Dehydrogenation of goethite in Earth's deep lower mantle.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 28143928.
- Also identified by DOI 10.1073/pnas.1620644114 and PMC identifier 5320987.
- No licence information is recorded for this record.
- Because redistribution is not established, this page shows the abstract only. Follow the links below for the full text.
Abstract
The cycling of hydrogen influences the structure, composition, and stratification of Earth's interior. Our recent discovery of pyrite-structured iron peroxide (designated as the P phase) and the formation of the P phase from dehydrogenation of goethite FeO<sub>2</sub>H implies the separation of the oxygen and hydrogen cycles in the deep lower mantle beneath 1,800 km. Here we further characterize the residual hydrogen, <i>x</i>, in the P-phase FeO<sub>2</sub>H<i>x</i> Using a combination of theoretical simulations and high-pressure-temperature experiments, we calibrated the <i>x</i> dependence of molar volume of the P phase. Within the current range of experimental conditions, we observed a compositional range of P phase of 0.39 < <i>x</i> < 0.81, corresponding to 19-61% dehydrogenation. Increasing temperature and heating time will help release hydrogen and lower <i>x</i>, suggesting that dehydrogenation could be approaching completion at the high-temperature conditions of the lower mantle over extended geological time. Our observations indicate a fundamental change in the mode of hydrogen release from dehydration in the upper mantle to dehydrogenation in the deep lower mantle, thus differentiating the deep hydrogen and hydrous cycles.