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

• Published in: Contributions to mineralogy and petrology Vol. 178; no. 12; p. 90
• Main Authors: Sokol, Alexander G., Kozmenko, Olga A., Kruk, Alexey N.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.12.2023; Springer Nature B.V
• Subjects: Aluminum oxide; Carbon dioxide; Carbonates; Chlorine; Coesite; Composition; Diamonds; Earth and Environmental Science; Earth Sciences; Fluids; Fluxing; Garnet; Geology; High temperature; Inclusions; Kyanite; Major elements; Mineral Resources; Mineralogy; Original Paper; Petrology; Potassium; Potassium oxides; Pyrite; Pyrrhotite; Residues; Rutile; Sediments; Silica; Silicon dioxide; Solutes; Subduction; Subduction (geology); Subduction zones; Supercritical fluids; Zircon; Slab sediments; High-pressure high-temperature experiments; Pelite; Fluid; Mantle metasomatism; Supercritical fluid; Element partitioning; Magma; Subduction
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-023-02074-0

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 H 2 O + CO 2 [at the weight ratio H 2 O/(H 2 O + CO 2 ) 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 CO 2 and 0.4–0.5 wt% Cl in the fluid. The reconstructed supercritical fluid composition includes components decreasing in the series SiO 2  > Al 2 O 3  > K 2 O > Na 2 O ≈ CaO ≈ MgO ≈ FeO. At 7.8 GPa, phengite becomes unstable, and K 2 O in the supercritical fluid increases from 3 to 8 wt% (on hydrous basis) while Al 2 O 3 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 H 2 O and CO 2 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.