Abstract
Subduction zone plays a significant role in deep carbon cycle. Solid-state reaction “dolomite (ankerite)=magnesite (siderite)+aragonite” has been first identified in two carbonated eclogites from southwestern Tianshan orogenic belt, China, which is the largest known ultra-high-pressure (UHP) oceanic subduction zone so far. In comparison to some other carbonates from high-pressure metamorphic rocks, carbonates from southwestern Tianshan carbonated eclogites and metapelites have high Fe content. In order to understand the effect of Fe on the stability of dolomite at high pressure, several experiments have been carried out using multi-anvil apparatus at pressures up to 8GPa and temperature between 600 and 1200°C. Both petrological observation and experimental study indicate that the stability of dolomite dramatically deceases with increasing Fe content in the solid solution of dolomite and ankerite at high pressure, and the peak pressure and temperature for the studied eclogites from southwestern Tianshan, China, are 2.5GPa and 550°C. The P–T conditions estimated according to dolomite decomposition in previous petrologic works should be revised by taking into account of the effect of Fe on the stability of dolomite at high pressure. More attention should also be paid to the effect of Fe on the stability of Fe-bearing carbonate during the decarbonation of the deep carbon cycle in the subduction zone.