An alternative explanation for the distribution of highly siderophile elements in the Earth

Abstract

The presence of highly siderophile elements (Ru, Rh, Pd, Re, Os, Ir, Pt, and Au) in the mantle has been a long standing enigma in the Earth sciences. Highly siderophile elements exhibit extremely low silicate/ metal partition coefficients and so should have partitioned into the core, leaving the mantle depleted and fractionated compared with precursor material. Late accretion of undifferentiated material after completion of core formation may have overprinted the residue inherited from metal-silicate equilibrium partitioning. Here, we present a model based on the osmium isotopic composition of the mantle that sheds new light on the distribution of highly siderophile elements in Earth. As the Earth grew from the accretion of chondritic material of unspecified nature, gravitational and radioactive heating permitted early segregation of metal from silicate. This resulted in fractionation of highly siderophile element abundances in the residual mantle relative to chondritic abundances. After completion of core formation, the model supposes that a late carbonaceous veneer delivered biogenic and highly siderophile elements to the Earth. This late veneer was mixed inhomogeneously with the fractionated residue left over after core formation. Part of the deep mantle was isolated early from shallow convection and thus preserved primordial noble gas and highly siderophile element signatures. In this scenario, the contrast between the ¹⁸⁷Os/¹⁸⁸Os ratio of the carbonaceous late veneer and that of fertile Iherzolites places strict constraints on the coupled silicate/ metal partition coefficients of Re and Os (D^Re=1.6 ± 1.2 × 10^-4 + D^Os). Similarly, the high ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁶Os/¹⁸⁸Os ratios observed in certain plumes impose restrictions on the coupled silicate/metal partition coefficients of Pt and Os (D^Pt = 8.0 ± 6.2 × 10^-4 + D^Os).