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High-pressure behaviour of carbonates and hydrates, and devolatilization of the subducted oceanic crust

Dolomite, magnesite and Mg-calcite are found to coexist with a variety of hydrous phases in basaltic and gabbroic rocks that underwent deep subduction processes. The transport of carbon and hydrogen to depth and the locus of fluid release to the overlying mantle wedge, source of arc magmatism, are controlled by phase relationships involving carbonates, hydrous phases and volatile-free minerals. Ca-Mg-Fe partitioning between garnet, clinopyroxene and carbonates defines the compositional space where lawsonite, epidote, talc are allowed to form in the presence of a C-O-H fluid. Experimental data [1] show that an increasing amount of C-O-H volatile components at a Ca/(Fe+Mg) ratio in clinopyroxene higher than unity promotes the crystallization of lawsonite at temperatures beyond the stability limit observed at H2O-saturated conditions. The abundance of lawsonite, and therefore, its ultimate desappearance in the subducted oceanic crust can be modelled by the normative anorthite content of the bulk composition. As a consequence lawsonite and dolomite/ magnesite are expected to coexist in deeply altered volcanoclastics or in differentiatied gabbroic rocks of noritic composition at temperatures approaching the basalt wet-solidus. New experimental data at 3.5 GPa are intended to unravel the effect of variable bulk and volatile compositions in two model compositions in the chemical system Na2O-CaO-FeOMgO-Al2O3-SiO2-C-O-H, enriched in the an component. Experiments are performed in a piston cylinder apparatus, at fH2 fixed at HM, OH (X, COH) buffer using a double capsule technique, after [2]. The small difference between the dP/dT slope of lawsonite devolatilization reactions and the dP/dT slope of P-T paths for subducting slabs greatly amplifies the role of bulk composition in generating the fluid that modifies the subarc mantle chemistry.