The transition of ringwoodite to bridgmanite and periclase (the post-spinel transition) is a strong control on the 660 phase discontinuity and the boundary between the transition zone and the lower mantle. The transition zone may contain significant amounts of water and thus the effect of water on the post-spinel transition must be known to correctly determine its properties. In this paper we examine the transition of ringwoodite to bridgmanite and periclase in both dry and wet conditions using density functional theory (DFT). In the dry case we calculate a high negative Clapeyron slope ($-3.19\pm 0.19$ MPa/K at 1873 K). We also find that the Clapeyron slope is significantly nonlinear with temperature and much lower at 1000 K (−1.31 MPa/K) or if determined by linear interpolation from 1000 K (−2.37 MPa/K). The addition of water causes a large broadening of the transition through the development of a phase loop. Seismic studies suggest that the 660 km discontinuity is narrower than 2 km. For this to be the case our results suggest that the water content at the bottom of the transition zone needs to be either less than ∼700 ppm or, alternatively, above ∼8000 ppm (assuming an effective transition width near the maximum transition width). In the latter case this is above the saturation limit for bridgmanite and so will be accompanied by the production of a free water phase/hydrous melt. The hydration of ringwoodite also causes the onset of the transition to deepen with 1 wt% water increasing the depth of the transition by about 8 km. This is relatively small compared to seismically observed variations in the 660 km discontinuity of around 35 km and so water alone cannot account for the observed 660 km discontinuity topography. Water causes no substantial changes to the Clapeyron slope of the transition, so the 660 km topography could be explained by thermal variations of ∼500 K.

从660的相间断面以及过渡带与下地幔之间的边界来看，林伍德石向Bridgmanite和镁长石的过渡（尖峰后过渡）是一个很强的控制力。过渡区可能包含大量的水，因此必须知道水对尖晶石后过渡的影响，才能正确确定其特性。在本文中，我们使用密度泛函理论（DFT）研究了在干燥和潮湿条件下，林伍德石向布里奇锰矿和钙镁石的过渡。在干燥的情况下，我们计算出较高的Clapeyron负斜率（$-3.19\pm 0.19$1873 K时的MPa / K）。我们还发现，Clapeyron斜率随温度明显呈非线性，并且在1000 K（−1.31 MPa / K）或通过1000 K（−2.37 MPa / K）的线性插值确定时，斜率要低得多。水的添加通过形成相环而导致过渡的大幅度拓宽。地震研究表明，660 km的不连续面比2 km的要窄。对于这种情况，我们的结果表明，过渡区底部的水含量应小于约700 ppm或大于约8000 ppm（假设有效过渡宽度接近最大过渡宽度）。在后一种情况下，这超过了桥锰矿的饱和极限，因此将伴随有游离水相/含水熔体的产生。林木的水合作用还导致过渡的开始加深1％（重量）的水，过渡深度增加了约8 km。与地震观测到的约35 km的660 km的不连续性相比，这相对较小，因此仅靠水就无法解释所观测到的660 km的不连续性地形。水对过渡的克拉珀龙斜率没有实质性影响，因此660 km的地形可以用约500 K的热变化来解释。