Property and composition of fluids from the subducting oceanic slab: Constraints from multiple veins from SW Tianshan, China

Abstract

Fluid properties and compositions play critical roles in element mobility and crust-mantle interactions in subduction zones. Veins within high/ultrahigh-pressure metabasites record fluid flow in subduction zones. A comprehensive study combining whole-rock and mineral major and trace element contents, halogen contents and zirconology was conducted on various veins—omphacite, garnet-quartz, quartz and epidote-quartz veins—from SW Tianshan. The results are used to quantitatively reconstruct the properties and compositions of slab-derived fluids during oceanic subduction and exhumation. Omp veins formed during prograde metamorphism (1.8–2.2 GPa; 466–544 °C), whereas Grt-Qz veins formed during early exhumation (1.2–1.4 GPa; 544–621 °C; 303 ± 4 Ma). Qz veins and some Ep-Qz veins formed at ∼287 Ma during eclogite- to amphibolite-facies retrogression (∼1.0 GPa; 493–557 °C), whereas the remaining Ep-Qz veins precipitated during late exhumation (0.6–0.8 GPa; 485–521 °C; 229 ± 7 Ma). Mineral element and isotope data indicate that Omp, Grt-Qz, and Ep-Qz veins formed from aqueous fluids derived from dehydration of metamafic rocks, whereas Qz veins involved serpentinite-derived fluids. Vein-forming aqueous fluids contained 0.04–0.88 wt% F and up to 4.56 wt% Cl. High-pressure fluids forming Omp and Grt-Qz veins exhibit elevated abundances of Ca, Al, Mg, Fe, LILE, HFSE, REE and halogens compared to low-pressure fluids forming Qz and Ep-Qz veins, confirming the role of halogens in enhancing element solubility in aqueous fluids. Whole-rock F/Cl ratios of the veins positively correlate with metal ion, LREE, MREE and HFSE contents. Apatite F/Cl ratios also correlate strongly with rutile Nb/Ta ratios across all vein types. These observations suggest that halogen compositions critically control element solubility in aqueous fluids, particularly for HFSE. As halogen signatures vary in slab-derived fluids during progressive oceanic subduction, they generate distinct element mobility patterns.