Using basaltic crystal mushes to understand the mechanisms of crust formation and the deep structure of basaltic volcanoes

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Most of the oceanic and continental Earth s crust is respectively made up of mafic or mafic-felsic crystalline rocks. The mechanisms of formation and the structure of the deep, mafic, crustal units are not well understood. In particular, we do not yet know how primary mantle-derived chemically change during differentiation in the lower crust and the timescales of mafic magma crystallization. The proportion of melt finally erupting at the surface of the Earth is also poorly constrained. An extremely useful way to decipher the large-scale lithological variability of the Earth s crust, the mechanisms of magma differentiation and to estimate mass transfer in the crust is to use crystal mush fragments commonly brought to the surface during eruptions. They represent cumulate rocks formed in the crust and therefore are a unique opportunity to determine the mineralogy of frozen, highly crystalline, parts of crustal magma chambers, inaccessible otherwise.The main objective of this project is to refine our understanding of deep magmatic processes beneath oceanic and continental basaltic volcanoes and to decipher the dominant mechanisms of crust formation. We will perform high-temperature experiments to constrain the formation and disaggregation of crystal mushes prior to eruption and determine their thermo-physical state in the crust. We will also use experimental data to propose new and accurate formulations of mineral-melt equilibria, applicable at crustal pressure (<7 kbar) that can be used to estimate magma storage conditions in the crust. These results will then be combined with a statistically-based, textural and geochemical analysis of phyric basalts from 4 tectonic settings: mid-ocean ridge (Galapagos Spreading Centre), oceanic plateau (Shatsky Rise), continental flood basalt (Snake River Plain) and continental arc (South Volcanic Zone of Chile). We will use high-quality imaging of basaltic samples together with geochemical mapping and crystal size distribution to discriminate between phenocrysts in equilibrium with the melt and crystals formed in deep crystal mushes. The composition (major and trace elements) and texture of crystals together with the compositions of their melt inclusions will then be used to investigate the deep plumbing systems and magma storage conditions of the basaltic volcanoes. In particular, we will identify the role of (heterogeneous) mantle melting and the processes and timescales of intra-crustal differentiation on the formation, growth and composition of the crust and on the geochemical variability of erupted magmas. All together, this project will improve our understanding of the lithological and geochemical variability of the crust in various tectonic settings and will add to the current understanding of crystal mush rheology, a critical parameter for magma differentiation. Our results will also be useful for other basaltic provinces where crustal processes play a major role on the diversity of erupted magmas.