Influence of Water on Major Phase Transitions in the Earth's Mantle
Influence of Water on Major Phase Transitions in the Earth's Mantle
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DOI：
10.1029/168gm08
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发表时间：
2013-03
期刊：
影响因子：
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通讯作者：
K. Litasov;E. Ohtani;A. Sano
中科院分区：
文献类型：
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作者：
K. Litasov;E. Ohtani;A. Sano
在这一章中，我们总结了水对地幔主要相变的影响的最新结果，以及对地震不连续结构和地幔动力学的启示。实验数据基于淬火多面顶锤和原位X射线衍射研究。橄榄石和水滑石之间以及环木石和镁钙钛矿+铁方镁石之间的水溶解度差异可能会使相变界面移位，这分别是410公里和660公里不连续的原因。结果表明，水使水滑石的稳定区向低气压方向扩展，这与地幔某些区域410公里长的不连续面的展宽相一致。水引起的瓦兹利石-环木岩相变向更高压力的显著转变也可能是深度变化或没有520公里不连续的原因。对含水热解石的后尖晶石相变的研究表明，相界也向高压方向移动。在1473K时，∼为2wt.%H2O的这条边界的位移相当于约15公里，因此，在该温度下俯冲带660公里的不连续处，水的存在可能占到观测到的30-40公里凹陷的一半。对无水和有水MORB中石榴石后相变的研究表明，加入2-5wt%的水后，石榴石相界面向低压方向移动了∼2 Gpa。这一观察表明，在含水条件下，在660公里附近的橄榄岩和玄武岩成分之间可能没有密度交叉，从而抑制了在660公里不连续面上这些成分的分离。
In this chapter we summarize recent results on the influence of water on major phase transformations in the Earth's mantle with implications for seismic discontinuity structure and mantle dynamics. The experimental data are based on quench multianvil and in situ X-ray diffraction studies. Differences in water solubility between olivine and wadsleyite, and between ringwoodite and Mg-perovskite + ferropericlase may displace the phase transition boundaries, which are responsible for the 410- and 660-km discontinuities, respectively. The results show that water expands the stability field of wadsleyite to lower pressures, which is consistent with broadening of the 410-km discontinuity in some regions of the mantle. A significant shift of the wadsleyite-ringwoodite phase transition to higher pressure caused by water may also be responsible for depth variations or absence of the 520-km discontinuity. Study of the post-spinel transformation in hydrous pyrolite indicates that the phase boundary also shifts to higher pressures. Displacement of this boundary with ∼2 wt.% H 2 O corresponds to about 15 km at 1473 K. Thus, presence of water could account for half of the observed 30-40 km depressions at the 660-km discontinuity in subduction zones at this temperature. Study of the post-garnet transformation in anhydrous and hydrous MORB show that this phase boundary shifts to the lower pressures by ∼2 GPa with the addition of 2-5 wt.% water. This observation demonstrates that the density crossover between peridotite and basaltic components near 660 km might be absent under hydrous conditions, inhibiting the separation of these components at the 660-km discontinuity.