Water Distribution In The Upper Mantle And Mantle Transition Zone:Constraints From Seismic Tomography Models And Mineral Physics Modeling

Water has important effects on the physical and chemical properties of the mantle.Studying the water content and distribution in the mantle can give us a better understanding of the mantle convection pattern in the Earth’s interior,and develop our understanding of the evolution history of the Earth.Although the hypothesis of a hydrous mantle has been verified by discovering hydrous ringwoodite inclusions and ice-VII inclusions in natural diamonds,the specific water content and water distribution in the mantle are less constrained,and results from different studies are often contradictory.In this study,we obtain the water distribution in the upper mantle by combining seismic radial anisotropy tomography and mineral physics modeling based on shear strain experimental results of lattice preferred orientation of olivine;we obtain the water distribution in the mantle transition zone(MTZ)by combining seismic isotropic velocity tomography and mineral physics modeling based on high pressures and temperatures experimental results of hydrous olivine polymorphs.In the upper mantle,most hydrous regions are close to subduction zones,where the shallow upper mantle is more hydrous than the deep upper mantle,which indicates a significant degree of dehydration occurs above 200 km depth.Moreover,in the deep upper mantle,most subducted slabs have higher water contents than the surrounding mantle,implying water can be transported below 200 km depth by slabs.Old subducted slabs are more hydrous in the deep upper mantle than young slabs.A pyrolite model interprets the observations of seismic anisotropy in the upper mantle near subduction zones with 0.075-0.15 wt.%water which is smaller than previously thought.We find that the MTZ is the main water reservoir of the Earth.The total water content of the MTZ is about 0.64-1 seawater.The shallow MTZ(410 km-520 km)and the deep MTZ(520 km-660 km)contain 0.3-0.5 wt.%and 0.15-0.2 wt.%water,respectively,implying water contents of the MTZ decrease with increasing depth.The most hydrous regions are mainly located near subduction zones,where the shallow MTZ and the deep MTZ can contain water up to 0.5-1 wt.%and 0.2-0.5 wt.%,respectively,which indicates water can be transported into the MTZ by slabs.In addition,old subducted slabs in the western Pacific subduction zone are more hydrous than young subducted slabs in the eastern Pacific subduction zone.Based on the inversion results of water distribution,we propose the water circulation model in Fig.3.13:The upper mantle beneath continents contains water less than 0.02 wt.%,and hydrous regions are near subduction zones,where slabs transported 0.075-0.15 wt.%water into the MTZ.Because plate tectonics began in the early Paleoproterozoic(2.2 Ga),the subduction of oceanic plates may have been going on for hundreds of millions of years.Deep mantle melting redistributed the water moving out of the MTZ,and made it return to the MTZ;hence,the MTZ have accumulated 0.15-0.5 wt.%water during the whole plate tectonic history,and can always maintain high water contents.Since the current subduction system has existed for 100 million years,the circum-Pacific subduction zone has higher water contents than the surrounding mantle,and 0.5-1 wt.%and 0.2-0.5 wt.%water have been accumulated in the shallow MTZ and the deep MTZ near subduction zones,respectively.