Identification of sulfur sources and isotopic equilibria in submarine hot-springs using multiple sulfur isotopes
Introduction
In deep-sea hydrothermal systems, circulating seawater extracts heat from young oceanic crust while undergoing a variety of water–rock reactions and entraining magmatic gases. The resultant hot, buoyant and chemically altered fluids are discharged as metal-rich vent fluids at the seafloor, where they precipitate metal-rich mineral deposits during cooling and mixing with ambient seawater. Collectively, these processes substantially influence global heat budgets, chemical fluxes, and the isotopic composition of seawater (Edmond et al., 1979, Staudigel and Hart, 1983, Von Damm et al., 1985a, Elderfield et al., 1993, Von Damm, 1995, Elderfield and Schultz, 1996, Jaffrés et al., 2007, Farquhar et al., 2010).
As the subsurface of active hydrothermal systems is rarely accessible to direct observation, inferences from vent fluid and mineral deposit chemistry are an important means of characterizing processes occurring in the shallow subsurface and at depth. In particular, the sulfur isotope signatures of hydrothermal vent fluids and associated mineral deposits can be used to identify sulfur sources and examine metal sulfide mineral precipitation processes in active deep-sea hydrothermal systems. As metal- and sulfide-rich, high-temperature (350–400 °C) ‘black smoker’ vent fluids ascend toward the seafloor and cool, metal sulfides may precipitate at and below the seafloor. Because metal sulfide deposits record the isotopic composition of hydrothermal fluids responsible for their formation, they can be used to assess temporal evolution within a hydrothermal system, provided that isotope systematics during mineral precipitation are well understood.
Sulfur sources in hydrothermal systems may include sulfide present in crustal host rocks, bacterial or thermochemical reduction of seawater-derived sulfate (SO4) to hydrogen sulfide (H2S), magmatic volatiles (H2S or SO2), and, at sedimented spreading centers, sedimentary biogenic pyrite and organic-derived sulfur. Several studies have utilized the two major stable sulfur isotopes (34S and 32S) to identify sources of sulfur and fluid-mineral interaction during hydrothermal circulation. These efforts have recognized that vent fluid H2S and co-existing metal sulfides are often not in isotopic equilibrium with respect to sulfur at measured vent temperatures (Kerridge et al., 1983, Bluth and Ohmoto, 1988, Woodruff and Shanks, 1988, Shanks, 2001, Ono et al., 2007, Rouxel et al., 2008a). Some authors have suggested that kinetic isotope effects occur during rapid mineral precipitation, while others have suggested that temporal variations in vent fluid δ34S produce isotopically variable micro-layers of precipitated sulfide that do not reflect the isotopic composition of concurrently sampled fluids. Both sulfur and iron isotopes have been used to demonstrate the importance of mineralogy and precipitation pathway to equilibrium versus disequilibrium effects in chimney environments. For example, chalcopyrite and sphalerite are often closer to isotopic equilibrium with fluid H2S, while other sulfide minerals, such as pyrite and marcasite, are further from equilibrium (Ono et al., 2007, Rouxel et al., 2008a).
Recent advances in multiple stable isotope analytical techniques now enable the precise determination of the four stable isotopes of sulfur: 32S, 33S, 34S, and 36S, with natural terrestrial abundances of approximately 95.02%, 0.75%, 4.21%, and 0.02%, respectively (MacNamara and Thode, 1950). Multiple sulfur isotope studies have shown that many modern biologically-mediated processes, particularly those involving reaction intermediates or branched reactions (e.g. microbial metabolism), are characterized by mass-dependent fractionation that is discernibly different than equilibrium predictions (Farquhar et al., 2003, Johnston et al., 2005, Sim et al., 2011). Temperature-dependent equilibrium isotopic exchange between sulfur-containing species (e.g. SO4–H2S) also generates differences in multiple sulfur isotope signatures, making it possible to distinguish between mixing and isotope exchange processes. For example, Ono et al. (2007) used multiple sulfur isotopes to demonstrate that fluid H2S and metal sulfide deposits at the 9°N East Pacific Rise hydrothermal field may be impacted by various degrees of isotope exchange with subsurface seawater-derived SO4 or anhydrite.
