Partitioning of Ru, Pd, Ag, Re, Pt, Ir and Au between sulfide-, metal- and silicate liquid at highly reduced conditions: Implications for terrestrial accretion and aubrite parent body evolution

Abstract

The abundances of highly siderophile elements (HSE) in planetary mantles and achondrites potentially provide important constraints on several aspects of planet formation, including the nature and composition of late accreted materials. Here, we experimentally and systematically assess the distribution of the HSE between silicate melts, sulfide and/or metal liquids at the highly to moderately reduced conditions thought to have characterized Earth accretion. The results show that the chalcophile behavior of all elements, except for Re, is strongly decreased at low FeO and/or high S concentrations in the silicate melt. There are considerable differences between how FeO and/or S contents of the silicate melt affect the D values of the various HSE, with the largest effects observed for Pd, Pt, Ir and Au. If liquid metal is Si-rich and S-poor, the siderophile behavior of the HSE mimics that in the presence of sulfide liquids, but with an offset due to differences in HSE activities in metal and sulfide liquids.Using our new experimental data, we quantify the relative effects of O in sulfide and S in silicate melt on the sulfide liquid-silicate melt partitioning behavior of the HSE using a thermodynamic approach. The resulting expressions were used to model the distribution of the HSE in highly reduced and differentiated EH- and EL chondritic parent bodies and during differentiation of the aubrite parent body. Our results show that even with their strongly decreased chalcophile and siderophile behavior at highly reduced conditions, HSE abundances in the mantles of these parent bodies remain extremely low. However, if such bodies accreted to Earth, any residual metal present in the parent body mantle and subsequently retained in Earth’s mantle would dramatically affect HSE abundances and produce chondritic ratios, making it impossible to track the potential accretion of a large reduced impactor to the BSE using HSE abundance systematics. In terms of the aubrite parent body, our results confirm previous hypotheses related to the importance of (un)differentiated core forming metals in establishing the HSE contents of unbrecciated aubrites. Finally, our results confirm that sulfides are likely a minor source of HSE abundances in aubrites, particularly for Re, consistent with sample observations.