Sulfur isotope compositions of pyrite from high-pressure metamorphic rocks and related veins (SW Tianshan, China): Implications for the sulfur cycle in subduction zones

Abstract

A detailed petrographic, microstructural and geochemical investigation of pyrites from four eclogites / blueschists and their veins in the Tianshan (NW China) have revealed the occurrence of four types of pyrites: δ34S values of the pyrites range from −24.7 to +13.2‰, whereby three populations can be distinguished that have distinct δ34S ranges corresponding to different paragenetic stages. Py1 pyrite grains contain micro- to nanoscale inclusions of pyrrhotite, chalcopyrite, bornite, sphalerite, galena, arsenopyrite, cobalt-pentlandite, enargite, titanite, biotite, barite, and have δ34S values of −3.8 to +2.7‰ (average δ34S = −0.6 ± 0.9‰), which is inherited from a magmatic and hydrothermal fluid input during alteration of the oceanic crust. Py2 pyrite occurs as inclusions within the cores of garnet where it coexists with fine-grained inclusions of rutile, glaucophane and rarely with omphacite. Thus, these pyrite inclusions likely formed during prograde evolution. Py2 is too small to be analyzed by SIMS and thus no isotopic compositions could be determined. The Py3 pyrite grains contain eclogite-facies mineral inclusions, such as omphacite, glaucophane, epidote, rutile and lawsonite, and are inferred to have formed during the late prograde evolution close to or at peak eclogite-facies conditions. Py3 pyrite display positive δ34S values ranging from +3.6 to +13.2‰ (average δ34S = +9.7 ± 2.8‰), suggesting the source of sulfur was either anhydrite incorporated during seafloor alteration or 34S-enriched sulfide from lower sections of the oceanic lithosphere. Py4 pyrite occurs as irregular patches, isolated euhedral to subhedral grains or as rims around Py1 or Py3 pyrites, associated with magnetite and ilmenite. Py4 grains have δ34S values of −24.7 to −3.9‰ (average δ34S = −5.0 ± 2.1‰), with the most likely source of sulfur being 32S-enriched pelagic sediments that in the Tianshan are interlayered with the metavolcanic lithologies and underwent amphibolite facies metamorphism during exhumation. This study provides insight into the sulfur cycle within subduction zones. We document that during subduction and exhumation fluids with distinct sulfur isotopic compositions driven by dehydration reactions in the downgoing slab cause metasomatism in the surrounding lithologies. In particular, we suggest that at least locally the plate interface is characterized by sediment-derived fluids that are characterized by 32S-enriched sulfur and comparatively higher oxygen fugacities.