Phase transformations of subducted basaltic crust in the upmost lower mantle

Abstract

Phase relations in natural mid-oceanic ridge basalt (MORB) were investigated to depths of 700–1400 km in the lower mantle, using a laser-heated diamond-anvil cell and synchrotron X-ray diffraction (XRD). The crystalline phases, their unit-cell volumes and proportions were determined from Rietveld refinement of the in situ XRD spectra. At the top of the lower mantle, Mg-rich perovskite is the most abundant phase, followed by Ca-rich perovskite and stishovite. Two Al-rich phases occur in smaller amounts, the Ca-ferrite type and the new aluminum-rich phase (or NAL). This assemblage is stable down to 1150 km, approximately. At greater depths, the new aluminum-rich phase disappears, and the assemblage of Mg-rich perovskite, Ca-rich perovskite, stishovite and Ca-ferrite phase is stable up to at least 1400 km. In view of the present phase diagram, we discuss the nature and relations of the lower mantle Al-phases in MORB composition, and their bearing on the density evolution of the oceanic crust.