Hydrogen-Mineral Reactions at High Temperatures and High Pressures

Abstract:

Hydrogen is the main constituent of gas giant planets. In these planets, hydrogen is in contact with rock and metal at very high pressure and high temperature. Recent data from the Juno mission suggest that the boundary between them may be fuzzy and a compositional gradient may exist over a large depth range in Jupiter. However, chemical origin for the hypothesized gradient is not well understood. In sub-Neptune exoplanets, hydrogen-mineral boundary may exist at much shallower depths. Some models have proposed that a large hydrogen ingassing to the silicate mantle could explain the observed abrupt decrease in sub-Neptune population at 3R_Earth. However, the assumed large ingassing of hydrogen is yet to be examined in high-pressure experiments. For rocky planets, recent studies have shown that a significant amount of nebular hydrogen could be ingassed in early magma oceans. Such ingassing would affect the redox conditions of the magma ocean, and consequently element partitioning during core formation and volatiles distribution in the interior. Despite the importance, laboratory data on reactions of hydrogen with silicates and iron alloys are sparse because of difficulty in heating hydrogen to sufficiently high temperatures expected for planetary interiors in static compression experiments. However, recent technical developments in pulsed laser heating synchronized with X-ray beams at the GSECARS sector of Advanced Photon Source enable us to study reactions of hydrogen with minerals at temperatures over 3000 K in a diamond-anvil cell. In this talk, we will report some recent experimental results on: (1) synthesis of super-stoichiometric iron-nickel-hydrogen alloys; (2) reaction of hydrogen with major planetary silicates and oxides (Mg₂SiO₄, MgO, and Fe₂O₃); and (3) possible redox control in a diamond-anvil cell using hydrogen-bearing media. We will also discuss implications of the results for the interiors of solar system planets and exoplanets.