10:15 〜 10:30
[SIT20-18] Hydrogen distribution during the core-mantle segregation
★Invited Papers
キーワード：Ab initio calcualtions, Hydrogen, Metal-silicate partition
Hydrogen is the most abundant volatile element in the solar system. D/H isotope ratios suggest that the present-day distribution of volatiles in the Earth may result from primordial accretion involving wet planetesimals, indicating that Earth's core-mantle differentiation potentially proceeded in a hydrogen-rich environment (Marty, 2012). As a life-forming volatile element, understanding the distribution of hydrogen between different reservoirs in Earth's early stages is essential for assessing the evolution of our planet's habitability. Although hydrogen has been thought to be a candidate light element in the Earth's core, its exact metal-silicate partitioning remains controversial at high pressure and temperature in both experimental (e.g. Clesi et al., 2018; Tagawa et al., 2021) and theoretical (Zhang and Yin, 2012; Li et al., 2020; Yuan and Steinle-Neumann, 2020) studies. In this study, we calculate the partition coefficient of hydrogen (DHmetal/silicate) between iron and silicate melts with various chemical compositions at 20 - 135 GPa and 3000 - 5000 K, using ab initio thermodynamic integration molecular dynamics (AITIMD) (Taniuchi and Tsuchiya, 2018). With the derived partition coefficients, we systematically examined the effects of light elements (O, S and Si) and FeO in the metal droplet-magma ocean equilibration system, which is lacking in previous theoretical studies, on the hydrogen partitioning behavior. Moreover, we further investigate the hydrogen partitioning in a scenario where hydrous silicate and metal melts engage in exchange and redox reaction simultaneously, an aspect not considered either.
Our results indicate that DHmetal/silicate increases with increasing pressure, suggesting that hydrogen may have been transported to the core as the Earth accreted. The presence of FeO in molten silicate significantly strengthens the siderophile nature of hydrogen, implying that more hydrogen could be trapped in the core when the metal equilibrates with a relatively oxidizing magma ocean. However, if considering the presence of light elements in the liquid metal, the amount of hydrogen being driven into the out core is difficult to simply estimate due to the mutual influences of partitioning among light elements. Light elements in the core sometimes decrease the DHmetal/silicate but sometimes increase it. This variation depends on the specific pressures and types of chemical reaction occurring when hydrous silicate and metals are in equilibrium. We find that hydrogen exhibits the lithophile behavior with lower DHmetal/silicate in the redox reaction compared with the exchange reaction. This demonstrates that a hydrous silicate-metal equilibrium model that considers only exchange or redox reactions is inadequate for exactly constraining the realistic partitioning behavior of hydrogen during the core-mantle segregation. Furthermore, the type of reaction occurring during core-mantle equilibrium controls the speciation of hydrogen (H2O/H2) in Earth's core. If considering both reactions, it can be expected that the mass fraction of oxygen becomes higher than that of hydrogen in the outer core. This disparity could have the significant implication for constraining the composition of the outer core. The light element composition of the outer core is difficult to reconcile with hydrogen solely even hydrogen can go into the core. Given the compositional and reaction type dependence of hydrogen partitioning, our study emphasizes that a multicomponent metal-silicate melts model where exchange and redox reaction coexist, instead of simple end-members system, are rendered necessary, in order to better understand the distribution and budget of volatile during Earth's accretion processes.
Our results indicate that DHmetal/silicate increases with increasing pressure, suggesting that hydrogen may have been transported to the core as the Earth accreted. The presence of FeO in molten silicate significantly strengthens the siderophile nature of hydrogen, implying that more hydrogen could be trapped in the core when the metal equilibrates with a relatively oxidizing magma ocean. However, if considering the presence of light elements in the liquid metal, the amount of hydrogen being driven into the out core is difficult to simply estimate due to the mutual influences of partitioning among light elements. Light elements in the core sometimes decrease the DHmetal/silicate but sometimes increase it. This variation depends on the specific pressures and types of chemical reaction occurring when hydrous silicate and metals are in equilibrium. We find that hydrogen exhibits the lithophile behavior with lower DHmetal/silicate in the redox reaction compared with the exchange reaction. This demonstrates that a hydrous silicate-metal equilibrium model that considers only exchange or redox reactions is inadequate for exactly constraining the realistic partitioning behavior of hydrogen during the core-mantle segregation. Furthermore, the type of reaction occurring during core-mantle equilibrium controls the speciation of hydrogen (H2O/H2) in Earth's core. If considering both reactions, it can be expected that the mass fraction of oxygen becomes higher than that of hydrogen in the outer core. This disparity could have the significant implication for constraining the composition of the outer core. The light element composition of the outer core is difficult to reconcile with hydrogen solely even hydrogen can go into the core. Given the compositional and reaction type dependence of hydrogen partitioning, our study emphasizes that a multicomponent metal-silicate melts model where exchange and redox reaction coexist, instead of simple end-members system, are rendered necessary, in order to better understand the distribution and budget of volatile during Earth's accretion processes.