Stability of Water‐Bearing Aluminous Silica Phases in Subducted Oceanic Crust: Implications for Water Transport to the Core‐Mantle Boundary

Abstract Geochemical evidence indicates the presence of H 2 O sources in the deep mantle that feed plume‐related magmatism. Understanding water storage and phase changes in multicomponent systems relevant to subducted oceanic crust would provide important constraints on the deep‐Earth water cycle. Here we conducted experiments in the MgO‐Al 2 O 3 ‐Fe 2 O 3 ‐SiO 2 ‐H 2 O systems over pressure‐temperature ( P ‐ T ) range of 94–134 GPa and 2,100–2,500 K. In our experiments, we found that NiAs‐type silica (Nt) phase is stable in H 2 O‐bearing basaltic composition along normal geotherm conditions of the deep lower mantle. We conducted additional experiments in an alumina‐poor H 2 O‐bearing system and obtained CaCl 2 ‐type silica (Ct) instead under P‐T conditions of the Nt phase stability field, indicating a phase boundary between Nt and Ct. The high alumina content in basaltic composition coupled with H 2 O is needed to stabilize the NiAs‐type structure of silica. Coexistence of aluminous silica and pyrite‐type FeOOH was observed under P‐T conditions corresponding to >2,300 km depth. Our results indicate that the water storage phases in the deep lower mantle are dependent on the bulk composition, H 2 O concentration and P‐T conditions. The presence of H 2 O greatly alters the mineralogy in the deep lower mantle and might play an important role in mantle dynamics.