北京高压科学研究中心
Center for High Pressure Science &Technology Advanced Research

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.


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


长久以来,由于水分子的化学结构相当稳定,地球内部的水循环与氢循环被认为是对等的。毛河光领导的研究小组发现,在下地幔条件下,含水矿物中的水分子的稳定性会下降,氢元素因此会独立于水,在高温高压下释放出来。通过x射线衍射和金刚石对顶压砧,小组将沼铁矿的主要成分(FeOOH)压缩至72万大气压以上,并通过激光加温,发现氢气会从样品中释放,这种脱氢速率与加热温度和时间息息相关。对地球化学家而言,下地幔一般认为是还原态的,而该工作表明在富含含水矿物的区域中,氢气会分离并向地幔上方迁移,而高度氧化的二氧化铁会留在地幔深处。这个新的发现将对地球内部化学环境的研究产生重要影响。