| Abstrakt: |
SiO2 is an important modally abundant component of terrestrial planets, while there are few studies that quantitatively determine the properties of silica under simultaneously high‐temperature and high‐pressure conditions. In this study, thermal properties of stishovite, CaCl2‐type SiO2 and seifertite have been obtained by using extensive first‐principles simulations. The quasi‐harmonic approximation and intrinsic anharmonic effects were considered, and a generalized rescaling method was adopted to eliminate systematic errors in the simulation results. Based on the simulation data, we have established new equations of state for the SiO2 polymorphs and determined their phase boundaries. Anharmonicity has been demonstrated to show noticeable effects on their thermal properties and phase boundaries. With the new results, we found that SiO2 polymorphs show increasing buoyancy in the subducted basaltic slabs at depths greater than ∼800–900 km. In the normal lower mantle, they are readily stable over a broad range of depths approximately from 1,500 km to 2,600 km. This implies that SiO2 polymorphs may contribute to the lateral heterogeneity in the deep lower mantle as observed by small‐scale seismic scatterers. Plain Language Summary: The solid Earth is mainly composed of silicates and SiO2 is an important endmember. Previous experiments and theoretical calculations show that SiO2 has three high‐pressure phases, stishovite, CaCl2‐type SiO2 and seifertite under lower mantle conditions. These phases may be related with the regional velocity anomalies in the lower mantle, so it is vital to accurately determine their thermal properties and the locations of phase transitions. In this study, based on extensive high quality first‐principles simulations, we have established new thermodynamic models for the three high‐pressure phases and predicted phase transition boundaries that are in good agreement with experiments. Our results show that SiO2 would meet its neutral buoyancy at about 800–900 km in the typical subduction slabs. Meanwhile, its density keeps being close to the averaged density in the normal lower mantle from ∼1,500 km to ∼2,600 km. These findings give important hints on the fate of SiO2 high‐pressure phases in the deep mantle. Key Points: We obtained the equations of state of stishovite, CaCl2‐type SiO2, and seifertite based on extensive first‐principles simulationsAnharmonicity is important at high temperatures and has noticeable effects on phase boundariesSilica polymorphs can be stable and related with the small‐scale seismic scatterers in the deep lower mantle [ABSTRACT FROM AUTHOR] |
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