Reduction of Iron by Hydrogen in Early Planetary Mantles

Hydrogen is the most abundant element in the universe. It is the main constituent of gas giant planets, and omnipresent in protoplanetary disks. In the new paradigm of "pebble accretion" [1], planetary embryos grow much more rapidly. Recent studies suggest magma oceans in these large embryos could ingass significant amounts of nebular hydrogen [2]. Therefore hydrogen-rich nebular gas may have a significant impact on formation and differentiation of planets.

We conducted experiments in the laser-heated diamond-anvil cell on Fe₂O₃ hematite and (Mg,Fe)O ferropericlase combined with H₂ at pressures and temperatures ranging from 20-50 GPa and 1000-4000 K respectively. At GSECARS sector of APS, data was collected using synchronized gated X-ray diffraction and pulsed laser heating to prevent the hydrogen medium from diffusing into the gasket and anvil. Fe₂O₃ samples, when heated, transformed into FeH in the fcc structure with FeO as an intermediate phase. Two different compositions of ferropericlase were used: (Mg_0.5,Fe_0.5)O, and (Mg_0.9,Fe_0.1)O. Both compositions when heated produced FeH in either the fcc or dhcp structure. Mg(OH)₂ brucite was also observed, likely due to the production of water:

Fe₂O₃+ 4H₂→2FeH + 3H₂O,

(Mg_x,Fe_1-x)O + (3(1-x)/2)H₂ → (1-x)FeH + xMgO + (1-x)H₂O

These reactions provide a mechanism to sequester and stabilize ingassed hydrogen in the interior of the planetary embryos. Iron from the oxide phase is reduced to metallic iron hydride which is much denser than the parent body and could sink toward the core. Producing iron hydride by the reduction of iron oxide rather than oxidation of iron metal by water allows the upper bound of hydrogen in the core to be much higher because it is no longer bounded by the FeO content of the mantle [3]. These reactions also provide a mechanism to produce water which can be stored in the mantle silicates.

[1] Lambrechts, M., & Johansen, A. 2012, Astronomy & Astrophysics, 544, A32

[2] Olson, P., & Sharp, Z. D. 2018, Earth and Planetary Science Letters, 498, 418

[3] Hirschmann, M. M., Withers, A., Ardia, P., & Foley, N. 2012, Earth and Planetary Science Letters, 345, 38