On hydrous reservoirs in the deep mantle in global-scale mantle convection simulations with water migration

Abstract:

Two possible hydrous reservoirs in the deep mantle have been suggested from mineral physics and/or geochemical experiments, which are the mantle transition zone and the core-mantle boundary (CMB) region [Bercovici and Karato, 2003; Hirshmann, 2006; Nishi et al., 2014; Ohira et al, 2014; Pamato et al., 2015]. To understand and quantify how these reservoirs might form and be preserved, however, requires global-scale mantle dynamics models that include some form of water migration process. Recently, Nakagawa et al. [2015] has provided the first global-scale models that include a simplified water cycle in numerical mantle convection simulations. These calculations suggest that dehydration-hydration processes may play a crucial role. However, that study did not include important processes related to partial melting, such as degassing-regassing and partitioning of water into the molten material. Here we present new numerical models that incorporate dehydration-hydration, degassing-regassing and water partitioning effects. A parameter study of those effects suggests that the dehydration-hydration process may still be the most crucial for developing two hydrous reservoirs in the deep mantle simultaneously. In addition, when dehydration-hydration is included, the residual water content in solid mantle is obtained as ~ 1 ocean mass. This result is consistent with experimental constraints [e.g. Hirshmann, 2006]. Without dehydration, the mean water content of the mantle is an order of magnitude higher in the models. Whether the mantle transition zone provides a significant reservoir for water, depends on the rheological properties of hydrous minerals as they a strongly control temperature in the mantle [Nakagawa et al., 2015]. However, hydrous reservoirs in the CMB region can be found, which are not dependent on the specific water migration mechanism.