Influence of C–O–H Volatiles on Ni, Co, and P Partition between Silicate Melt and Liquid Metal Fe Alloy at 4 GPa, 1550°C

A set of experiments was carried out in the system NaAlSi3O8–FeO–NiO–CoO–SiC–NaH2PO4 at 1550°C, 4 GPa, and oxygen fugacity (fO2) 0.5–2.9 log. units below the iron–wustite (IW) buffer to estimate how C–O–H components can affect Ni, Co, and P partition between silicate melt and a liquid metallic phase at redox conditions under which the metallic phase is segregated into melting products of the early reduced mantles of the Earth and other planetary bodies. It has been established that the Ni, Co, and P partition coefficients D(М)met/sil between silicate melts saturated with carbon and containing dissolved C–O–H volatiles (mainly in the form of OH groups, H2 and CH4) at relatively oxidized conditions (fO2 > IW – 1.5) correspond to D(М)met/sil values expected of metal–silicate melt equilibrium in volatile-free systems at analogous P, T, fO2, and nbo/t parameters. Under more reduced conditions (fO2 ≤ IW – 2), the presence of C–O–H volatiles leads to a decrease in D(М)met/sil for Ni and P compared to that in “dry” melts. This difference increases with decreasing fO2 and reaches ~0.5 and more than one order of magnitude for Ni and P, respectively, at fO2 = IW – 2.9. The effect of volatiles on D(Co)met/sil is much weaker, and hence, a decrease in fO2 leads to that D(Ni)met/sil and D(Co)met/sil converge. The Raman spectra of the experimental glasses and their SIMS analyses for hydrogen show that water content (OH + H2O) in the melts decreases with decreasing fO2, whereas the contents of CH4 and complexes with C–H bonds significantly increases. The likely reasons for the decrease in D(М)met/sil under strongly reduced conditions may be changes in the structure of the silicate melts and the origin of complex compounds of siderophile elements with volatiles in these melts.