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.