钙钛矿（Davemaoite）的四方-立方相变：对下地幔地震异常的意义

中文摘要 AI 辅助

钙钛矿（CaSiO3钙钛矿）是地球下地幔中第三丰富的矿物，也是俯冲洋壳中的主要相。其晶体结构在环境条件下发生畸变（四方或正交），但被认为在高温下转变为立方结构。先前的实验报道了较低且几乎与压力无关的转变温度（约600 K），这与大多超过1000 K的理论预测存在长期矛盾。在此，我们利用激光加热金刚石压砧结合同步辐射X射线衍射，在同时高温高压条件下测定了CaSiO3钙钛矿及其含钛固溶体[Ca(Si0.75,Ti0.25)O3]的相稳定性和热状态方程。我们发现四方到立方的转变发生在比先前报道高得多的温度下，且钛替代进一步稳定了四方相，并将转变边界推移至更高温度。这些发现表明，俯冲洋壳中的CaSiO3钙钛矿可能在下地幔中部经历其铁弹相变，而富钛钙钛矿可能在下地幔大部分区域保持四方相，直至核幔边界附近才转变为立方相。我们的结果表明，钙钛矿的成分依赖性相行为可以解释在中下地幔和最下地幔观测到的地震异常。

英文摘要

Davemaoite (CaSiO3 perovskite) is the third most abundant mineral in Earth's lower mantle and a dominant phase in subducted oceanic crust. Its crystal structure is distorted (tetragonal or orthorhombic) at ambient conditions but is considered to transform to cubic at high temperatures. Previous experiments reported low, nearly pressure-independent transition temperatures (approximately 600 K), demonstrating a long-standing discrepancy with theoretical predictions that mostly exceed 1000 K. Here, we determine the phase stability and thermal equation of state of CaSiO3 davemaoite and its titanium-bearing solid solution [Ca(Si0.75,Ti0.25)O3] under simultaneous high-pressure and high-temperature conditions using a laser-heated diamond anvil cell combined with synchrotron X-ray diffraction. We find that the tetragonal-to-cubic transition occurs at substantially higher temperatures than previously reported, with the titanium substitution further stabilizing the tetragonal phase and shifting the transition boundary to even higher temperatures. These findings indicate that CaSiO3 davemaoite in subducted oceanic crust likely undergoes its ferroelastic transition in the mid-lower mantle, whereas Ti-rich davemaoite may remain tetragonal throughout most of the lower mantle, transforming to cubic near the core-mantle boundary. Our results demonstrate that the compositionally dependent phase behaviour of davemaoite can account for the seismic anomalies observed in both the mid-lower mantle and the lowermost mantle.

发表机构

• Hawai'i Institute of Geophysics and Planetology, University of Hawai'i at Manoa（夏威夷大学马诺阿分校地球物理与行星学研究所）
• Department of Earth and Planetary Sciences, Institute of Science Tokyo（东京科学大学地球与行星科学系）
• SPring-8, Japan Synchrotron Radiation Research Institute（日本同步辐射研究机构 SPring-8）
• National Synchrotron Light Source II, Brookhaven National Laboratory（布鲁克海文国家实验室国家同步光源二号）
• Center for Advanced Radiation Sources, The University of Chicago（芝加哥大学先进辐射源中心）

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