Hydrous phases in the Earth’s lower mantle

The origin and survival of life on Earth depend on the availability of water. Plate tectonics connects the Earth’s surface with its deep interior and promotes exchange of H2O between the surface and the Earth’s mantle. The resultant deep H2O cycle has affected the evolution of the oceans and atmosphere on Earth and supplies H2O to the mantle. Rocks of the upper mantle may store H2O as defects in the crystal structures of nominally anhydrous minerals (NAMs). The NAMs of the lower mantle, however, incorporate significantly less H2O. As a consequence, the contribution of the lower mantle to the deep H2O cycle has been discounted as minor. HYDROSPHEAR challenges this paradigm by investigating the storage of H2O in solid hydrous phases (SHPs) and hydrous melts of the lower mantle.Phase D and phase H are SHPs that form at pressures and temperatures of the lower mantle. The stability limits of phase D and phase H in complex chemical systems and in realistic phase assemblages remain largely unconstrained, in particular, at pressures of the deeper lower mantle. HYDROSPHEAR will determine the stability limits of these SHPs in rocks of the lower mantle by performing high-pressure melting experiments.Melting reactions will be sampled in laser-heated diamond anvil cells (DACs). We will detect the formation of a melt phase by Brillouin spectroscopy and, in the same experiment, measure the speed of sound in the hydrous melt. Brillouin spectroscopy will also be used to determine the elastic properties of single crystals of SHPs at high pressures and high temperatures. Complementary high-pressure experiments in DACs and multi-anvil presses will constrain the equations of state and phase relations of SHPs.The experimental results will be integrated into a systematic search for hydrous rocks and hydrous melts in the lower mantle by analysing seismic observations. The emerging picture of H2O storage and transport in the lower mantle will reshape our image of the deep H2O cycle.