PHASE-TRANSFORMATIONS IN SUBDUCTED OCEANIC-CRUST AND BUOYANCY RELATIONSHIPS AT DEPTHS OF 600-800 KM IN THE MANTLE

Phase transformations in a representative oceanic crust composition (MORB) have been studied at pressures up to 28 GPa, at 1200-degrees-C and 1500-degrees-C using a multi-anvil apparatus. The assemblage of majorite + CaSiO3-rich perovskite + stishovite which is displayed by the MORB composition at pressures above 20 GPa was found to transform further into an assemblage containing a new aluminous phase at pressures above 25 GPa. This Al-phase has a complex composition and is also relatively sodium rich. Its X-ray diffraction pattern is similar to that observed for MgAl2O4 (calcium ferrite type), and is tentatively indexed on the basis of an orthorhombic cell, yielding a zero-pressure density of 3.87(2) g/cm3. Majorite garnet was found to remain as the most abundant phase at the highest pressure studied (28 GPa) and does not appear likely to transform to perovskite until pressures exceed 29-30 GPa. These results imply that the basaltic crust component of subducted oceanic lithosphere would be 0.1-0.2 g/cm3 less dense than surrounding mantle pyrolite between the depths of 660-800 km, thereby confirming an earlier hypothesis of Anderson. Moreover, the underlying layer of harzburgite would be approximately 0.05 g/cm3 less dense than pyrolite throughout most of this depth interval. These buoyancy relationships contribute to a barrier which impedes and may sometimes prevent the entry of slabs into the lower mantle.