Experimental evidence for hydrogen incorporation into Earth’s core

Abstract

Hydrogen is one of the possible alloying elements in the Earth’s core, but its siderophile (iron-loving) nature is debated. Here we experimentally examined the partitioning of hydrogen between molten iron and silicate melt at 30–60 gigapascals and 3100–4600 kelvin. We find that hydrogen has a metal/silicate partition coefficient DH ≥ 29 and is therefore strongly siderophile at conditions of core formation. Unless water was delivered only in the final stage of accretion, core formation scenarios suggest that 0.3–0.6 wt% H was incorporated into the core, leaving a relatively small residual H2O concentration in silicates. This amount of H explains 30–60% of the density deficit and sound velocity excess of the outer core relative to pure iron. Our results also suggest that hydrogen may be an important constituent in the metallic cores of any terrestrial planet or moon having a mass in excess of ~10% of the Earth.

A speciation model linking the fate of carbon and hydrogen during core – magma ocean equilibration

“…These studies reported significant effects of P, T and fO2 on H metalsilicate partitioning that are consistent with molecular dynamics calculations at extreme P and T (Zhang and Yin, 2012;Li et al 2020, Yuan andSteinle-Neumann, 2020). Finally, empirical relationships have also been established to predict the molten metalsilicate liquid partitioning of H (Clesi et al, 2018;Malavergne et al, 2019;Tagawa et al 2021).…”

“…The entire database (but Clesi et al, 2018) gives the C and H content of silicate melts at C-saturation. We have selected the experimental studies in which both H and C contents in the silicate melt were determined, thus the recent high-P work by Fischer et al (2020), Fichtner et al (2021 and Tagawa et al (2021) could not be used here (see the 2-next paragraphs). Clesi et al (2018) did not measure the C-content in the silicate melts but C-contents in silicate melts at C-saturation under the P-T-fO2-fH2O conditions of their experiments is mostly provided by other studies.…”

“…MD data of Li et al (2020) and that experimental data of Tagawa et al (2021) are not shown for sake of simplicity and because these were not used to calibrate our model.…”

“…The cause of mantle depletion of moderately siderophile elements with respect to chondritic abundances is generally attributed to the sequestration of the depleted elements into the core. Given the growing interest in the conditions defining habitable worlds, the distribution of C and H, two life-essential elements, during the magma ocean stage has been thrust into the scientific limelight (Okuchi, 1996;Hirschmann, 2012a;Sarafian et al, 2014;Li et al, 2016;Clesi et al, 2018;Malavergne et al, 2019;Greenwood et al, 2018;Grewal et al, 2019;Fischer et al, 2020;Li et al, 2020;Tagawa et al, 2021;Gaillard et al, 2021). At the pressure (P)temperature (T)redox conditions (oxygen fugacity fO2) of modern magmatic and volcanic settings, C and H are considered as volatile elements or atmophile, ie.…”

“…Most of these experimental studies also involved H (ie. often reported as water) and determined the H-content in the silicate melts, but so far, only three studies measured the partitioning of H between metal and silicate melts (Okuchi, 1996;Clesi et al, 2018;Malavergne et al, 2019) to 21 GPa, and recently, Tagawa et al (2021) indirectly estimated D H at 30-60 GPa 3100-4600 K (using the phase proportions and the cell volume of FeHx et -FeOOH to get the H content in metals). These studies reported significant effects of P, T and fO2 on H metalsilicate partitioning that are consistent with molecular dynamics calculations at extreme P and T (Zhang and Yin, 2012;Li et al 2020, Yuan andSteinle-Neumann, 2020).…”