Seminar of Mathieu Rosbapé

Interactions between magmatism and high temperature hydrothermalism under the ocean ridges: the example of Oman ophiolite

An important part of the chemical and thermal exchanges between the internal and external envelopes of the Earth (mantle, crust, hydrosphere) takes place at the level of the ocean ridges, in particular at the transition between mantle and oceanic crust (MTZ for Moho Transition Zone). MTZ acts as a reactive filter: the reactions between fluids and / or magmas resulting from the deep partial melting of the mantle and residual harzburgites lead to the resorption of the restitic orthopyroxenes and to the formation of a residue essentially formed of olivine, the dunites . This reaction is accompanied by an increase in porosity which favors the migration and extraction of magmas towards the crust.

Recent work on the study of Oman ophiolite has shown that high temperature hydrothermal processes, transient between anhydrous magmatism and late low temperature weathering, have affected MTZ and the upper part of the mantle section (Python et al., 2007, 2011; Akizawa et al., 2016; Rospabe et al., 2017). The occurrence of orthopyroxene and amphibole among the mineral impregnation phases within the MTZ (ie products of fractional crystallization from the interstitial magmas) provides information on the hydrated nature of the magma (s) enriched in silica. parent (s), contrasting with the MORB character expressed in crustal cumulates and mantle veins. The spatial distribution of these phases, more abundant near the base of the crust, suggests more crystallisation from a hybrid magma between ascending MORB and downstream hydrothermal front rather than a magma resulting from the partial melting of a deep hydrated source (Rospabé et al., 2017; Rospabe et al., 2018b). The correlation between spatial variation of the geochemical signatures and the presence of faults or synmagmatic fractures, which can be related to the process of accretion to the dorsal, provides information on the early establishment of these tectonic accidents, and on their important influence on structuration. Petrology and geochemistry of MTZ (Rospabé et al., submitted). The drilling of the CM1A and CM2B cores (400 + 300 m) through the Oman Oversight Oman Drilling Project (MTZ) aims at a sampling resolution never reached so far for this interface, as well as better quantification of elementary fluxes, in particular Cr, involved during such hybridization processes.