Freeing hydrogen in Earth's lower mantle - Dr. Ho-Kwang Mao
FEBURARY 04, 2017
In Earth interior, water (H2O) plays an important role in rock physics but geoscientists rarely treat water in its decomposable forms, like hydrogen plus oxygen. However, new work from a team led by HPSTAR director, Dave Mao, has identified the hydrogen can escape from the water under lower mantle conditions. Their results were published in Proceeding of the National Academic Science, U.S.A.
In the atmosphere, hydrogen is a colorless, transparent gas. It bonds with oxygen to form water which fuels the biosphere on the Earth surface. Deep in the rocky world beneath our feet, so-called hydrous minerals enriched with water mainly exist in the subduction slabs that are relatively colder than average geotherm. 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.
“The stability of water in hydrous mineral might break at the lower mantle”, said Dave. Using advanced diffraction tools, the 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 (>720,000 atmosphere, and ~2,000 K), Dave’s team was able to identify hydrogen released and the hydrogen loss closely related with the heating temperature and duration.
“Oxygen and hydrogen cycles are separated above the Earth’s core.” suggested by Qingyang Hu, the leading author, “Hydrogen is not freezing, instead, freeing from rocks.”
The team’s results have led to a new paradigm change in the planetary science of lower mantle chemistry. 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.
Caption: The hydrogen cycling in the deep Earth. Image courtesy Qingyang Hu.