Geophysical evidence of a nearly dry bridgmanite in the Earth's lower mantle

Water in Earth's mantle plays a crucial role in shaping geological and geochemical processes. While previous electrical conductivity measurements on major mantle minerals have constrained water content in the Earth's mantle down to the mantle transition zone, the water content of the lower mantle remains experimentally unconstrained. This study measures the electrical conductivity of hydrous (Fe,Al)-bearing bridgmanite, synthesized with Fe and Al compositions representative of peridotitic lithology, at pressures up to 47 GPa and temperatures up to 2210 K. The results demonstrate that proton conduction is the predominant charge transport mechanism in hydrous (Fe,Al)-bearing bridgmanite under the investigated pressure-temperature conditions. Water incorporation markedly enhances the electrical conductivity of bridgmanite, increasing it by approximately an order of magnitude relative to that of dry samples. Extrapolation of the experimental data suggests that bridgmanite in the lower mantle contains less than 40-70 wt. ppm H₂O near the top and at mid-mantle depths. These findings offer insight into potential hidden water reservoirs in Earth's lower mantle.