17:15 〜 18:45
[SIT15-P07] Water in davemaoite inferred from water contents of CaTiO3-bearing CaSiO3 perovskite up to uppermost lower mantle conditions
キーワード：水、マントル、デイブマオアイト、ペロブスカイト、マルチアンビルプレス
Water is transported into the deep mantle by subducting slabs and can significantly affect chemical and physical properties of mantle minerals. Our recent study reported that aluminous silica minerals such as stishovite and CaCl2-type phase can accommodate water up to weight percent levels even at hot plume conditions unlike major lower mantle minerals such as bridgmanite and ferropericlase (<0.1 wt.%), suggesting deep water cycle by silica minerals in crustal materials.
CaSiO3 cubic perovskite, davemaoite, is one of the abundant minerals in peridotite, subducted basaltic crusts, and sediments/continental crust materials in deep mantle conditions. The stability and physical properties such as elasticity and rheology of dry davemaoite have intensively investigated, suggesting important implications with lower mantle structure and dynamics. The recent experimental and theoretical studies proposed high water contents up to 0.5-2 wt.% in davemaoite. However, there is still no clear evidence of such significant solubility of water because davemaoite is unquenchable and becomes amorphous during recovery, hampering to analyze its water content precisely.
In this study, we have determined water solubility of CaTiO3-bearing CaSiO3 perovskite up to 25 GPa and 2100 K by means of Kawai-type multi-anvil press in combination with Fourier Transform Infrared Spectroscopy (FTIR) at ambient pressure. It is known that davemaoite can make a solid solution with CaTiO3 component, which becomes quenchable by the incorporation more than 40 mol%, allowing us to precisely estimate its water content by FTIR.
Single crystals of CaTiO3-bearing CaSiO3 perovskite were synthesized using Kawai-type multi-anvil presses installed at Institute for Planetary Materials, Okayama University, Japan. A powdered mixture of natural suolunite [Ca2Si2O5(OH)2·H2O]:CaTiO3=6:4 (mol) was prepared as a starting material. The sample was put in a welded Pt tube capsule. Recovered single crystals were analyzed by scanning electron microscopy with energy-dispersive X-ray spectroscopy, electron probe micro-analysis, micro-focused X-ray diffraction (XRD) and FTIR for phase identification and chemical composition. Single crystal XRD structure analysis was also performed.
We recovered single crystals of ~40 mol%CaTiO3-bearing CaSiO3 perovskite with dimensions up to 200 microns together with hydrous silicate melt at 20-25 GPa and 1700-2100 K. The single crystal structure analysis showed that the space group of Fm-3m and double cubic unit cell, indicating the crystal structure is double cubic perovskite-type. Previous studies for CaSiO3-H2O system suggested a possible water incorporation mechanism of Ca2+ = 2H+, contributing volume reduction of davemaoite. Our volume data by XRD showed no volume reduction compared to those of dry perovskites in CaSiO3-CaTiO3 system. FTIR spectra of these perovskite samples showed no OH-bands, indicating water content is less than 1 ppm. Thus, CaSiO3 davemaoite would be dry in mantle conditions and therefore we suggest that water is cycled only by crustal materials including aluminous silica minerals in the lower mantle.
CaSiO3 cubic perovskite, davemaoite, is one of the abundant minerals in peridotite, subducted basaltic crusts, and sediments/continental crust materials in deep mantle conditions. The stability and physical properties such as elasticity and rheology of dry davemaoite have intensively investigated, suggesting important implications with lower mantle structure and dynamics. The recent experimental and theoretical studies proposed high water contents up to 0.5-2 wt.% in davemaoite. However, there is still no clear evidence of such significant solubility of water because davemaoite is unquenchable and becomes amorphous during recovery, hampering to analyze its water content precisely.
In this study, we have determined water solubility of CaTiO3-bearing CaSiO3 perovskite up to 25 GPa and 2100 K by means of Kawai-type multi-anvil press in combination with Fourier Transform Infrared Spectroscopy (FTIR) at ambient pressure. It is known that davemaoite can make a solid solution with CaTiO3 component, which becomes quenchable by the incorporation more than 40 mol%, allowing us to precisely estimate its water content by FTIR.
Single crystals of CaTiO3-bearing CaSiO3 perovskite were synthesized using Kawai-type multi-anvil presses installed at Institute for Planetary Materials, Okayama University, Japan. A powdered mixture of natural suolunite [Ca2Si2O5(OH)2·H2O]:CaTiO3=6:4 (mol) was prepared as a starting material. The sample was put in a welded Pt tube capsule. Recovered single crystals were analyzed by scanning electron microscopy with energy-dispersive X-ray spectroscopy, electron probe micro-analysis, micro-focused X-ray diffraction (XRD) and FTIR for phase identification and chemical composition. Single crystal XRD structure analysis was also performed.
We recovered single crystals of ~40 mol%CaTiO3-bearing CaSiO3 perovskite with dimensions up to 200 microns together with hydrous silicate melt at 20-25 GPa and 1700-2100 K. The single crystal structure analysis showed that the space group of Fm-3m and double cubic unit cell, indicating the crystal structure is double cubic perovskite-type. Previous studies for CaSiO3-H2O system suggested a possible water incorporation mechanism of Ca2+ = 2H+, contributing volume reduction of davemaoite. Our volume data by XRD showed no volume reduction compared to those of dry perovskites in CaSiO3-CaTiO3 system. FTIR spectra of these perovskite samples showed no OH-bands, indicating water content is less than 1 ppm. Thus, CaSiO3 davemaoite would be dry in mantle conditions and therefore we suggest that water is cycled only by crustal materials including aluminous silica minerals in the lower mantle.