Formation of an Al-Rich Niccolite-Type Silica in Subducted Oceanic Crust: Implications for Water Transport to the Deep Lower Mantle

Subducted oceanic crust is enriched in free silica. Although being one of the silica polymorphs at lower-mantle pressures, niccolite-type phase (Nt-phase) has not been documented in multicomponent metabasaltic or metasediment compositions relevant to subducting oceanic crust. Here, we report the formation of an Al-rich Nt-phase (similar to 24.4 to 32.4 wt% Al2O3), coexisting with Al-depleted bridgmanite (similar to 6.4 to 7.6 wt% Al2O3), delta-phase, and iron-rich phase in model hydrated basalts over the pressure-temperature range of 84-113 GPa and 1,800-2,200 K. Infrared spectroscopy of a pure synthetic Al-rich Nt-phase shows OH bending and stretching vibrations at high pressures, indicative of its hydrous nature. This study suggests that Al-rich Nt-phase can serve as a potential water carrier in subducted oceanic crust to the deep lower mantle