Sulphur isotopic investigation of dissolved and solid sulphur phases in fluids, mineral precipitates, sediments and rock samples from the Mid-Atlantic Ridge
Final Report Abstract
At mid-ocean ridges, circulation of seawater through the oceanic crust and intense water-rockinteraction between progressively heated seawater and mafic and ultramafic rocks of the oceanic crust alters the chemical and isotopic composition of both, fluids and rocks. During hydrothermal circulation, cold and oxic, sulfate-bearing ambient seawater turns into a hot, reducing, acidic, sulfide- and metalbearing hydrothermal solution. In addition, rocks display variable degrees of hydrothermal alteration. It was the central objective of this research project to achieve a qualitative and quantitative understanding with regard to the cycling of sulfur during hydrothermal circulation. As the principal analytical approach multiple sulfur isotopes (32S, 33S, 34S) were measured for dissolved seawater sulfate, dissolved hydrothermal sulfide, sulfate and sulfide precipitates from hydrothermal chimneys, sulfur contained in mafic and ultramafic host rocks, and sulfide dissolved in pore waters or finely disseminated in sediments from the vicinity of hydrothermal vent sites. In addition, the fate of hydrothermal sulfide during microbial utilization (sulfur/sulfide oxidizers) was studied. Multiple sulfur isotope measurements were supplemented by H, O, and C isotope analyses of the hydrothermal fluids as a measure of water-rock-interaction. Hydrothermal vent sites along the Mid-Atlantic Ridge at 14°45’N (the Logatchev Hydrothermal Field) and between 4° and 11°S (collectively called as the southern MAR) were the principal study areas. Multiple sulfur isotope data were utilized to quantitatively discuss the two principal models of hydrothermal sulfur cycling, i.e. the two-component-mixing model and the anhydrite buffer model. Principal endmembers of these models, i.e. seawater and host rock sulfur, were isotopically characterized, providing crucial information for interpreting the sulfur isotopic composition of dissolved hydrothermal sulfide and resulting mineral precipitates. Results from this study clearly indicate that the isotopic composition of hydrothermal sulfide cannot be explained by a single model. For the SMAR study site, a contribution from seawater sulfate between 25 and 33% was calculated, comparable to other vent sites along the Mid-Atlantic Ridge but clearly different from the fast-spreading East Pacific Rise. Moreover, different vent sites reflect isotopic disequilibrium between dissolved sulfate and sulfide in the subsurface at variable, site-specific temperatures prior to fluid discharge into the ocean. This study clearly demonstrated/confirmed the significance of stable isotope analyses for hydrothermal research. In particular, the newly developed multiple sulfur isotope approach was successfully applied here and substantially enhanced the existing database and with it our understanding of hydrothermal sulfur cycling at mid-ocean ridges.
Publications
- (2008) Hydrothermal venting at pressure-temperature conditions above the critical point of seawater, 5°S on the Mid-Atlantic Ridge. Geology 36: 615-618
Koschinsky, A., Garbe-Schönberg, D., Sander, S., Schmidt, K., Gennerich, H.H., and Strauss, H.
- (2009) Short-term microbial and physico-chemical variability in low-temperature hydrothermal fluids near 5°S on the Mid-Atlantic Ridge. Environ. Microbiol. 11: 2526-2541
Perner, M., Bach, W., Hentscher, M., Koschinsky, A., Garbe-Schönberg, D., Streit, W.R., and Strauss, H.
- (2010) Sulfur cycling at the Mid-Atlantic Ridge: a multiple sulfur isotope approach. Chem. Geol. 269: 180-196
Peters, M., Strauss, H., Farquhar, J., Ockert, C., Eickmann, B., and Jost, C.L.
- (2011) Fluid elemental and stable isotope composition of the Nibelungen hydrothermal field (8°18′S, Mid-Atlantic Ridge): Constraints on fluid–rock interaction in heterogeneous lithosphere. Chem. Geol. 280: 1-18
Schmidt, K., Garbe-Schönberg, D., Koschinsky, A., Strauss, H., Jost, C.L., Klevenz, V., Königer, P.
