High-pressure single-crystal elasticity of wadsleyite and the seismic signature of water in the shallow transition zone
High-pressure single-crystal elasticity of wadsleyite and the seismic signature of water in the shallow transition zone
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DOI：
10.1016/j.epsl.2018.06.027
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发表时间：
2018-09-15
影响因子：
5.3
通讯作者：
Kurnosov, Alexander
中科院分区：
文献类型：
--
作者：
Buchen, Johannes;Marquardt, Hauke;Kurnosov, Alexander
在410千米至660千米深度之间的地球过渡带在地球的深水循环中起着关键作用，因为大量的氢可以储存在名义上无水的矿物wadsleyite和ringwoodite， （Mg,Fe）(2)SiO4中。以往对无铁瓦德斯莱岩的矿物物理实验提出，低地震速度是过渡带水化的指示特征。本文报道了含铁wadsleite单晶在0.24 wt-% H2O条件下的声速和密度测量结果。通过与早期研究的比较，我们发现压力抑制了含铁瓦德利岩中较高水化程度引起的速度降低，最终导致纵波和横波的速度交叉。基于实验结果的建模表明，过渡带内的波速变化和410 km地震不连续处的速度跳变是较差的水传感器，这两种方法在以前的工作中用于探测地幔水化。相反，在存在水的情况下，410公里地震不连续区的阻抗对比减小，可以作为过渡带含水部分的更可靠指标。(C) 2018 Elsevier B.V.版权所有
Earth's transition zone at depths between 410 km and 660 km plays a key role in Earth's deep water cycle since large amounts of hydrogen can be stored in the nominally anhydrous minerals wadsleyite and ringwoodite, (Mg,Fe)(2)SiO4. Previous mineral physics experiments on iron-free wadsleyite proposed low seismic velocities as an indicative feature for hydration in the transition zone. Here we report simultaneous sound wave velocity and density measurements on iron-bearing wadsleyite single crystals with 0.24 wt-% H2O. By comparison with earlier studies, we show that pressure suppresses the velocity reduction caused by higher degrees of hydration in iron-bearing wadsleyite, ultimately leading to a velocity cross-over for both P-waves and S-waves. Modeling based on our experimental results shows that wave speed variations within the transition zone as well as velocity jumps at the 410-km seismic discontinuity, both of which have been used in previous work to detect mantle hydration, are poor water sensors. Instead, the impedance contrast across the 410-km seismic discontinuity that is reduced in the presence of water can serve as a more robust indicator for hydrated parts of the transition zone. (C) 2018 Elsevier B.V. All rights reserved.