In Situ Observation of Pressure‐Enhanced Hydrous–Anhydrous Solid Solution in Lower‐Mantle SiO 2 –AlOOH Phases

Abstract Water plays a critical role in Earth's mantle dynamics and evolution, yet lower‐mantle storage mechanisms remain debated. δ‐AlOOH and SiO 2 phases have been independently proposed as high‐pressure water hosts. Here, we conducted in situ synchrotron X‐ray diffraction experiments with a multi‐anvil apparatus and sintered diamond anvils to investigate phase relations in the SiO 2 –AlOOH system up to 45 GPa and high temperatures. We found that SiO 2 and δ‐AlOOH form extensive, stable CaCl 2 ‐type solid solutions across a broad compositional range under lower‐mantle conditions. The solid‐solution extent increases strongly with pressure, indicating enhanced H 2 O storage in dense SiO 2 ‐rich phases and stabilization of SiO 2 ‐enriched AlOOH‐rich hydrous phases at greater depths. These results suggest that water released from less favorable peridotitic hosts during lower‐mantle transport can be redistributed into subducted crustal lithologies, where pressure‐stabilized complementary CaCl 2 ‐type phases serve as water hosts.