Anhydrite replacement reaction in nodular pyrite breccia and its geochemical controls on the δ34S signature of pyrite in the TAG hydrothermal mound, 26° N Mid Atlantic Ridge

• Published in: Lithos Vol. 400-401; p. 106357
• Main Authors: Pujatti, Simone, Klyukin, Yury, Steele-MacInnis, Matthew, Tutolo, Benjamin M.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-11-2021
• Subjects: Mineral replacement reaction; Nodular texture; Seafloor hydrothermal system; Sulfur isotopes; TAG; Sulfur isotopes; TAG; Seafloor hydrothermal system; Mineral replacement reaction; Nodular texture
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2021.106357

The Trans-Atlantic Geotraverse (TAG) active mound is a seafloor hydrothermal deposit that has been sampled by drilling to the base of its alteration zone, providing a rare opportunity for in situ investigation of the effects of water-rock interactions. The δ34S signature of sulfide minerals in seafloor hydrothermal settings is commonly explained by invoking isotopic mixing and fractionation based endmember models. Here, we present textural evidence of the replacement reactions that underlie the fractionation model endmember, found in core samples recovered from the TAG active mound during ODP expedition 158. The samples were obtained at the TAG-1 area, at the transition between shallower pyrite-silica (± anhydrite) breccias and deeper pyrite-silica breccias. The shallower breccias show porous domains associated with anhydrite, quartz and pyrite. Similar textures were also observed in the deeper samples, in which anhydrite is absent. Our results suggest that at stockwork depths, anhydrite is removed during its interaction with hydrothermal fluids, leaving behind pores lined by quartz crystals and partially filled by pyrite. The textural evolution suggests that pyrite grows at the expense of anhydrite, leading to δ34S values in pyrite controlled by the isotopic fractionation factor between SO42− and H2S. Our petrographic observations are supported by geochemical modelling at in situ temperatures, estimated from analysis of fluid inclusions in quartz. Our interpretation is in agreement with previously published δ34S data from the TAG active mound. Both the highest δ34S signatures in disseminated pyrite and the lowest δ34S signatures in vein-related mineralization, as well as the progressive 34S-enrichment with depth recorded by the isotopic stratification at the TAG active mound can be explained based on the temperature at which water-rock interactions occurred. This research provides direct evidence of the mineral replacement processes occurring within the active TAG sulfide mound and their link to established models of isotopic fractionation. The textural evidence for these processes is characterized in detail and can aid the interpretation of the evolution of other seafloor hydrothermal deposits.
•Pyrite from the subseafloor at TAG shows a distinctive nodular texture.•A genetic model is proposed for the formation of the nodular texture.•The nodular texture is the petrologic expression of the anhydrite buffer model.