Transient water storage in the mantle transition zone governed by subduction and water-induced buoyancy

The nominally unhydrous wadsleyite and ringwoodite present in the mantle transition zone (MTZ), can contain up to 1–2 wt% of water, which creates large potential water storage capacity of this upper mantle zone. However, whether these water reservoirs in the MTZ can be eventually filled remains debatable. We developed new empirical model of deep hydrous mantle melting and performed systematic investigation of water dynamics in the MTZ by using new 2D thermo-hydro-mechanical-chemical (THMC) upper mantle models. Our results suggest that relatively cold solid-state mantle upwellings can start from thermally relaxed hydrated stagnant subducted slabs present at the bottom of the MTZ. These water-bearing plumes rise to and interact with the wadsleyite-olivine phase transition. Depending on the water content and temperature of these thermal-chemical plumes, they may trigger hydrous melting by water release from the wadsleyite upon its conversion to olivine. The hydrous melts are less dense than the solid matrix and rise upward in the form of either melt diapirs or porosity waives. Similar dehydration-induced melting process is also documented for subducting slabs crossing the lower MTZ boundary, where they can generate buoyant melt diapirs rising through the MTZ. Based on the investigated water dynamics, we propose that relatively small amounts of water (<0.1 wt%, <0.2 ocean masses) and a geologically moderate duration (<500 Myr) of the transient water residence should be characteristic for the MTZ. These findings also have implications for the long-term stability of the surface ocean mass on Earth and Earth-like rocky exoplanets due to rather small dynamic water storage in the MTZ.