Stability of the high-pressure polymorph of zircon (ZrSiO4) in the deep mantle

The decomposition reaction of ZrSiO4 (scheelite structure) to ZrO2 (cotunnite structure) and SiO2 (stishovite) was studied at temperatures between 1500 and 1800 °C in the pressure range 15–25 GPa using a Kawai-type multi-anvil apparatus. In order to ensure the validity of the experimental results, two types of starting materials (synthetic ZrSiO4 and mixture of ZrO2 and SiO2) were set in the same furnace assembly and the results were compared in all the experimental runs. Reversed experimental results were obtained for the decomposition reaction. It was revealed that the break down would occur at 20.5 GPa and 1500 °C (equivalent to 600 km deep in the Earth) and the boundary has a positive Clapeyron slope extending to 23.0 GPa at 1800 °C. Our experiments indicate that zircon (ZrSiO4) has limited stability only within the upper mantle.