Presentation Information
[R3-03]Re-evaluation of hydrogen concentrations of deep-mantle hydrous minerals by a combination analysis of secondary ion mass spectrometry and neutron structure analysis
*Takuo OKUCHI1, Kenji Shimizu2, Naotaka Tomioka2, Narangoo Purevjav3, Shoichi Itoh4, Takayuki Ushikubo2, Akimasa Suzumura5 (1. Kyoto University, IIRNS, 2. JAMSTEC Kochi, 3. Seoul National Univ, 4. Kyoto University, Grad Sch Sci, 5. Kyoto University, Grad Sch Hum Env)
Keywords:
deep-mantle hydrous minertals,neutron diffraction,seconday ion mass spectrometry
Deep-mantle hydrous minerals play important roles in the evolution of the Earth. The total amount of hydrogen in these minerals has been discussed to easily exceed the entire mass of surface oceans. Therefore, hydrogen concentration analysis in these minerals synthesized at variable pressure and temperature conditions is a fundamental topic for considering the planet’s physical, chemical, and biological evolutions related to hydrogen’s storage and cycling. Our neutron diffraction studies quantitatively demonstrated the hydrogen site locations and concentrations in these minerals. On the other hand, Secondary Ion Mass Spectrometry (SIMS) has been extensively applied in previous works to determine hydrogen concentrations in various types of mineral species; this method is capable of analyzing even smaller amounts of hydrogen than neutron diffraction. Recently, we quantitatively determined the hydrogen site and concentration of a synthetic lower-mantle bridgmanite, (Mg0.88Fe0.10Al0.03)(Si0.88Al0.11H0.01)O3, by a combination analysis of SIMS and neutron diffraction. As in this case, this combination analysis of the other deep-mantle hydrous minerals, which are synthesized under variable pressure-temperature conditions and chemical compositions, would be a very effective method for constraining the hydrogen budget within the Earth in a more robust manner. We thus analyzed our synthetic deep-mantle mineral crystals by SIMS, in addition to the previous neutron diffraction analyses. Representative results are shown in the Figure. It was originally expected that all mineral species would yield self-consistent results between SIMS and neutron diffraction; however, the results differed. The hydrous wadsleyite species shows its own trend, and it obviously deviates from the ringwoodite one that closely follows the general trend of the previously defined calibration line for hydrogen. It was thus inferred that the hydrogen concentrations in wadsleyite, as determined by SIMS analysis, had been extensively overestimated by about 30% at most. A more careful, crystal-structure-dependent calibration line system for future SIMS analysis is now under construction.
