Redox changes in a seafloor hydrothermal system recorded in hematite-chalcopyrite chimneys

Hydrothermal chimneys composed of hematite and chalcopyrite grow at the Irina II vent site (Logatchev hydrothermal field, Mid-Atlantic Ridge). Hematite and chalcopyrite form layers that repeatedly alternate within the chimney wall. The repeating alternation of hematite and chalcopyrite implies for repeating changes in the physico-chemical parameters of the vent fluid. The modeled stability diagrams of hematite and chalcopyrite at the physico-chemical conditions of the Irina II hydrothermal site showed that the precipitation of these two Fe-minerals is primarily governed by the activity of H2S and pH. Fe-isotope studies suggest that the successive deposition of chalcopyrite (δ56/54Fe = ~0.20‰) and hematite (δ56/54Fe = ~0.60‰) layers in the Irina II chimneys is a result of redox changes in the hydrothermal fluid. The replacement of hematite by either magnetite or chalcopyrite at some places also suggests for a redox inversion in the hydrothermal system from oxic to reduced. Chondrite-normalized rare earth element distribution patterns of the hematite layers show a strong negative Eu anomaly, which is interpreted to be a result of crystal-chemical control on the rare earth element fractionation during hematite precipitation. Sr isotope systematics indicate significant and variable incorporation of seawater (from ~40% to ~80%) in the hydrothermal system. Nd-Pb-isotope studies show that Nd and Pb in chalcopyrite and hematite layers are derived mainly from the underlying mafic and ultramafic rocks. The chimneys are young (5.13 ± 1.28 kyrs B.P.) and their wall thickened inward. The duration of redox cycles (oxic and reducing) within the scale of a single chimney is about several hundred years. The hematite layers are enriched in W, In, Sn and U indicating that primary hematite hydrothermal deposits at the seafloor may be a potential source for these strategic trace elements. High concentrations of these elements in the studied hematite layers can be explained by their (easy) ion substitution for Fe3+ in the hematite crystal lattice. Whereas the most plausible source of the elevated W, In and Sn contents are the hydrothermal fluids, the seawater is the possible major source of U.