Water solubility in the dominant lower-mantle bridgmanite phase remains controversial. Discrepancies between previous results highlight the importance of the growth high-quality single crystals of bridgmanite under high-pressure and high-temperature conditions corresponding to the top of the lower mantle. Here we synthesized high-quality single crystals of aluminous bridgmanite up to 300 μm in size that were saturated with hydrous melt at 24–26 GPa and 1700–1900 K using both stoichiometric and MgO-rich non-stoichiometric hydrous starting materials in a multi-anvil press. Fourier-transform infrared spectroscopy measurements on clear and pure spots of the single-crystal bridgmanites did not detect any pronounced OH-stretching bands, which were prominent in some earlier studies. The present results support that the lower-mantle dominated bridgmanite is nearly dry, at least at the top of the lower mantle, and that Al3+ and Fe3+ cannot enhance water incorporation into the crystal structure even in the presence of oxygen vacancies. Large partition coefficients of water between transition-zone minerals and dry lower-mantle dominated bridgmanite further support dehydration melting at the top of the lower mantle. We suggest that the majority of the top of a pyrolitic lower mantle is nearly dry based on the dry principal minerals and stability of hydrous phases in this region.
Water solubilityin bridgmanite (Brg), garnet (Gar), stishovite (Sti), and peri-clase (Per)/ferropericlase (Fp) synthesized from various hydrous compositions from our study and earlier studies under the pressure and temperature conditions of the top of the lower mantle.
Liu, Z., Fei, H., Chen, L., McCammon, C., Wang, L., Liu, R., Wang, F., Liu, B., Katsura, T., Bridgmanite is nearly dry at the top of the lower mantle, Earth Planet. Sci. Lett., 570, 117088, 2021.