Evidence of former stishovite in metamorphosed sediments, implying subduction to > 350 km

Deep subduction of continental rocks or sediments has until recently been considered impossible because they are buoyant relative to mantle compositions. Nevertheless, it has been shown over the last 20 yr that during continental collision, such rocks can be subducted to > 200 km and returned to the surface. Delineation of the depth to which continental materials can be subducted is important for understanding continental collision processes and interpretation of the geochemical signals in mantle magmas. Here we report oriented aluminum- and iron-bearing oxide inclusions in quartz of “ultra-high-pressure” rocks from the Altyn Tagh, western China, already known to have seen depths of more than 200 km. Laboratory experiments at high pressure show that inclusion abundances are not consistent with precipitation from quartz or coesite but are consistent with precipitation from stishovite (SiO2 polymorph stable only above ∼ 9 GPa pressure). Further, geometrical relationships between the oriented inclusions demonstrate that the original host mineral must have had tetragonal symmetry and lattice spacings consistent with those of stishovite. The sum of these data establish that sediments can be subducted to at least 350 km and returned to the surface, a depth close to the “point of no return” where continental materials would have density comparable to the mantle and would “fall” to at least the base of the mantle transition zone where they could contribute to the continental trace element “signal” in ocean island basalts.