Textures Induced by the Coesite‐Stishovite Transition and Implications for the Visibility of the X‐Discontinuity
Textures Induced by the Coesite‐Stishovite Transition and Implications for the Visibility of the X‐Discontinuity
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DOI：
10.1029/2022gc010544
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发表时间：
2022-09
期刊：
影响因子：
3.7
通讯作者：
Matthias Krug;M. Saki;E. Ledoux;J. Gay;J. Chantel;A. Pakhomova;R. Husband;A. Rohrbach;S. Klemme;C. Thomas;S. Merkel;C. Sanchez‐Valle
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文献类型：
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作者：
Matthias Krug;M. Saki;E. Ledoux;J. Gay;J. Chantel;A. Pakhomova;R. Husband;A. Rohrbach;S. Klemme;C. Thomas;S. Merkel;C. Sanchez‐Valle
柯石英-斯石英相变被认为是解释在各种构造环境中约300 km深度处观察到的X-不连续性的最合理的候选者。在这里，我们研究了SiO2在单轴压缩实验中穿过柯石英-斯石英转变的微观结构。我们在激光加热金刚石砧室（LH-DAC）中应用多晶粒晶体学技术（MGC）来识别地幔中过渡和SiO2含量的地震特征。柯石英在转变前表现出较弱的晶格择优取向（LPO），而斯石英则表现出较强的LPO，其特征是[112]轴平行于压缩方向排列。然而，LPO对整个过渡区的阻抗对比几乎没有影响，在沿着正常地幔地热（10 GPa-1700 K）的300 km深度处，对于大洋中脊玄武岩（MORB）成分中的S波，阻抗对比高达8.8%。因此，10-50体积%的MORB组分，对应于0.6-3.2体积%需要SiO2与热解地幔机械混合，以解释X不连续性观察到的阻抗（和速度）对比范围。基于柯石英-中石英过渡计算的反射系数，我们表明入射角或震中距离对于上地幔含硅岩性的检测至关重要，小入射角的检测概率最高。因此，X不连续面的间歇性可见性可以通过柯石英-斯石英过渡的地震可探测性来解释，而不是在某些特定构造环境中缺乏过渡或化学不均匀性。
The coesite‐stishovite phase transition is considered the most plausible candidate to explain the X‐discontinuity observed at around 300 km depth in a variety of tectonic settings. Here, we investigate the microstructure in SiO2 across the coesite‐stishovite transition in uniaxial compression experiments. We apply the multigrain crystallography technique (MGC) in a laser‐heated diamond‐anvil cell (LH‐DAC) to identify the seismic signature of the transition and the amount of SiO2 in the mantle. While coesite displays weak lattice‐preferred orientations (LPO) before the transition, stishovite develops strong LPO characterized by the alignment of [112] axes parallel to the compression direction. However, LPO has little effect on the impedance contrast across the transition, which is up to 8.8% for S‐waves in a mid‐ocean ridge basalt (MORB) composition at 300 km depth along a normal mantle geotherm, 10 GPa‐1700 K. Therefore, 10–50 vol.% of a MORB component, corresponding to 0.6–3.2 vol.% SiO2, mechanically mixed with the pyrolytic mantle would be required to explain the range of impedance (and velocity) contrasts observed for the X‐discontinuity. Based on the reflection coefficients computed for the coesite‐stishovite transition, we show that the incidence angle or epicentral distance is critical for the detection of silica‐containing lithologies in the upper mantle, with highest detection probabilities for small incidence angles. The intermittent visibility of the X‐discontinuity may thus be explained by the seismic detectability of the coesite‐stishovite transition rather than by absence of the transition or chemical heterogeneities in some specific tectonic settings.