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Abstract

The highly siderophile elements (HSE's: Ru, Rh, Pd, Re, Os, Ir, Pt and Au) and those elements with distribution coefficients between Fe-rich metal and silicate phases which exceed 104. The large magnitude of these distribution coefficients makes them exceedingly difficult to measure experimentally. We describe a new experimental campaign aimed at obtaining reliable values of DMmets/sil melt for selected HSE's indirectly, by measuring the solubilities of the pure metals (or simple HSE alloys) in haplobasaltic melts as a function of oxygen fugacity.Preliminary results for Pd, Au, Ir and Re indicate that the HSE's may dissolve in silicate melts in unusually low valence states, e.g., 2+ for Ir and 1+ for the others. These unusual valence states may be important in understanding the geochemical properties of the HSE's. Inferred values of DMmet/sil melt from the solubility data at 1400°C and IW −1 are ∼107 for Pd and Au, and 109−1012 for Ir. Metal/silicate partition coefficients are thus confirmed to be very large, and are also different for the different HSE's.A review of the abundance of the HSE's in the Earth's upper mantle shows that they are all present at ∼0.8% of chondritic, i.e. they have the same relative abundance, and the ratios of their concentrations are chondritic (e.g., Re/Os). Both the low degree of depletion (compared to the high values of DMmet/sil melt) and the chondritic relative abundances support the idea that the mantle's HSE's were added in a “late veneer” after the cessation of core formation. Sulfur is even more depleted in the mantle relative to CI chondrites than the HSE's: this implies a late veneer which was depleted in volatile elements, and which was added to a mantle stripped of S. Since considerable S dissolves in silicate melt, this further implies that core formation in the Earth either occurred under P−T conditions below the solicate solidus, or, if the process occurred over a range of temperatures in a cooling Earth, then the process continued down to conditions below the silicate solidus.The chondritic relative abundances of the HSE's in the upper mantle argue for a chemically unstratified primitive mantle, unless the late veneer was mixed only into the upper mantle.