New MgSiO4H2 Phases as Potential Primary Water Carriers into the Deep Earth

Dense hydrous magnesium silicate ${\mathrm{MgSiO}}_{4}{\mathrm{H}}_{2}$ is widely regarded as a primary water carrier into the deep Earth. However, the stability fields of ${\mathrm{MgSiO}}_{4}{\mathrm{H}}_{2}$ based on the prevailing structure model are narrower than experimental results at relevant pressure and temperature (P-T) conditions, casting doubts about this prominent mineral as a water carrier into the great depths of the Earth. Here, we report on an advanced structure search that identifies two new crystal structures, denoted as $\ensuremath{\alpha}$- and $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{MgSiO}}_{4}{\mathrm{H}}_{2}$, that are stable over unprecedentedly wide P-T conditions of 17--68 GPa and up to 1860 K, covering the entire experimentally determined range. Moreover, we performed x-ray diffraction measurements with backscattering electron image, combined with ab initio simulations, to demonstrate the formation of ${\mathrm{MgSiO}}_{4}{\mathrm{H}}_{2}$ and AlOOH solid solutions that exhibit further enhanced P-T stability fields, making them robust carriers of water into the deepest lower mantle. These findings establish and elucidate the new ${\mathrm{MgSiO}}_{4}{\mathrm{H}}_{2}$ phases as potential primary water carriers into the vast depths of the lower mantle, creating a distinct paradigm for the deep Earth water cycle.