The fate of subducted Upper Continental Crust: An experimental study

The occurrence of ultra-high pressure metamorphic (UHPM) index minerals within rocks from continental collision zones indicates that continental rocks may descend into the Earth's upper mantle and leave specific geochemical signatures during the continental collision. It has long been suggested that continental crust is buoyant than that of the mantle rocks, and therefore might be gravitationally trapped during subduction. In order to understand the fate and exhumation of subducted continental crust, we conducted experimental studies up to 24 GPa and 1800 °C on natural UHPM gneisses in bulk compositions corresponding to average upper continental crust and terrigenous rocks, using piston-cylinder and multi-anvil apparatus. We found that the upper continental crust is no longer buoyant with respect to the surrounding mantle when it has been transported to a depth of 250 km (~ 8-9 GPa), which defines the “depth of no return” for the subducted upper continental crust. This in turn is consistent with the depth limit for most exposed UHPM rocks. Our experimental results suggest that the transformed subducted upper continental crust with a jadeite stishovitite lithology has the potential to sink into the transition zone. However, this density relationship is reversed at the base of the transition zone, leaving subducted upper continental crust being trapped at the bottom of the upper mantle, due to the presences of perovskite and magnesiowustite in the lower mantle.