Survival of pyropic garnet in subducting plates

An experimental study has been conducted to clarify the conditions and kinetic aspects of the pyroxene–garnet transformation by using synthetic polycrystals (two pyroxenes + garnet) in the CaO–MgO–Al2O3–SiO2 system. The run products recovered from the in situ X-ray diffraction experiments in a double-stage multi-anvil system at 15.5–20.4 GPa and 800–1600 °C were carefully analyzed by scanning electron microscopy. Our results show that the pyroxene–garnet transformation proceeds by overgrowth of garnet absorbing the surrounding pyroxene components above 1550 °C. At 18.4 GPa and 1320 °C, pyroxenes (enstatite + diopside) directly decomposed to the assemblages (wadsleyite + stishovite + Ca-perovskite) without dissolving in garnet. The pyroxene–garnet transformation requires much higher temperatures than those needed for transformation of olivine to wadsleyite (∼1000 °C) and of pyroxenes to their high-pressure phases (∼1300 °C) in laboratory time (∼103 s) and grain (∼10−5 m) scales. Therefore, the pyroxene–garnet transformation is likely to be kinetically inhibited at low temperatures in the subducting plate, and the pyropic garnet could survive in the transition zone instead of equilibrium majoritic garnet.