Trace element partitioning between sulfide-, metal- and silicate melts at highly reduced conditions: Insights into the distribution of volatile elements during core formation in reduced bodies

Chalcophile and siderophile element abundances are used to provide important constraints on the interior compositions of planetary bodies as well as the pressure (P) - temperature (T) conditions that prevailed during core formation. The oxygen fugacity (fO(2)) during core formation varied considerably between the various terrestrial planets and asteroidal bodies in our solar system. Mercury, the aubrite parent body (AuPB) and some terrestrial precursor bodies may have differentiated at highly reduced conditions. At present knowledge about how the metal liquid-silicate melt and sulfide liquid-silicate melt partitioning behavior of major and trace elements are affected by high S concentrations in the silicate melt at highly reducing conditions is incomplete. Here, we experimentally study the metal-silicate and sulfide-silicate partitioning behavior of trace elements in reduced silicate melts over a wide range of S contents as a function of redox state at 1 GPa and 1833-1883 K. Silicate melt S contents ranged between similar to 0.5 and similar to 20 wt%, with a corresponding silicate FeO range of similar to 0.4 to similar to 17.5 wt%, in a fO(2) range between 1 and 9 log units below the iron-wustite buffer. Our results reproduce the decrease of the S concentration at sulfide saturation (SCSS) with decreasing FeO contents down to similar to 3 wt%, as well as its strong increase at <3 wt% FeO. At S contents exceeding >6-9 wt% S, the FeO contents increase again. Results show that most elements (Mg, Ti, V, Cr, Mn, Cu, Zn, Se, Nb, Cd, Sb, Te, Ta, Tl, Pb and Bi) are more chalcophile than siderophile at reducing conditions, whereas Si, Co, Ni, Ga, Ge, Mo and W preferentially partition into Fe-rich melts instead of sulfide liquids. Silicon, Ti, Se, and Te preferentially partition into Fe-S over (Fe,Mg,Ca)-S liquids, whereas Mn, Zn and Cd are more compatible in the latter. As proposed by Wood and Kiseeva (2015), chalcophile elements such as Cu, Se and Te behave less chalcophile with increasing S concentrations of the silicate melt, whereas the opposite is observed for nominally lithophile elements such as Mg, Ca and Ti. The results can be used to improve interpretations of the observed trace element systematics of aubrites and other reduced achondrites. All of the volatile elements considered here behave chalcophile at the reducing conditions inferred for differentiation of the AuPB. A significant degree of the observed volatile element depletions in aubrites may therefore reflect their preferential partitioning into sulfide liquids, rather than degassing during or after differentiation of the AuPB. These results suggest that, depending of the extent of core merging, precursor body differentiation and the efficiency of sulfide liquid segregation, reduced precursor bodies that were incorporated in the early Earth were likely more rich in volatile elements than currently assumed.