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Title
Hydrogen in the Nominally Anhydrous Phases and Possible Hydrous Phases in the Lower Mantle
Description
The transport of hydrogen to the Earth’s deep interior remains uncertain. The upper mantle minerals have very low hydrogen solubilities (hundreds of ppm). The hydrogen storage capability in the transition zone minerals (2 wt%) is high compared to those of the upper mantle. The hydrogen storage in the lower mantle is not well known. The main minerals in the lower mantle bridgmanite and ferropericlase have very low hydrogen storage capacities (less than 20 ppm). In order to further understand the hydrogen storage in the lower mantle, a series of experiments had been conducted to simulate the environment similar to the Earth’s mantle. The experiments with hydrous Mg2SiO4 ringwoodite (Rw) show that it converts to crystalline dense hydrous silica, stishovite (Stv) or CaCl2-type SiO2(mStv), containing ∼1 wt% H2O together with bridgmanite (Brd) and MgO at the pressure-temperature conditions expected for lower mantle depths between approximately 660 to 1600 km. Brd would break down partially to dense hydrous silica (6–25 mol%) and(Mg,Fe)O in mid-mantle regions with 0.05–0.27 wt% H2O. The hydrous stishovite has a CaCl2 structure, which is common among hydrous minerals in the lower mantle. Based on this observation, I hypothesize the existence of hydrous phases in the lower mantle. The experiments found a new hexagonal iron hydroxide (η-Fe12O18+x/2Hx) between the stability fields of the epsilon and pyrite-type FeOOH at 60–80 GPa and high temperature. The new phase contains less H2O, limiting the H2O transport from the shallow to the deep mantle in the Fe–O–H system. Possible hydrogen storage in Ca-perovskite was studied. CaPv could contain 0.5–1 wt% water and the water in CaPv could distort the crystal structure of CaPv from cubic to tetragonal structure. In conclusion, hydrogen can be stored in hydrous stishovite in the shallower depth of the lower mantle. At greater depth, the new η phase and pyrite-type phase would take over the hydrogen storage. The role of CaPv in deep water storage needs to be considered in future studies.
Date Created
2019
Contributors
- Chen, Huawei (Author)
- Shim, Sang-Heon (Thesis advisor)
- Garnero, Edward (Committee member)
- Bose, Maitrayee (Committee member)
- Li, Mingming (Committee member)
- Leinenweber, Kurt (Committee member)
- Arizona State University (Publisher)
Topical Subject
Resource Type
Extent
153 pages
Language
eng
Copyright Statement
In Copyright
Primary Member of
Peer-reviewed
No
Open Access
No
Handle
https://hdl.handle.net/2286/R.I.54920
Level of coding
minimal
Note
Doctoral Dissertation Natural Science 2019
System Created
- 2019-11-06 03:39:47
System Modified
- 2021-08-26 09:47:01
- 3 years 2 months ago
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