Gréaux Steeve, Irifune Tetsuo, Higo Yuji, Tange Yoshinori, Arimoto Takeshi, Liu Zhaodong, Yamada Akihiro
Geodynamics Research Center, Ehime University, Matsuyama, Japan.
Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo, Japan.
Nature. 2019 Jan;565(7738):218-221. doi: 10.1038/s41586-018-0816-5. Epub 2019 Jan 9.
Laboratory measurements of sound velocities of high-pressure minerals provide crucial information on the composition and constitution of the deep mantle via comparisons with observed seismic velocities. Calcium silicate (CaSiO) perovskite (CaPv) is a high-pressure phase that occurs at depths greater than about 560 kilometres in the mantle and in the subducting oceanic crust. However, measurements of the sound velocity of CaPv under the pressure and temperature conditions that are present at such depths have not previously been performed, because this phase is unquenchable (that is, it cannot be physically recovered to room conditions) at atmospheric pressure and adequate samples for such measurements are unavailable. Here we report in situ X-ray diffraction and ultrasonic-interferometry sound-velocity measurements at pressures of up to 23 gigapascals and temperatures of up to 1,700 kelvin (similar to the conditions at the bottom of the mantle transition region) using sintered polycrystalline samples of cubic CaPv converted from bulk glass and a multianvil apparatus. We find that cubic CaPv has a shear modulus of 126 ± 1 gigapascals (uncertainty of one standard deviation), which is about 26 per cent lower than theoretical predictions (about 171 gigapascals). This value leads to substantially lower sound velocities of basaltic compositions than those predicted for the pressure and temperature conditions at depths between 660 and 770 kilometres. This suggests accumulation of basaltic crust in the uppermost lower mantle, which is consistent with the observation of low-seismic-velocity signatures below 660 kilometres and the discovery of CaPv in natural diamond of super-deep origin. These results could contribute to our understanding of the existence and behaviour of subducted crust materials in the deep mantle.
通过与观测到的地震波速度进行比较，对高压矿物声速的实验室测量为深入了解地幔的组成和结构提供了关键信息。硅酸钙（CaSiO）钙钛矿（CaPv）是一种高压相，出现在地幔中深度大于约560公里处以及俯冲的洋壳中。然而，此前尚未在如此深度所对应的压力和温度条件下对CaPv的声速进行测量，因为该相在大气压力下不可淬火（即无法物理恢复到室温条件），并且没有足够的样本用于此类测量。在此，我们报告了使用由块状玻璃转化而成的立方CaPv烧结多晶样本和多砧装置，在高达23吉帕斯卡的压力和高达1700开尔文（类似于地幔过渡区底部的条件）的温度下进行的原位X射线衍射和超声干涉声速测量。我们发现立方CaPv的剪切模量为126±1吉帕斯卡（一个标准偏差的不确定性），比理论预测值（约171吉帕斯卡）低约26%。该值导致玄武岩成分的声速显著低于在660至770公里深度处的压力和温度条件下的预测值。这表明玄武岩地壳在最上部的下地幔中堆积，这与在660公里以下观测到的低地震波速度特征以及在超深天然金刚石中发现CaPv相一致。这些结果有助于我们理解俯冲地壳物质在地幔深处的存在和行为。
