Sulfur cycling at the Mid-Atlantic Ridge: A multiple sulfur isotope approach |
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| Authors: | Marc Peters Harald Strauss James Farquhar Charlotte Ockert Benjamin Eickmann Cristiane L Jost |
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| Institution: | 1. Westfälische Wilhelms-Universität Münster, Institut für Geologie und Paläontologie, Corrensstr. 24, 48149 Münster, Germany;2. University of Maryland, Department of Geology and ESSIC, College Park, Maryland 20742, USA;3. Jacobs University Bremen, Earth and Space Sciences, Campus Ring 1, 28725 Bremen, Germany;1. Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, MNR, Hangzhou 310012, China;2. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China;3. Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal;1. Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543, United States;2. Marine Geosciences Unit, Institut Français de Recherche pour l''Exploitation de la Mer, Centre de Brest, Technopôle Brest Iroise, Plouzané, France;1. Department of Geology and ESSIC, University of Maryland, College Park, MD 20740, USA;2. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA;1. Paleoproterozoic Mineralization (PPM) Research Group, Department of Geology, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa;2. Department of Geological Sciences, Stockholm University, 106 91 Stockholm, Sweden;3. GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1-3, D-24148 Kiel, Germany;1. Laboratoire de Géochimie des Isotopes Stables, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, 75238 Paris, France;2. Department of Earth Science, University of California Santa Barbara, Santa Barbara, CA, 93109-9630, USA |
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| Abstract: | The role of sulfur in two hydrothermal vent systems, the Logatchev hydrothermal field at 14°45′N/44°58′W and several different vent sites along the southern Mid-Atlantic Ridge (SMAR) between 4°48′S and 9°33′S and between 12°22′W and 13°12′W, is examined by utilizing multiple sulfur isotope and sulfur concentration data. Isotope compositions for sulfide minerals and vent H2S from different SMAR sites range from + 1.5 to + 8.9‰ in δ34S and from + 0.001 to + 0.051‰ in Δ33S. These data indicate mixing of mantle sulfur with sulfur from seawater sulfate. Combined δ34S and Δ33S systematics reveal that vent sulfide from SMAR is characterized by a sulfur contribution from seawater sulfate between 25 and 33%. This higher contribution, compared with EPR sulfide, indicates increased seawater sulfate reduction at MAR, because of a deeper seated magma chamber and longer fluid upflow path length, and points to fundamental differences with respect to subsurface structures and fluid evolution at slow and fast spreading mid-ocean ridges.Additionally, isotope data uncover non-equilibrium isotopic exchange between dissolved sulfide and sulfate in an anhydrite bearing zone below the vent systems at fluid temperatures between 335 and 400 °C. δ34S values between + 0.2 to + 8.8‰ for dissolved and precipitated sulfide from Logatchev point to the same mixing process between mantle sulfur and sulfur from seawater sulfate as at SMAR. δ34S values between ? 24.5 and + 6.5‰ and Δ33S values between + 0.001 and + 0.125‰ for sulfide-bearing sediments and mafic/ultramafic host rocks from drill cores taken in the region of Logatchev indicate a clear contribution of biogenic sulfides formed via bacterial sulfate reduction. Basalts and basaltic glass from SMAR sites with Δ33S = ? 0.008‰ reveal lower Δ33S lower values than suggested on the basis of previously published isotopic measurements of terrestrial materials.We conclude that the combined use of both δ34S and Δ33S provides a more detailed picture of the sulfur cycling in hydrothermal systems at the Mid-Atlantic Ridge and uncovers systematic differences to hydrothermal sites at different mid-ocean ridge sites. Multiple sulfur isotope measurements allow identification of incomplete isotope exchange in addition to isotope mixing as a second important factor influencing the isotopic composition of dissolved sulfide during fluid upflow. Furthermore, based on Δ33S we are able to clearly distinguish biogenic from hydrothermal sulfides in sediments even when δ34S were identical. |
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