Low osmium solubility in silicate at high pressures and temperatures

A new methodology to experimentally investigate metal/silicate partitioning for Os (DOsmetal/silicate) under high P–T conditions was devised that utilizes metal and silicate components with natural Os abundances and disparate 187Os/188Os. The isotopic contrast in starting components allows examination of isotopic exchange between metal and silicate. As is common for metal–silicate partitioning experiments involving highly siderophile elements (HSE), tiny nuggets of metal appeared in the silicate melts. Linear trends in the 187Os/188Os versus 1/188Os diagram for different silicate chunks from individual experimental charges permit projections of data to nugget-free silicate concentrations. The Os concentrations and isotope compositions in pure silicates reveal that the nuggets remove Os from the silicate melt while percolating through the melt, but contribute limited Os back to the silicate melt. Our data suggest that the rate of chemical equilibration regarding Os concentration between metal and silicate is much faster than that for isotopic exchange. Results for multiple experiments indicate extremely low Os solubility in silicate melt (DOsmetal/silicate > 2 × 105) at pressures as high as 2 GPa, and temperatures as high as 2400 °C. These results indicate that metal–silicate segregation during planetary differentiation at high temperatures and pressures may leave silicate that is extremely depleted in Os compared to the mantle abundances of Os observed, and that isotopic exchange of HSE between planetary cores and silicate mantles subsequent to primordial differentiation may be limited.