11:30 〜 11:45
[SIT17-10] Water solubility of aluminous stishovite coexisting with hydrous phase δ: Implication for water cycle in the lower mantle
キーワード：スティショバイト、含水相δ、下部マントル、水循環、CaCl2型アルミナ含有シリカ
It is considered that water is transported into the deep mantle, playing important roles on mantle dynamics and evolution. Hydrous minerals discovered as diamond inclusion such as hydrous ringwoodite suggests that water is transported at least into the mantle transition zone. However, water cycle in the lower mantle is not clear yet. Our recent study found that aluminous silica minerals of stishovite and CaCl2-type phase can accommodate significant amounts of water up to ~1 wt.% in their crystal structures at uppermost lower mantle pressures above temperatures of the average mantle, compared with other lower-mantle minerals (<0.1 wt.%) (Ishii et al. 2022). Along subducting slab geotherm (1000-1200 ℃), aluminous stishovite coexists with hydrous phase δ. Although hydrous phase δ should mainly transport water in this temperature range, stishovite may also accommodate a significant amount of water. In order to better understand water carriers in the lower mantle, in this study, water partitioning experiments between stishovite and hydrous phase δ were carried out at 24-28 GPa and 1000-1200 ℃ by means of multi-anvil press in combination with Fourier transform infrared spectroscopy (FTIR) at ambient conditions.
Single crystals of aluminous stishovite was synthesized by multi-anvil press at 24 GPa and 2000 ℃ for 2 h using a mixture of regent grade SiO2 and Al(OH)3 with a molar ratio of 9:1. The single crystal was packed in a gold capsule together with a powdered mixture of regent grade SiO2 and AlOOH with a molar ratio of 9:1. After annealing for 24 h at a desired pressure and temperature, the sample was quenched and recovered. Water contents of the starting and recovered aluminous stishovites were measured by FTIR.
The starting aluminous stishovite has ~700 ppmH2O. The matrix of the powdered mixture was changed to stishovite and hydrous phase δ after the partitioning experiments. Water contents of aluminous stishovites decreased by 100-200 ppm H2O after the partitioning experiments, indicating aluminous stishovite is nearly dry when coexisting with hydrous phase δ. Thus, water is transported into the lower mantle by hydrous phase δ along low-temperature slab geotherm and by aluminous silica polymorphs in high-temperature regions.
Reference
Ishii, T., Criniti, G., Ohtani, E., Purevjav, N., Fei, H., Katsura, T., & Mao, H. K. (2022). Superhydrous aluminous silica phases as major water hosts in high-temperature lower mantle. Proceedings of the National Academy of Sciences, 119(44), e2211243119.
Single crystals of aluminous stishovite was synthesized by multi-anvil press at 24 GPa and 2000 ℃ for 2 h using a mixture of regent grade SiO2 and Al(OH)3 with a molar ratio of 9:1. The single crystal was packed in a gold capsule together with a powdered mixture of regent grade SiO2 and AlOOH with a molar ratio of 9:1. After annealing for 24 h at a desired pressure and temperature, the sample was quenched and recovered. Water contents of the starting and recovered aluminous stishovites were measured by FTIR.
The starting aluminous stishovite has ~700 ppmH2O. The matrix of the powdered mixture was changed to stishovite and hydrous phase δ after the partitioning experiments. Water contents of aluminous stishovites decreased by 100-200 ppm H2O after the partitioning experiments, indicating aluminous stishovite is nearly dry when coexisting with hydrous phase δ. Thus, water is transported into the lower mantle by hydrous phase δ along low-temperature slab geotherm and by aluminous silica polymorphs in high-temperature regions.
Reference
Ishii, T., Criniti, G., Ohtani, E., Purevjav, N., Fei, H., Katsura, T., & Mao, H. K. (2022). Superhydrous aluminous silica phases as major water hosts in high-temperature lower mantle. Proceedings of the National Academy of Sciences, 119(44), e2211243119.