Prograde P-T Evolution of a Lawsonite Eclogite from the Monviso Meta-ophiolite (Western Alps): Dehydration and Redox Reactions during Subduction of Oceanic FeTi-oxide Gabbro

Lawsonite eclogites represent fossil records of processes occurring during subduction of cold- and fast-subducting slabs, and provide an opportunity to investigate the thermal and metamorphic evolution of palaeosubduction zones. Occurrences of lawsonite eclogites are rare because lawsonite is often replaced during exhumation. We report here, for the first time, the occurrence of a lawsonite eclogite from the Monviso meta-ophiolite (Western Alps), representing the product of the Alpine metamorphism of a FeTi-oxide gabbro. The prograde metamorphic evolution of this lawsonite eclogite has been investigated using the petrological approach of pseudosections. The reconstruction of its prograde P-T evolution allowed us to monitor changes in chemical and physical properties (mineral assemblages and compositions, density, H2O content, oxygen fugacity) during subduction. The pseudosection modelling suggests peak metamorphic conditions of T >= 550°C, P = 25-26 kbar, and a decrease in the thermal gradient during subduction from about 9°C/km to less than 7°C/km, which may be interpreted as related to an increase in the subduction rate. During its prograde evolution, at a depth of about 65-70 km, the eclogite-facies metagabbro experienced a significant dehydration passing from the Grt(1) + Omp(1) + Lws + Chl + Qtz + Rt assemblage (stage I) to the Grt(2) + Omp(2) + Tlc + Qtz + Rt assemblage (stage II). The breakdown of lawsonite and chlorite caused the release of up to 3 wt % of H2O and also of oxygen, as modelled by the redox equilibrium Lws + Qtz + Chl + Omp(1) = Grt + Omp(2) + H2O + O2.This redox-equilibrium represents the boundary between an earlier, more oxidized assemblage (stage I), stable at lower T, and a later, more reduced assemblage (stage II), stable at higher T. These results have possible implications for the understanding of the complex interactions between crust and mantle in subduction zones, especially in clarifying the processes locally involved in the oxidation of the mantle wedge overlying the subducting slab.