Stability of Fe–Ni hydride after the reaction between Fe–Ni alloy and hydrous phase (δ-AlOOH) up to 1.2 Mbar: Possibility of H contribution to the core density deficit

The hydrous mineral, δ-AlOOH, is stable up to at least the core-mantle boundary, and therefore has been proposed as a water carrier to the Earth’s deep mantle. If δ-AlOOH is transported down to the core-mantle boundary by a subducting slab or the mantle convection, then the reaction between the iron alloy core and δ-AlOOH is important in the deep water/hydrogen cycle in the Earth. Here we conducted an in situ X-ray diffraction study to determine the behavior of hydrogen between Fe–Ni alloys and δ-AlOOH up to near the core-mantle boundary conditions. The obtained diffraction spectra show that fcc/dhcp Fe–Ni hydride is stable over a wide pressure range of 19–121 GPa at high temperatures. Although the temperature of formation of Fe–Ni hydride tends to increase up to 1950 K with increasing pressure to 121 GPa, this reaction temperature is well below the mantle geotherm. δ-AlOOH was confirmed to coexist stably with perovskite, suggesting that δ-AlOOH can be a major hydrous phase in the lower mantle. Therefore, when δ-AlOOH contacts with the core at the core-mantle boundary, the hydrogen is likely to dissolve into the Earth’s core. Based on the present results, the amount of hydrogen to explain the core density deficit is estimated to be 1.0–2.0 wt.%.