Sulfur cycling at the Mid-Atlantic Ridge: A multiple sulfur isotope approach

Peters M, Strauss H, Farquhar J, Ockert C, Eickmann B, Jost CL

Abstract

The role of sulfur in two hydrothermal vent systems, the Logatchev hydrothermal field at 14 degrees 45'N/44 degrees 58'W and several different vent sites along the southern Mid-Atlantic Ridge (SMAR) between 4 degrees 48'S and 9 degrees 33'S and between 12 degrees 22'W and 13 degrees 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%o in delta S-34 and from +0.001 to +0.051 parts per thousand in Delta S-33. These data indicate mixing of mantle sulfur with sulfur from seawater sulfate. Combined delta S-34 and Delta S-33 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 degrees C. delta S-34 values between +0.2 to +8.8 parts per thousand for dissolved and precipitated sulfide from Logatchev point to the same mixing process between mantle sulfur and sulfur from seawater sulfate as at SMAR. delta S-34 values between -24.5 and +6.5 parts per thousand and Delta S-33 values between +0.001 and +0.125 parts per thousand 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 Delta S-33 = -0.008 parts per thousand reveal lower Delta S-33 lower values than suggested on the basis of previously published isotopic measurements of terrestrial materials. We conclude that the combined use of both delta S-34 and Delta S-33 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 Delta S-33 we are able to clearly distinguish biogenic from hydrothermal sulfides in sediments even when delta S-34 were identical. (C) 2009 Elsevier B.V. All rights reserved.