MINERAL CHEMISTRY AND DENSITY SUBDUCTED BASALTIC CRUST AT LOWER-MANTLE PRESSURES

SUBDUCTED slabs are less dense than the surrounding mantle near the base of the transition zone (similar to 660 km depth) because of the survival of garnet in former basaltic crust: by this depth mantle peridotite has transformed to denser perovskitite(1,2). The buoyancy of the former basaltic crust may contribute to the observed accumulation or horizontal displacement of many slabs at the base of the transition zone(3). Here we report experimental confirmation of the widely held belief that the basaltic crust of slabs eventually transforms to a dense perovskititic lithology, stable in the lower mantle. Synthetic mid-ocean-ridge basalt (MORB) glass subjected to pressures of 45, 80 and 100 GPa in a laser-heated diamond anvil cell transforms to an assemblage of aluminous Mg,Fe silicate perovskite, non-quenchable CaSiO3 perovskite, stishovite and a sodic, aluminous phase with the Ca-ferrite structure (Fig. 1). Perovskititic MORB is about 0.06 g cm(-3) more dense than a model lower mantle (PREM) derived from seismological data. Thus even thermally equilibrated perovskititic slabs should encounter no significant hindrance to subduction and convection in the lower mantle.