Since high-temperature hydrothermal processes facilitate equilibrium isotope fractionations, high-precision measurements of 33S may provide additional information identifying biogenic sulfide sources when δ34S values are inconclusive (Ono et al., 2007). Moreover, sulfur isotopes can be used to constrain the relative contributions of magmatic volatiles, sediment interaction, and fluid/rock reactions to the chemistry of circulating fluids, which remains poorly understood in hydrothermal systems. Magmatic degassing and sediment interactions contribute a variety of chemical species to hydrothermal fluids and have important implications for chemical fluxes and fluid/rock reactions. Magmatic SO2 disproportionation represents a potentially large source of acidity in back-arc systems that may influence crustal alteration and metal mobilization, for example. In the present study, multiple sulfur isotopes of inner-wall chalcopyrite, fluid H2S, and elemental sulfur (S0) were determined to examine the systematics of fractionation during mineral precipitation and identify sulfur sources in a broad spectrum of deep-sea hydrothermal systems.
Section snippets
Sample description
The collection of vent fluids and mineral deposits through which they flow (fluid-mineral pairs) allows evaluation of the degree to which fluid temperature and chemistry influences the mineralogy and isotope composition of a deposit. This study includes well-characterized fluid-mineral pairs from a variety of deep-sea hydrothermal systems, including two back-arc spreading centers at Lau Basin and Manus Basin, the mid-ocean ridge spreading center at the Southern East Pacific Rise (SEPR), and
Sulfide minerals
Picked chalcopyrite grains were placed in HCl-cleaned Teflon® vials and cleaned in 1 N HCl for several hours to remove surface tarnish before grinding to a fine powder in ethanol. Following established methods, chalcopyrite was dissolved in 5 mL aqua regia to oxidize sulfur and evaporated to dryness at 150 °C (Shanks, 2001, Rouxel et al., 2008a). The dry residue was re-dissolved in water, and a 0.4 M BaCl2 solution was added to precipitate BaSO4. After drying overnight at 80 °C, BaSO4 was reduced to
Results
The values of and δ34Schalcopyrite for all measured fluids and minerals (−5.5‰ to +6.1‰) span the range of previously measured values at deep-sea hydrothermal vents (Fig. 1) (Shanks, 2001). All measured fluids and minerals are characterized by Δ33S values equal to zero within error (Fig. 1).
Among the five sites in this study, the back-arc PACMANUS vent field in the Eastern Manus Basin shows the largest isotopic variability, with vent chimney δ34Schalcopyrite values ranging from −0.8‰ to
Unsedimented Southern East Pacific Rise mid-ocean ridge
A number of physical and chemical processes influence the abundance and isotopic composition of sulfur species during the chemical evolution of submarine hot-spring fluids. The major contributors to H2S in vent fluids and associated mineral deposits at unsedimented mid-ocean ridge hydrothermal systems are host rock sulfide, with an average δ34S value of +0.1 ± 0.5‰ for mid-ocean ridge basalt (Sakai et al., 1984), and reduced seawater SO4, with a δ34S value of +21.0 ± 0.2‰ (Rees et al., 1978). A
Summary
Fluid H2S, chalcopyrite, and S0 samples from two back-arc systems, an unsedimented mid-ocean ridge, and a sedimented mid-ocean ridge show Δ33S values close to zero, which implicates the abiotic processes of SO4 reduction, leaching of host rock, and SO2 disproportionation as the major contributors to sulfur content in high temperature and acid spring-type hydrothermal fluid circulation, with lesser contributions of biogenic pyrite as indicated by a lack of positive Δ33S values. At the heavily
Acknowledgements
We thank the captain and crew of the R/V Thompson, R/V Atlantis, R/V Melville, and R/V Western Flyer and the ROV Jason II, HOV Alvin, and ROV Tiburon teams for their dedication, expertise, and assistance during five successful field programs. We wish to thank J. Alt, B. Wing, and two anonymous reviewers for comments that greatly improved this manuscript, as well as S. Herrera and S. Sylva for their recommendations on earlier versions. This study received financial support from NASA Grant
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