P13-YANG Xiaozhi

Hydrogen in the mantle

Xiaozhi Yang

Nanjing University, China (xzyang@nju.edu.cn)

The cycling of hydrogen (or more commonly water) between the mantle and the exosphere is one of the most important factors in affecting Earth’s climate, habitability, geochemical evolution, geophysical properties and geodynamical processes. However, the species and inventory of hydrogen in the mantle are not fully known. It is widely believed that hydrogen is stored in the mantle mostly as OH groups structurally bound in the lattice structure of mantle minerals. Studies in the past decades have provided some constraints on the storage capacity of OH groups in the mantle, the maximum OH content that can be dissolved in a hydrogen-saturated system, by equilibrating minerals with H2O under experimentally simulated conditions. However, fluids in the mantle are not H2O only but complex C-O-H mixtures with their species determined largely by the prevailing redox state, e.g., CO2-H2O at oxidizing conditions and CH4-H2O-H2 at reducing conditions. Taking this into consideration, we have shown by experimental studies at elevated conditions that the storage capacity of OH groups in mantle minerals is much smaller, e.g., by a factor of up to ~20, than previous estimates. Also, we have shown that, under reducing conditions, molecular hydrogen (H2) has an appreciable solubility in mantle minerals, suggesting the possible storage of H2 in the deep mantle. It appears therefore that the storage of hydrogen in the mantle differs greatly from available knowledge, and so does the impact on many aspects of the Earth (and other planets).