Slab dehydration in the Earth's mantle transition zone

Because of the high water solubility in wadsleyite and ringwoodite, the mantle transition zone is possibly a large water reservoir. The potentially high water content of the Earth's mantle transition zone is a key element of several global mantle dynamic models. Nevertheless, to keep the transition zone relatively wet, the tendency for convection to distribute water over the entire mantle has to be offset by other mechanisms. One such mechanism could be linked to the dehydration of the slab. Studies of slab dehydration mainly focus on hydrous minerals phase diagrams that provide the depth of water exsolution. In this study, we investigate the water diffusion process that occurs prior to such exsolution. The diffusion process is controlled by both chemical gradients and thermal gradients via the temperature dependence of water solubility. We have addressed how this diffusion phenomenon influences slab water transport into the transition zone by developing a theoretical model of coupled thermal and chemical diffusion in a flat lying slab stalled in the transition zone.Model solutions demonstrate that even if intrinsic water solubility is decreasing with increasing temperature (which normally would act to impede a chemical diffusion from the slab), the water concentration profile adjusts to reduce the adverse effects of temperature-dependent solubility. The self-adjustment entails the concentration of water reaching a local maximum below the top edge of the stagnant slab; under specific conditions, this concentration maximum can reach the solubility limit and thus exsolve fluid (i.e. create of hydrous melt or aqueous silicate fluid). Consequently, model water flux computations are relatively independent of solubility variation and indicate that much of the water flux introduced by relatively well hydrated slabs is taken up into the transition zone by diffusion.