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.