Muir, JMR; Zhang, F; Brodholt, JP; (2021) The effect of water on the post-spinel transition and evidence for extreme water contents at the bottom of the transition zone. Earth and Planetary Science Letters , 565 , Article ARTN 116. 10.1016/j.epsl.2021.116909.

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Abstract

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 ( 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.

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