The Earth's mantle is primarily composed of peridotites, which are magnesium and iron silicate rocks that often contain 5 to 10% heterogeneities by volume. These heterogeneities can be very variable in composition but pyroxene-rich rocks, and in particular websterites (those that have additional calcium, sodium and aluminum in the silicate crystal structure) appear to be the dominant type of heterogeneity associated with peridotites exhumed at the Earth?s surface. No study has so far explained why and how this type of heterogeneity is so widespread in mantle rocks. Importantly, it might be an indication that a ubiquitous process is significantly modifying the composition of the mantle and its derived lavas. A multidisciplinary study will be conducted, which will combine field observations, geochemical and petrophysical analyses of selected websterites from two locations: the Lherz Massif (FR) and the Josephine Peridotite (USA), that are believed to represent subcontinental and subarc mantle, respectively. Trace and major element variations will be measured and modeled in order to constrain the process(es) responsible for the formation of heterogeneities in the mantle. Additionally geodynamical conditions of emplacement of the websterites will be constrained by measuring small-scale variations of lattice-preferred orientations of minerals. The results of this study will benefit a broad community of scientists due to its multidisciplinary approach that can reconcile observations of interest for mantle geochemists, rheology experimentalists and geophysicists.
The mantle represents about 80% of the volume of our planet, lying underneath the crust between approximately 30 km and 2900 km depth. A large part of the thermal and chemical variations of the Earth is controlled by the mantle, which locally melts and gives rise to most of the volcanic activity visible at the surface of the planet. In order to better predict volcanic hazards, it is fundamental to constrain what type of material is melting at depth, because the composition of the lava affects the explosivity of volcanic eruptions. In order to answer this question, the composition of the mantle should be well constrained. Exposures of mantle rock at the Earth?s surface are rare but where we can obtain access, we find that many aspects of its composition remain uncertain. We know that the Earth's mantle is not uniform in composition and that it contains heterogeneities, however the nature and the distribution of these heterogeneities are poorly constrained. For instance, there could be blobs of crustal material, recycled in the deep Earth by plate tectonics. This project will focus on the fundamental processes responsible for the formation of these heterogeneities.
