Unravelling slab delta 34S compositions from in-situ sulphide delta 34S studies of high-pressure metamorphic rocks

摘要

Sulphur is of particular importance in the subduction zone system, as it is thought to be intimately related to the arc redox state and metallogenesis above the slab. Knowing the sulphur isotope compositions of the subducted slab, and identifying the delta S-34 variations between the pre-subduction oceanic lithosphere and its high-pressure (HP) equivalents is of great importance for understanding the sulphur behaviour in subduction zones and the long-term sulphur recycling of the Earth. In this study, we undertook secondary ion mass spectroscopy (SIMS) in-situ delta S-34 analyses of sulphides from metasediments, mafic blueschists/eclogites, and ultramafic slab serpentinites from the Southwestern Tianshan HP-UHP metamorphic belt, which represents one of the deepest fossil subduction zones on Earth. By integrating the in-situ delta S-34 compositions of sulphides studied here and those reported from worldwide HP-UHP metamorphic terranes, we found that prograde to peak metamorphic sulphides of metasediments, metabasites/metagabbros, and slab HP serpentinites have delta S-34 values ranging from -33 to -6 parts per thousand, -4 to +4 parts per thousand, and +2 to +18 parts per thousand, respectively. However, retrograde metasomatism of HP rocks during exhumation may alter the delta S-34 compositions of sulphides. The integrated dataset proposes that sulphides in subducting slab sequences retain their protolith's delta S-34 signatures during HP metamorphism. The delta S-34 inheritance indicates that no large isotope fractionation is expected to occur in subduction zones. It further suggests that evaluations of slab subduction effects on the delta S-34 signature of arc magmatism and the deep mantle should consider pre-subduction processes such as ridge geodynamic setting and hydrothermal alteration.