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 col
lision, such rocks can be subducted to > 200 km and returned to the
surface. De
lineation
of the depth to which continental materials can be subducted is important for understanding continental col
lision
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 (SiO
2 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 estab
lish 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.