9:30 AM - 9:45 AM
[SMP26-03] The stability of hydrous SiO2 stishovite in the deep mantle
Keywords:Silica mineral, Mantle transition zone, Lower mantle
Some minerals that compose the mantle or slab can contain small amounts of water as impurities in the crystal lattice. SiO2 silica minerals, which are typical minerals in crustal materials such as sediments and basalts, are stable as stishovite under a wide range of pressures from 10 to 70 GPa in the deep mantle and can contain small amounts of water. Earlier studies reported that water content in stishovite is related to Al content and to the maximum water content in stishovite would be limited to 0.3 wt% even for 4.4 wt% Al2O3 (Litasov et al., 2007). On the other hand, recent studies have indicated that much larger amounts of water (>3 wt%) can dissolve in the high-pressure phase of pure SiO2 silica (e.g., Spektor et al., 2016; Lin et al., 2020). The amount of water stored in stishovite is important because it significantly effects water distribution and water transport efficiency in the deep mantle. Quench experiments using a multi-anvil apparatus suggest that SiO2 stishovite with high water content may be metastable, however experiments using a laser-heated DAC indicated that it can be a major hydrous phase in the lower mantle. Thus, its stability under high-pressure and -temperature is controversial. In this study, we measured the cell volume of the SiO2 high-pressure phase under hydrous conditions and investigated its stability by in situ high-pressure and -temperature X-ray experiments using a multi-anvil apparatus.
The experiments were conducted using silicic acid (SiO2・xH2O) with a water content of 9.8 wt% as a starting material. The sample was heated up to 1000°C at 13-29 GPa using the multi-anvil apparatus SPEED-1500 installed at BL04B1, SPring-8. The X-ray diffraction patterns of the samples were acquired by an energy-dispersive system under high pressure and high temperature. Observed SiO2 phase after heating was always stishovite. At initial stage of heating, cell volume at low temperature was much larger than that of anhydrous stishovite, with a maximum difference of +2.6%. The difference rapidly decreased as the temperature increased, and the cell volume became close to that of anhydrous stishovite at temperatures above 550°C. Time-resolved X-ray observations at constant temperature showed that the cell volume decreased continuously with time. These observations suggest that stishovite contains a large amount of water (>1 wt%) only metastably. Therefore, we concluded that SiO2 stishovite is unlikely to be the main hydrous phase, at least at conditions up to the top of the lower mantle.
The experiments were conducted using silicic acid (SiO2・xH2O) with a water content of 9.8 wt% as a starting material. The sample was heated up to 1000°C at 13-29 GPa using the multi-anvil apparatus SPEED-1500 installed at BL04B1, SPring-8. The X-ray diffraction patterns of the samples were acquired by an energy-dispersive system under high pressure and high temperature. Observed SiO2 phase after heating was always stishovite. At initial stage of heating, cell volume at low temperature was much larger than that of anhydrous stishovite, with a maximum difference of +2.6%. The difference rapidly decreased as the temperature increased, and the cell volume became close to that of anhydrous stishovite at temperatures above 550°C. Time-resolved X-ray observations at constant temperature showed that the cell volume decreased continuously with time. These observations suggest that stishovite contains a large amount of water (>1 wt%) only metastably. Therefore, we concluded that SiO2 stishovite is unlikely to be the main hydrous phase, at least at conditions up to the top of the lower mantle.