Effect of water on garnet-perovskite phase transformation in MORB system

Garnet lithology in Mid-Ocean Ridge basaltic (MORB) component of the subducting slab transforms to perovskite lithology at the top of the lower mantle whereas peridotite component has transformed to perovskite at shallower depths. This difference causes density crossover between MORB layer and peridotite layer in the slabs and induces the buoyancy force. Although some studies have been made to determine the phase boundary of the garnet-perovskite transformation in dry-MORB system, little is known about an influence of water. We performed in-situ X-ray diffraction experiments to examine the effect of water on the garnet-perovskite transformation in a hydrous MORB composition. High pressure and high temperature in-situ X-ray diffraction experiments were performed using a uniaxial 1500-ton press (SPEED1500) and 800-ton press (MAX III) installed in the synchrotron radiation beamline BL04B1 at SPring-8 and BL14C at the Photon Factory (PF) at the National Laboratory for High Energy Physics (KEK), respectively. The crystallization of CMA (Ca-Mg-Al) - perovskite from glass starting material was observed at lower temperatures (1273-1373K) during heating. This observation is in agreement with the previous diamond-anvil cell experiments. Then, it decomposed into orthorhombic Mg-Al-perovskite, Ca-perovskite and aluminous phase at 21.5-23.1GPa and 1473-1773K. The phase transformation of garnet lithology into perovskite-bearing assemblage in hydrous conditions is about 2GPa lower than that of the dry condition even if the discrepancy among pressure scales are taken into account. These results suggest that the conditions where the buoyancy occurs are restricted within a narrow depth region by the presence of water. Water can play an important role in dynamics of subducting slab.