Mineral chemistry and density of subducted basaltic crust at lower-mantle pressures

Abstract

SUBDUCTED slabs are less dense than the surrounding mantle near the base of the transition zone (~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 accu-mulation or horizontal displacement of many slabs at the base of the transition zone³. 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 (MORE) 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). Per-ovskititic MORE 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 sig-nificant hindrance to subduction and convection in the lower mantle.