The seismic discontinuity around 520 km is believed to be caused by the phase transition from wadsleyite to ringwoodite, the dominant minerals in the mantle transition zone (MTZ). Both wadsleyite and ringwoodite can contain more than 1.0 wt% water at MTZ’s conditions, but it is not well known how water affects the wadsleyite-ringwoodite transition. Here we investigated water partitioning between wadsleyite and ringwoodite and the water effect on this phase boundary using first-principles calculations. Our results show that the presence of water will shift the phase boundary to higher pressures, and the width of the two-phase coexistence domain in the Mg2SiO4-H2O system is insignificant at mid-MTZ conditions. For the (Mg0.9Fe0.1)2SiO4 system, the incorporation of 1.0 wt% water can narrow the effective width of two-phase coexistence by two-thirds. Together with elastic data, we find that velocity and impedance contrasts are only mildly changed by the water partitioning. We suggest that compared to the anhydrous condition, the presence of 1.0 wt% water will increase velocity gradients across the wadsleyite-ringwoodite transition by threefold, enhancing the detectability of the 520 km discontinuity.

中文翻译：

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wadsleyite 和 ringwoodite 中水的第一性原理研究：对 520 km 不连续性的影响

大约 520 km 的地震不连续性被认为是由地幔过渡带 (MTZ) 中的主要矿物从瓦德利石到菱镁矿的相变引起的。在 MTZ 的条件下，硅钙石和林伍德石都可以含有超过 1.0 wt% 的水，但水如何影响硅钙石-林伍德石的转变尚不清楚。在这里，我们使用第一性原理计算研究了wadsleyite 和ringwoodite 之间的水分配以及水对该相界的影响。我们的结果表明，水的存在将使相界向更高的压力移动，并且在 Mg2SiO4-H2O 系统中的两相共存域的宽度在中 MTZ 条件下是微不足道的。对于(Mg0.9Fe0.1)2SiO4体系，加入1.0 wt%的水可以使两相共存的有效宽度缩小三分之二。结合弹性数据，我们发现速度和阻抗对比仅因水的分配而发生轻微变化。我们建议，与无水条件相比，1.0 wt% 的水的存在将增加 3 倍穿过 wadsleyite-ringwoodite 过渡的速度梯度，从而提高 520 km 不连续性的可探测性。