[S-GC37] Volatiles in the Earth - from Surface to Deep Mantle

[SGC37-03] Large-scale loss of the Earth's primordial atmosphere implied by carbon metal/silicate partitioning

Shengxuan Huang¹, *Taku Tsuchiya¹ (1.Geodynamics Research Center, Ehime University)

Keywords:Ab initio free energy calculations, Carbon, Metal-silicate partition

Carbon, as an Earth's major volatile element, plays a key role in regulating the climate and habitable surface environment, and is extensively involved in mantle dynamical processes. The abundance and distribution of carbon in the Earth is a fundamental issue regarding the evolution of Earth's volatiles and the chemistry of the Earth's core. The bulk silicate Earth (BSE) is characterized as super-chondritic C/N and chondritic C/S ratios [Hirschmann, 2016]. Understanding the origin of such features is essential to unveil the origin of life-essential volatile elements and requires the knowledge of their partitioning behaviors during core-mantle differentiation. Among these elements, the results of carbon partition coefficients under high-pressure and high-temperature are highly controversial. For instance, previous experiments conducted at relatively low pressures (< 8 GPa) have shown that carbon is strongly siderophile and becomes more with increasing pressure [Chi et al., 2014; Grewal et al., 2021]. In contrast, many lines of evidence demonstrate that carbon is much less siderophile at pressures of 35~60 GPa, though those data scatter more than an order of magnitude [Zhang & Yin, 2012; Fischer et al., 2020; Blanchard et al.. 2022]. In this study, we investigate the partitioning behavior of carbon between liquid iron and molten silicate to 135 GPa and 5000 K using ab initio molecular dynamics simulations combined with the thermodynamic integration technique. We find that pressure and temperature have complex effects on carbon partitioning, primarily depending on the bonding features of carbon in molten silicate. These results help to reconcile the large discrepancies among experimental results. Combining geochemical modeling and observations, we find that the super-chondritic C/N and chondritic C/S ratios of the BSE could have been established by deep core-mantle differentiation and large-scale loss of the primordial atmosphere, which sheds light on the composition and evolution of the Earth's atmosphere.