'Forbidden zone' subduction of sediments to 150 km depth - the reaction of dolomite to magnesite plus aragonite in the UHPM metapelites from western Tianshan, China

The solid-state reaction magnesite (MgCO3) + calcite (aragonite) (CaCO3) = dolomite (CaMg(CO3)(2)) has been identified in metapelites from western Tianshan, China. Petrological studies show that two metamorphic stages are recorded in the metapelites: (1) the peak mineral assemblage of magnesite and calcite pseudomorphs after aragonite which is only preserved as inclusions within dolomite; and (2) the retrograde glaucophane-chloritoid facies mineral assemblage of glaucophane, chloritoid, dolomite, garnet, paragonite, chlorite and quartz. The peak metamorphic temperatures and pressures are calculated to be 560-600 degreesC, 4.95-5.07 GPa based on the calcite-dolomite geothermometer and the equilibrium calculation of the reaction dolomite = magnesite + aragonite, respectively. These give direct evidence in UHP metamorphic rocks from Tianshan, China, that carbonate sediments were subducted to greater than 150 km depth. This UHP metamorphism represents a geotherm lower than any previously estimated for subduction metamorphism (<3.7 degrees C km(-1)) and is within what was previously considered a 'forbidden' condition within Earth. In terms of the carbon cycle, this demonstrates that carbonate sediments can be subducted to at least 150 km depth without releasing significant CO2 to the overlying mantle wedge.