Water and partial melting in the Earth's mantle

With the investigation of water in natural samples, experiments on the water solubility in mantle minerals, and geophysical surveys on the deep mantle structure since the 1990s, it has been gradually known that the mass of water preserved in the deep mantle in the form of point defects in crystal lattice could be comparable with that of the present ocean. Besides its remarkable impacts on the physical properties of minerals such as thermal and electrical conductivity, viscosity, and elemental diffusivity, water could also affect the production of mafic magmatism from different geological backgrounds. Water plays a role as a strong incompatible element during mantle melting, and its preferential partitioning into the melts than the residue solid can largely affect the structure of the melts and consequently impact the melt-solid equilibrium. In principle, for a mantle under specific pressure and temperature, water can lower the solidus temperature and increase the partial melting degree. In contrast, for an adiabatically decompressing mantle, water can deepen the initial melting depth and decrease the average partial melting degree. During the past several years, more and more evidence supports that water accounts for not only the low degrees of partial melting at the lithosphere-asthenosphere boundary, but also the high degrees of partial melting required for the formation of large igneous provinces. In this work, the new progresses on the water contents of mantle-derived magmas and their effects on partial melting in different tectonic settings (e. g. mid-ocean ridge, island arc, mantle transition zone, large igneous provinces, continental intraplate, oceanic island) were briefly introduced. We also point out the limitations of the present works and the challenges in the future