Gravitationally Unstable Hydrous Melts at the Base of the Upper Mantle

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Fei, Hongzhan ; Chen, Juan ; Wang, Fei ; Zhang, Baohua ; Xia, Qunke ; Katsura, Tomoo:
Gravitationally Unstable Hydrous Melts at the Base of the Upper Mantle.
In: Journal of Geophysical Research: Solid Earth. Bd. 130 (2025) Heft 4 . - e2024JB030737.
ISSN 2169-9356
DOI: https://doi.org/10.1029/2024JB030737

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Abstract

The water-rich mantle transition zone in contrast to the low water storage capacity of the upper mantle suggests the presence of hydrous melts near the 410-km discontinuity. However, the gravitational stability of the melts is under debate. Since melt density is strongly correlated to its H2O content, we systematically determined the pressure, temperature, and compositional dependences of H2O contents in hydrous melts by high-pressure experiments combined with mass balance calculations. Subsequently, we estimated the density of hydrous melts at 410-km depth based on the H2O content and equation of state of H2O at high pressure. The hydrous melts are found to be buoyant near the 410-km discontinuity. Therefore, as far as melts are formed, they may migrate upward, resulting in the water circulation maintained by slab subduction. The upwelling melts may hydrate the upper mantle minerals continuously, leading to a low seismic velocity, high electrical conductivity, and water-saturated deep upper mantle.