In situ X-ray diffraction study of the effect of water on the garnet-perovskite transformation in MORB and implications for the penetration of oceanic crust into the lower mantle

The phase boundary of the transformation of garnet to perovskite in hydrous mid-ocean ridge basalt (MORB) was determined by in situ high-pressure and high-temperature X-ray diffraction (XRD) experiments. The garnet-bearing assemblage was formed in experiments at pressures below 21 GPa. In contrast, an assemblage of Mg-perovskite, Ca-perovskite, stishovite, CF phase and/or NAL phase is stable at pressures above 21 GPa. Garnet- and perovskite-bearing assemblages were synthesized at a pressure range of 20-25 GPa and a temperature range of 1000-1600 °C. On the basis of our results, we have located the garnet-perovskite phase boundary and find that it can be described by the following equation: P (GPa) = 0.004T (°C) + 17.5, which positions it 2 GPa lower under dry conditions. If the top 7 km of the slab is colder than 1200 °C in the transition zone, perovskite will appear at a depth shallower than that where ringwoodite decomposes. This result indicates that the MORB layer becomes denser than the peridotite portion of the slab throughout the transition zone and the lower mantle, and thus no density crossover would exist between basalt and underlying peridotite along the cold subducting slab geotherm.