Stable Isotope Constraints on the Origin of Sulfur-bearing Minerals in the Seawater Hydrothermal System of Surtsey Volcano, Iceland

Abstract

At mid-ocean ridges, seawater carrying SO4 infiltrates the oceanic crust. However, hydrothermal fluid emissions from such systems predominantly contain H2S. The absence of SO4 may be explained by the reduction of seawater SO4 to H2S and/ or by the immediate precipitation of anhydrite upon temperature increase. These contrasting hypotheses highlight the need to explore sulfur cycling in the oceanic crust. Surtsey volcano, Iceland, offers an exceptionally well-constrained environment to study sulfur cycling in an exposed section of the oceanic crust. Here, anhydrite was measured for sulfur and oxygen isotope compositions to distinguish different sources of sulfur to the oceanic crust. Geochemical isotope modelling was used to (1) assess the major sources of sulfur to the system and (2) investigate hydrothermal processes (e.g., alteration, redox reactions) affecting isotope systematics of anhydrite at various temperatures. Comparison of model results with our dataset reveals that observed deviations in δ34S and δ18O of anhydrite from the original isotope composition of seawater SO4 can be attributed to equilibrium isotope fractionation upon progressive seawater-rock alteration. Most of the sulfur derives from the seawater whereas the origin of oxygen changes from seawater- to a basalt-dominated source with progressive alteration. Reduction of SO4 to sulfide was found to be limited below 150°C. Our modelling approach shows that low-temperature seafloor alteration may have the potential of significantly modifying the δ34S value of the oceanic crust.