Phase transformations of subducted basaltic crust in the upmost lower mantle

Jean-Philippe Perrillat (1) , Angèle Rocolleau (2) , Isabelle Daniel (1) , Guillaume Fiquet (2) , Mohamed Mezouar (3) , Nicolas Guignot (3) , Hervé Cardon (1)

1 LST - Laboratoire de Sciences de la Terre
2 LMCP - Laboratoire de minéralogie, cristallographie de Paris
3 ESRF - European Synchrotron Radiation Facility

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.

Mots clés: High pressure; Lower mantle; Mid-oceanic ridge basalts; Subduction; X-ray diffraction

Domaines: Minéralogie

Jean-Philippe Perrillat, Angèle Rocolleau, Isabelle Daniel, Guillaume Fiquet, Mohamed Mezouar, et al.. Phase transformations of subducted basaltic crust in the upmost lower mantle. Physics of the Earth and Planetary Interiors, 2006, 157, pp.139-149. ⟨10.1016/j.pepi.2006.04.001⟩. ⟨hal-00093977⟩