Sulfur Isotopes in Subduction Systems and the Global Sulfur Cycle


DOEI Funded Research: 2010

Background and Aims of the Study

Sulfur is one of the six most abundant elements in the Earth (~2 wt. %) with four stable isotopes.  34S/32S ratios (expressed as δ34S relative to a meteorite standard) show a fractionation of ~150 ‰ in nature. Isotopic fractionation is due to (i) kinetic effects during bacterial reduction of sulfate, and (ii) chemical exchange between sulfate (SO42-) and sulfide (S2-) species and between different sulfides.

Sulfur is an essential ingredient of life and its isotopes are used as tracers in marine geochemistry, in the investigation of secular changes of seawater chemistry and the composition of the atmosphere, but also in ore geology, in diamond research and in coal and oil geology.  The surface sulfur cycle has been investigated in detail for many decades [e.g., Canfield, 2004; Seal, 2006].  Studies have revealed feedback mechanisms between oxygenation of the atmosphere and the deep seawater and the biogenic and chemical fixation of sulfur in sediments and altered igneous rocks of the oceanic crust.  The deep cycle of sulfur between crust and mantle is controlled by subduction of oceanic lithosphere (mantle input) and by volcanic eruptions and degassing along plate margins and intra-plate volcanoes (mantle output; Figure 1).  The budget and cycling of sulfur among the surface reservoirs, such as the oceans, is strongly influenced by mantle input and output rates via subduction and volcanism [e.g., Canfield, 2004].

Primitive undergassed basalts, as well as sulfide inclusions in mantle xenoliths and in eclogite-type diamonds display a relatively large variation in δ34S (Seal, 2006) providing evidence for significant mantle heterogeneities in S isotopes.  It has been speculated that recycling of sedimentary sulfur in subduction zones is in part responsible for these variations.  However, all models on the deep mantle sulfur cycle presented so far include the compositions of un-metamorphosed seafloor sediments to estimate the input into the mantle.  They simply ignore any processes operating during subduction.  Despite the unique geochemical importance of sulfur, surprisingly little is known on the behavior of sulfur and sulfur isotopes during high-pressure metamorphism and subduction zone devolatilization.  The proposed study aims to close this gap (Figure 1), and investigate the S isotope composition of sulfides and sulfate components in natural high-pressure rock samples.  Studies of sulfur isotopes on non-volcanic subduction-related material are very rare, and no data on any HPM rocks have been published to date.  Hence, models on the global sulfur cycle and on secular changes in the surface sulfur budget are afflicted with a complete lack of knowledge on the efficiency and isotopic effects of subduction (Figure 2).