High temperature equilibrium Cr isotopic fractionation between metal and silicate melts

Core formation was one of the most important event in the history of planet formation and evolution. Recently, stable isotopes have emerged as new tracers to constrain the conditions of core formation process [1, 2]. Moynier et al. (2011) [2] reported Cr isotopic compositions for chondrites with δ⁵³Cr values of -0.2‰ to -0.4‰, about 0.1‰ to 0.3‰ lighter than those of the terrestrial rocks, and suggested that the light Cr isotopes preferentially entered the metal phase during the core formation under relatively oxidizing conditions. However, later studies reported δ⁵³Cr for chondrites indiscernible from that of BSE [3, 4]. High P-T experiments were previously conducted to verify whether there is Cr isotopic fractionation between silicate and metal, and the preliminary results showed no significant Cr isotopic fractionation between metal and silicate at 1.5GPa and 1923K [3]. Here, we systematically tested the effects of temperature, oxygen fugacity and compositions on Cr isotopic fractionation between metal and silicate. We found that the oxygen fugacity, silicate composition and S content in the metal do not have significant effects on Cr isotopic fractionation, but increasing Ni content in the metal slightly increases the fractionation. ally heavier metal phase. ITAll our experiments except one BN-capsuled experiment have an isotopically heavier metal phase. Our experiments gave an average Δ⁵³Cr_{metal-silicate} of 0.08±0.01‰ at 1873K, 1GPa and about 6% Ni in the metal. The results suggest that the core formation will leave the terrestrial mantle with slightly lighter Cr isotopic composition compared to chondrites assuming the Earth has the same Cr isotopic composition as chondrites.

[1] Georg et al. (2007) Nature 447, 1102-1106

[2] Moynier et al. (2011) Science 331, 1417-1420

[3] Bonnand et al. (2016) EPSL 435, 14-21

[4] Schoenberg et al. (2016) GCA 183, 14-30