The Fate of Subducted Basaltic Crust in the Earth's Lower Mantle : an Experimental Petrological Study

Several models have attempted to reconcile observed geochemical data with a whole mantle convection. Such a convection pattern would be in agreement with geochemical constraints, provided the crustal component of subducted lithospheric slabs is denser than the surrounding mantle and reaches the lowermost mantle, where it could accumulate and eventually generate ascending plumes, producing the particular geochemical signatures observed at the surface [Albarède, Chem. Geol. 145, 413-429, 1998; Coltice and Ricard, Earth Planet. Sci. Lett. 174, 125-137, 1999]. In such models, the key point is that the relatively minor crustal component of the subducting lithosphere is able to reach the base of the mantle. The density differential between the oceanic crust and the mantle is thus a crucial parameter we would like to address experimentally.

In this work, we describe synchrotron X-ray diffraction experiments carried out in situ at high-pressure and high-temperature (to 90 GPa and 2800 K), and coupled with the study of recovered samples by Analytical Transmission Electronic Microscopy (ATEM). Quenched samples were prepared with the FIB (Focused Ion Beam) technique for ATEM investigations, so as to obtain chemical compositions of observed phases identified by selected area electron diffraction (SAED) patterns study. Under lower mantle pressure and temperature conditions, the high aluminium budget of MORB cannot be totally accommodated in an orthorhombic perovskite structure and two Al-rich phases are observed in our experiments, in addition to Mg-perovskite, Ca-perovskite, and stishovite. The chemical compositions of individual phases were taken into account in the Rietveld refinements made to the X-ray diffraction patterns, from which phase relative abundances, molar volumes, hence densities could be extracted.

We provide an estimate for the density profile of a MORB oceanic crust at lower mantle pressure and temperature conditions as well as a detailed phase relationships picture for this chemical composition. We show that the density profile of a MORB oceanic crust may intersect that of a normal lower mantle at depths greater than 2000 km.