[S-IT20] Deep Earth Sciences

[SIT20-15] Water Incorporation in the Mantle Transition Zone During Magma Ocean Crystallization

*Longjian Xie^1,2,3,4, Michael Walter², Tomoo Katsura³, Fang Xu⁵, Jianhua Wang², Yingwei Fei² (1.Center for High Pressure Science & Technology Advanced Research, Shanghai, 201203, P.R. China, 2.Earth & Planets Laboratory, Carnegie Science, Washington DC 20015, USA, 3.Bayerisches Geoinstitut, University of Bayreuth, Bayreuth, 95440, Germany., 4.Department of Earth Sciences, University College London, London WC1E6BS, UK., 5.School of Earth Sciences, Zhejiang University, Hangzhou 310058, China)

Keywords:water, mantle transition zone, liquidus phase relations, magma ocean

The mantle transition zone (MTZ) is known for its high water storage capacity and has been proposed to be hydrous (Ohtani, 2021). While slab dehydration is a well-studied mechanism for hydrating the MTZ, with water originating from the surface ocean (Kuritani et al., 2011; Richard et al., 2006), primary water in the deep mantle (Hallis et al., 2015) suggests an alternative source. Residual melt from a deep magma ocean is a potential candidate for this primary water. To investigate the evolution of hydrous magma ocean melts at the boundary between the MTZ and the top of the lower mantle, we determined liquidus phase relations in the MgO-FeO-CaO-Al₂O₃-SiO₂-H₂O system at 24 GPa. Our results show that the bridgmanite (brg) + stishovite (st) + melt and bridgmanite (brg) + ferropericlase (fp) + melt boundary curves shift toward Mg-rich melt compositions with decreasing temperature and extend to extremely high H₂O contents (~80 mol% H₂O). Density calculations reveal that melts along these boundary curves are neutrally buoyant relative to upper lower mantle and MTZ minerals at H₂O contents up to ~25 mol%. During late-stage crystallization of a mushy magma ocean, a transient melt-rich layer can form at the top of the lower mantle when melt percolation dominates. As crystallization of this layer exceeds ~98%, hydrous melts (>25 mol% H₂O) become buoyant and migrate upward. This process provides a mechanism for hydrating the MTZ from a primary water source, offering new insights into the deep Earth's water cycle.

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