Dynamics in the Uppermost Lower Mantle: Insights into the Deep Mantle Water Cycle Based on the Numerical Modeling of Subducted Slabs and Global-Scale Mantle Dynamics

Abstract

In this review, we address the current status of numerical modeling of the mantle transition zone and uppermost lower mantle, focusing on the hydration mechanism in these areas. The main points are as follows: ( a) Slab stagnation and penetration may play significant roles in transporting the water in the whole mantle, and ( b) a huge amount of water could be absorbed into the deep mantle to preserve the surface seawater over the geologic timescale. However, for further understanding of water circulation in the deep planetary interior, more mineral physics investigations are required to reveal the mechanism of water absorption in the lower mantle and thermochemical interaction across the core–mantle boundary region, which can provide information on material properties to the geodynamics community. Moreover, future investigations should focus on determining the amount of water in the early planetary interior, as suggested by the planetary formation theory of rocky planets. Moreover, the supplying mechanism of water during planetary formation and its evolution caused by plate tectonics are still essential issues because, in geodynamics modeling, a huge amount of water seems to be required to preserve the surface seawater in the present day and to not be dependent on an initial amount of water in Earth's system. ▪ Slab stagnation and penetration of the hydrous lithosphere are essential for understanding the global-scale material circulation. ▪ Thermal feedback caused by water-dependent viscosity is a main driving mechanism of water absorption in the mantle transition zone and uppermost lower mantle. ▪ The hydrous state in the early rocky planets remains to be determined from cosmo- and geochemistry and planetary formation theory. ▪ Volatile cycles in the deep planetary interior may affect the evolution of the surface environment.