Low hydrogen contents in the cores of terrestrial planets

Abstract

During planetary accretion, hydrogen behaves as a lithophile element and is unlikely to be a major element in planetary cores.

Origin of Earth's Water: Chondritic Inheritance Plus Nebular Ingassing and Storage of Hydrogen in the Core

“…This situation is in contrast to previous experiments that used no graphite and found higher solubilities (Okuchi, ; Shibazaki et al, ). In addition, Clesi et al () inferred carbon concentrations of 3 to 7 wt %, a significant fraction of the saturation concentration of carbon, 6.67 wt %, as in Fe 3 C. Actually, Clesi et al () did not measure carbon directly, instead calculating it as the balance after all other elements were accounted for, so it is possible that the carbon concentrations were even closer to saturation than their estimates. Percent‐level concentrations of carbon would likely decrease D H appreciably, possibly owing to the limiting effect of carbon on hydrogen solubility in iron, which has been experimentally confirmed both in Fe‐C melts at 1865 K and 1 bar (Weinstein & Elliott, ) and in carbon steels at 500–900 K and 0.1–7 bar (Gadgeel & Johnson, ).…”

“…Several factors suggest that carbon played a role in Clesi et al () measurements. Their experimental design used (inner) graphite capsules and temperatures above 2000 K, providing an opportunity for iron in their samples to be saturated with carbon.…”

“…Both theoretical and experimental analyses have demonstrated that the logarithm of the partition coefficient, log 10 D H , is linearly proportional to the inverse of temperature, and to the abundance of incompatible elements, in many geological solution systems (McIntire, ). The partition coefficients reported by Clesi et al () for two iron specimens at ~20 GPa and ~2735 K with carbon concentrations of 7.50 and 5.44 wt % (whose average is close to the 6.67 wt % of Fe 3 C), can be taken as an approximation of D H between carbon‐saturated iron liquid and silicate melt. These numbers yield D H ≈ 0.56.…”

“…Likewise, McDonough (2003) estimated~0.06 wt %, and Zhang and Yin (2012) calculated~0.02 wt % hydrogen in the core. In contrast, Clesi et al (2018) recently argued for much lower hydrogen contents in the cores of terrestrial planets based on the low hydrogen solubility that they measured in iron quenched from 5 to 20 GPa. However, their samples appear to have contained 3 to 7 wt % carbon, and carbon is known to significantly lower solubility of hydrogen in Fe 3 C and related iron carbides (Litasov et al, 2016;Terasaki et al, 2014).…”

Hydrogen Limits Carbon in Liquid Iron

“…The previous metal-silicate partitioning experiments by Clesi et al (2018) and Malavergne et al (2019) argued that hydrogen is least partitioned into molten iron. However, they found at most only~500 ppm H in quenched molten Fe, because the vast majority of hydrogen had been lost during decompression.…”