Global seismic data reveal little water in the mantle transition zone
Global seismic data reveal little water in the mantle transition zone
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DOI：
10.1016/j.epsl.2016.04.018
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发表时间：
2016-08-15
影响因子：
5.3
通讯作者：
Houser, C.
中科院分区：
文献类型：
--
作者：
Houser, C.
了解地球目前的含水量对于限制地球在形成过程中获得的水和其他挥发物的数量以及从地表循环回内部的数量是必要的。本研究将410 km和660 km的不连续深度与地幔过渡带内的横波层析成像进行比较，以确定地震信号与水一致的区域。410公里和660公里不连续面深度是根据SS- s410s和SS- s660s差分走时数据集（称为SS前兆）确定的。将SS前兆数据进行分组和叠加得到的不连续深度与过渡区剪切速度模型HMSL-S06进行了比较。在绘制所有可能的组合图时，很少有位置符合矿物物理学对水对不连续深度和剪切速度的影响的预测。虽然尚未在实际的过渡带温度和压力下测量，但预测结果是410公里的浅层不连续，660公里的深层不连续，以及缓慢的剪切速度。只有8%具有高质量数据的桶与这些预测一致，这些桶内计算的平均含水量约为0.6 wt.%。少数孤立地点的速度/地形模式与水相一致，而大区域尺度的模式与冷/热温度异常相一致。结合对全球长周期地震数据的分析和目前对过渡带矿物中水的矿物物理预测，我发现地幔过渡带通常是干燥的，包含的水少于一个地球海洋。虽然俯冲带可能是局部水化的，但综合不连续和速度数据显示，没有证据表明，由于俯冲或初始地球条件，瓦德斯莱岩或环伍德岩已经在全球水化。(C) 2016 Elsevier B.V.版权所有
Knowledge of the Earth's present water content is necessary to constrain the amount of water and other volatiles the Earth acquired during its formation and the amount that is cycled back into the interior from the surface. This study compares 410 and 660 km discontinuity depth with shear wave tomography within the mantle transition zone to identify regions with seismic signals consistent with water. The depth of the 410 and 660 km discontinuities is determined from a large updated dataset of SS-S410S and SS-S660S differential travel times, known as SS precursors. The discontinuity depths measured from binning and stacking the SS precursor data are then compared to the shear velocity model HMSL-S06 in the transition zone. Mapping all the possible combinations, very few locations match the predictions from mineral physics for the effects of water on discontinuity depth and shear velocity. The predictions, although not yet measured at actual transition zone temperatures and pressures, are a shallow 410 km discontinuity, a deep 660 km discontinuity, and a slow shear velocity. Only 8% of the bins with high quality data are consistent with these predictions, and the calculated average water content within these bins is around 0.6 wt.%. A few isolated locations have patterns of velocity/topography that are consistent with water, while there are large regional-scale patterns consistent with cold/hot temperature anomalies. Combining this global analysis of long period seismic data and the current mineral physics predictions for water in transition zone minerals, I find that the mantle transition zone is generally dry, containing less than one Earth ocean of water. Although subduction zones could be locally hydrated, the combined discontinuity and velocity data show no evidence that wadsleyite or ringwoodite have been globally hydrated by subduction or initial Earth conditions. (C) 2016 Elsevier B.V. All rights reserved.