17:15 〜 18:45
[SIT15-P09] In situ lattice volume observation of davemaoite in a water-saturated system up to uppermost lower mantle conditions
キーワード：水輸送、名目上無水鉱物、CaSiO3ペロブスカイト、マントル遷移層、下部マントル
The water solubility of davemaoite (CaSiO3 perovskite), a major mineral in the lower mantle, is important for an accurate understanding of the distribution and transportation of water in the lower mantle. Recently, some studies reported that the lattice volume shrinks by incorporating water in davemaoite based on laser-heated diamond anvil cell experiments and theoretical calculation [1][2], and proposed that solubility of water in davemaoite as high as 0.5-2 wt.%. However, the relationship between water content and lattice volume change is not established due to the paucity of experimental data, and thus the actual value of water solubility in davemaoite is still unclear. In this study, we investigated change of the lattice volume of davemaoite in a water-saturated system under high pressure-temperature conditions
The experiments were performed using a natural mineral suolunite (Ca2Si2O5(OH)2・H2O with a water content of ~14 wt.%) mixed with a gold powder as a starting material using a multi-anvil apparatus, SPEED-MkII combined with in situ X-ray observation technique at BL04B1 in SPring-8. X-ray diffraction (XRD) patterns of the sample and gold pressure marker were obtained by an energy-dispersive system under high-pressure and high-temperature. The sample was oscillated during the diffraction measurements to obtain well averaged XRD patterns even after significant grain-growth. The lattice volume of davemaoite was calculated based on the XRD patterns taken at pressures of 10-30 GPa and temperatures up to 1500°C.
At ~15 GPa, davemaoite first appeared at 500°C, and its lattice volume was found to be almost equal to that of anhydrous davemaoite [3][4] up to 1300°C. On the other hand, at 20-30 GPa, the lattice volume of davemaoite was excessively larger than that of anhydrous when it first appeared during heating. However, the excess volume decreased with increasing temperature and time. These indicate that davemaoite metastably incorporates water only at low temperatures at these pressures, but dehydration occurs immediately at higher temperatures. After approaching the lattice volume of anhydrous value, the lattice volumes became lower than but almost the same as that of anhydrous values with increasing temperature. This observation is qualitatively consistent with the previous experimental study [1], but the volume difference was much smaller (max. -3%). The reason for this difference and the mechanism of the volume contraction are so far unclear. Our tentative conclusion is that davemaoite is expected to be almost anhydrous at lower mantle temperatures.
References
[1] Chen et al.: Physics of the Earth and Planetary Interiors, 299, 106412 (2020)
[2] Shim et al.: American Mineralogist: Journal of Earth and Planetary Materials, 107(4), 631-641 (2022)
[3] Sueda: PhD Thesis (2005)
[4] Sueda et al.: Geophysical Research Letters, 33.10 (2006)
The experiments were performed using a natural mineral suolunite (Ca2Si2O5(OH)2・H2O with a water content of ~14 wt.%) mixed with a gold powder as a starting material using a multi-anvil apparatus, SPEED-MkII combined with in situ X-ray observation technique at BL04B1 in SPring-8. X-ray diffraction (XRD) patterns of the sample and gold pressure marker were obtained by an energy-dispersive system under high-pressure and high-temperature. The sample was oscillated during the diffraction measurements to obtain well averaged XRD patterns even after significant grain-growth. The lattice volume of davemaoite was calculated based on the XRD patterns taken at pressures of 10-30 GPa and temperatures up to 1500°C.
At ~15 GPa, davemaoite first appeared at 500°C, and its lattice volume was found to be almost equal to that of anhydrous davemaoite [3][4] up to 1300°C. On the other hand, at 20-30 GPa, the lattice volume of davemaoite was excessively larger than that of anhydrous when it first appeared during heating. However, the excess volume decreased with increasing temperature and time. These indicate that davemaoite metastably incorporates water only at low temperatures at these pressures, but dehydration occurs immediately at higher temperatures. After approaching the lattice volume of anhydrous value, the lattice volumes became lower than but almost the same as that of anhydrous values with increasing temperature. This observation is qualitatively consistent with the previous experimental study [1], but the volume difference was much smaller (max. -3%). The reason for this difference and the mechanism of the volume contraction are so far unclear. Our tentative conclusion is that davemaoite is expected to be almost anhydrous at lower mantle temperatures.
References
[1] Chen et al.: Physics of the Earth and Planetary Interiors, 299, 106412 (2020)
[2] Shim et al.: American Mineralogist: Journal of Earth and Planetary Materials, 107(4), 631-641 (2022)
[3] Sueda: PhD Thesis (2005)
[4] Sueda et al.: Geophysical Research Letters, 33.10 (2006)