Cycling of hydrogen in the deep earth

Figure Caption: The hydrogen cycling in the deep Earth. Image courtesy Q. Hu
Figure Caption: The hydrogen cycling in the deep Earth. Image courtesy Q. Hu

At ambient condition, the chemistry of water is so stable that it is considered as the unit of hydrogen carrier in the hydrosphere. For a long time, cycling of hydrogen is equivalent to the cycling of water in the Earth’s solid crust and mantle. Geoscientists rarely treat water in its decomposable forms, like hydrogen plus oxygen. Using HPCAT facility, a team discovered hydrogen was freed from hydrous minerals in its elemental form. Starting from a typical hydrous mineral (goethite) and placing it under lower mantle pressure-temperature conditions, they have observed hydrogen released and the hydrogen loss closely related with the heating temperature and duration. The released hydrogen can directly source the liquid outer core, which is considered to have a substantial amount of hydrogen. Free hydrogen can also rise upwards to partially recover its loss from the subduction of hydrous minerals, and completes its journey in deep Earth. More in Q. Hu et al., PNAS, 114, 1498, 2017.