Partitioning of H and C between the mantle and core during the core formation in the Earth: Its implications for the atmospheric evolution and redox state of early mantle

Abstract

Partitioning of H and C among fluid, silicate melt, and molten metallic iron within a growing Earth at temperatures 2000–2500 K and pressures 0.2–5 GPa is estimated by using a thermodynamic model based on the recent knowledge on gas solubility into silicate melts and molten metallic iron. The repulsive interactions among H, C, and S dissolved in molten metallic iron are taken into account. It is shown that partition coefficient of H between molten metallic iron and silicate melt increases with pressure and temperature. Under the presence of Fe‐rich metal, CO2content in silicate melt is suggested to be very low because of low oxygen fugacity under such condition. Assuming a homogeneous accretion of planetesimals with the composition given by the two‐component model slightly modified fromRingwood[1977] andWäanke[1981], it is shown that C is preferentially partitioned to molten metallic iron and quite less to silicate melt, whereas a substantial proportion of H is partitioned to silicate melt as H2O and also to molten metallic iron as interstitial atoms. For such concentrations of H, C, and S, the effect of thermodynamic interaction among them in molten metallic iron is not strong enough to cause the oversaturation of graphite. H and C partitioned to molten metallic iron may account for a significant portion of the density deficit in Earth's core. The estimated amount of H2O partitioned to silicate melt is possibly large enough to explain the sources for (1) H2O in the hydrosphere and mantle, (2) oxygen which partially oxidizes ferrous iron to ferric iron in the mantle, and (3) oxidant for metals which may fail to segregate to the core and act as the source of the highly siderophile elements in the mantle of the present Earth. The higher H/C and the lower C/36Ar ratios in the silicate Earth including the hydrosphere compared to various classes of meteorites are possibly explained if these elements are derived from the early mantle material after the H and C partitioning to molten metallic iron and core segregation. Accretion of the late veneer material such as the highly oxidized, CI chondrite‐like material seems difficult to explain such elemental abundance pattern without invoking unknown large volatile reservoir in the mantle.