Os isotope compositions between many basalts (e.g., MORB and OIB) and common depleted mantle peridotites. Moreover, the presence of these enriched hybrid components in the mantle, evolved over sufficient timescales and in geologically reasonable proportions, may account for the ¹⁸⁶Os-¹⁸⁷Os enrichment of plume lavas without requiring a chemical contribution from the core.