Molecular hydrogen in mantle minerals

Abstract

doi: 10.7185/geochemlet.1616Current models assume that hydrogen was delivered to Earth already in oxidised form as water or OH groups in minerals; similarly, it is generally believed that hydrogen is stored in the present mantle mostly as OH. Here we show by experiments at 2-7 GPa and 1100-1300 °C that, under reducing conditions, molecular hydrogen (H 2 ) has an appreciable solubility in various upper mantle minerals. This observation suggests that during the accretion of the Earth, nebular H 2 could have been delivered to the growing solid planet by direct dissolution in a magma ocean and subsequent incorporation in silicates. Moreover, the presence of dissolved molecular H 2 in the minerals of the lower mantle could explain why magmas sourced in this region are rich in hydrogen, despite the fact that lower mantle minerals contain almost no OH groups.

Experimental constraints on the fate of H and C during planetary core-mantle differentiation. Implications for the Earth

“…These chondritic materials might contain significant amounts of Hydrogen (H) and Carbon (C) as observed, for example, in carboneous chondrites (e.g. Morbidelli et al, 2012;Marty, 2012;Yang et al, 2016;Iizuka-Oku et al, 2017). Unfortunately, the exact nature of the terrestrial precursor materials and the redox conditions at early stages of the Earth formation (e.g.…”

“…In case of incomplete devolatilization of the Earth during its accretion (e.g. Albarede et al 2013;Yang et al, 2016;Iizuka-Oku et al, 2017;Tikoo and Elkins-Tanton, 2017), the mantle (e.g. Frost et al, 2008;Yang et al, 2016) and possibly the core (e.g.…”

“…Albarede et al 2013;Yang et al, 2016;Iizuka-Oku et al, 2017;Tikoo and Elkins-Tanton, 2017), the mantle (e.g. Frost et al, 2008;Yang et al, 2016) and possibly the core (e.g. Okuchi, 1997;Ohtani et al, 2005;Ohtani, 2015;Iizuka-Oku et al, 2017) likely trapped H and C. Defouiloy et al, (2013) have found around 100 -200 wt ppb of H in Fe-Ni alloys of iron meteorites, implying that H was present during the formation of their parental body and could potentially enter the core.…”

Reduced methane-bearing fluids as a source for diamond

“…In addition to the presence of OH groups (Fig. S2), Matjuschkin et al 33 report Raman spectra that also indicate incorporation of H 2 into olivine rather than just in fluid inclusions, as was first described by Yang & Keppler 55 . This provides a further mechanism to crystallize diamond by driving both equilibria (2) and (3) to the right.…”

Molecular hydrogen in minerals as a clue to interpret ∂D variations in the mantle

“…At such fugacity, a substantial amount of H should be present in a reduced form 15 . Reducing conditions were demonstrated experimentally to greatly decrease the solubility of OH in olivine 16 and it was recently discovered that H 2 could also be dissolved in NAMs, while it had remained undetectable due to its low infra-red extinction coefficient 17 .…”