Composition of supercritical fluid in carbonate- and chlorine-bearing pelite at conditions of subduction zones

The composition of fluid from carbonate- and chlorine-bearing pelite has been studied at 5.5 GPa, 850–1030 °C and at 7.8 GPa, 940–1090 °C using the diamond trap method. The run products include a residue composed of an eclogitic assemblage (garnet, coesite, clinopyroxene, and kyanite ± phengite and accessory pyrite/pyrrhotite, rutile, and zircon) and fluid solutes captured in the trap. The new data show that the pelite-derived supercritical fluid, with nearly equal amounts of solutes and H2O + CO2 [at the weight ratio H2O/(H2O + CO2) from 0.8 to 0.9], is stable at the applied P–T conditions. At higher temperatures, the amount of solutes in the supercritical fluid increases only slightly, apparently, due to the presence CO2 and 0.4–0.5 wt% Cl in the fluid. The reconstructed supercritical fluid composition includes components decreasing in the series SiO2 > Al2O3 > K2O > Na2O ≈ CaO ≈ MgO ≈ FeO. At 7.8 GPa, phengite becomes unstable, and K2O in the supercritical fluid increases from 3 to 8 wt% (on hydrous basis) while Al2O3 decreases from 8 to 5–6 wt%. Among the elements that fractionated into the fluid, B, Sr, Rb and P, as well as K at 7.8 GPa and 1090 °C, are the least compatible. The fluid-residue Sr partition coefficient varies from 4 to 10 and is notably lower at higher temperatures. Thus, supercritical fluids can form in carbonate- and chlorine-bearing sediments under subduction back arc-conditions, in cases of fluid-fluxing of the slab. Such supercritical fluids penetrating into the mantle together with H2O and CO2 can transport large amounts of major elements, B, Sr, Rb and P. The formation of potassium-rich silicic supercritical fluids is possible during subduction of pelite to ~ 250 km depths. They can be important agents in metasomatism of lithospheric mantle, with its composition reconstructed from data on micro-inclusions from fibrous diamond.