The elasticity of lawsonite at high pressure and the origin of low velocity layers in subduction zones
The elasticity of lawsonite at high pressure and the origin of low velocity layers in subduction zones
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DOI：
10.1016/j.epsl.2012.06.034
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发表时间：
2012-10-01
影响因子：
5.3
通讯作者：
Frost, Daniel J.
中科院分区：
文献类型：
--
作者：
Chantel, Julien;Mookherjee, Mainak;Frost, Daniel J.
由于相对较冷的海洋岩石圈的再循环，俯冲带的地震波速度比周围的地幔快。然而，在某些俯冲带，已推断出沿着俯冲板片的顶面存在5-10 km厚的低速层（LVL），深度可达250 km。剪切波速度，特别是在这些层内，已被估计为高达10%，比周围mantle.We进行了高压超声干涉测量，以了解弹性的硬铝石[CaAl 2（Si 2 O 7）（OH）（2）- H2O]，一种含水矿物相稳定在冷俯冲带条件下的性质。此外，我们已经计算了全弹性常数张量在升高的压力和温度，使用静态电子结构计算和分子动力学模拟。理论计算和实验结果吻合较好。我们发现硬毛钠石在高压下具有异常低的剪切模量，它在俯冲洋壳中的形成可以解释一些地震证据LVL在深度超过100公里。为了接近估计的LVL速度，需要在俯冲地壳中形成硬铝石，这是由于其他含水矿物（如蛇纹石）的分解而导致的流体流入的结果。硬毛钠石的形成还降低了地震速度，因为它是以石榴石为代价形成的，石榴石是一种具有相对较快地震速度的矿物。因此，LVL观测可以用来限制俯冲到深部地幔中的H2O的量。(C)2012爱思唯尔有限公司版权所有。
Subduction zones exhibit faster seismic wave velocities compared to the surrounding mantle due to the recycling of relatively cold oceanic lithosphere. In certain subduction zones, however, a 5-10 km thick low velocity layer (LVL) has been inferred to exist along the top surface of the subducting slab at depths of up to 250 km. Shear-wave velocities, in particular, within these layers have been estimated as up to 10% slower than the surrounding mantle.We have conducted high-pressure ultrasonic interferometric measurements to gain insight into the elastic properties of lawsonite [CaAl2(Si2O7)(OH)(2)- H2O], a hydrous mineral phase stabilized under cold subduction zone conditions. In addition, we have computed the full elastic constant tensor at elevated pressures and temperature, using static electronic structure calculations and molecular dynamics simulations. The bulk and shear modulus obtained from theory and experiments are in good agreement. We find that lawsonite has an unusually low shear modulus at high pressure and its formation in subducted oceanic crust can explain some seismic evidence for LVL at depths exceeding 100 km. To approach estimated LVL velocities requires lawsonite to form in the subducting crust as a result of a fluid influx due to the breakdown of other hydrous minerals such as serpentine. The formation of lawsonite additionally lowers seismic velocities because it forms at the expense of garnet, a mineral with relatively fast seismic velocities. LVL observations may therefore be used to place constraints on the amount of H2O subducted into the deep mantle. (C) 2012 Elsevier B.V. All rights reserved.