Uncovering and quantifying the subduction zone sulfur cycle from the slab perspective

Abstract

Sulfur belongs among H 2 O, CO 2 , and Cl as one of the key volatiles in Earth's chemical cycles. High oxygen fugacity, sulfur concentration, and δ 34 S values in volcanic arc rocks have been attributed to significant sulfate addition by slab fluids. However, sulfur speciation, flux, and isotope composition in slab-dehydrated fluids remain unclear. Here, we use high-pressure rocks and enclosed veins to provide direct constraints on subduction zone sulfur recycling for a typical oceanic lithosphere. Textural and thermodynamic evidence indicates the predominance of reduced sulfur species in slab fluids; those derived from metasediments, altered oceanic crust, and serpentinite have δ 34 S values of approximately −8‰, −1‰, and +8‰, respectively. Mass-balance calculations demonstrate that 6.4% (up to 20% maximum) of total subducted sulfur is released between 30-230 km depth, and the predominant sulfur loss takes place at 70-100 km with a net δ 34 S composition of −2.5 ± 3‰. We conclude that modest slab-to-wedge sulfur transport occurs, but that slab-derived fluids provide negligible sulfate to oxidize the sub-arc mantle and cannot deliver 34 S-enriched sulfur to produce the positive δ 34 S signature in arc settings. Most sulfur has negative δ 34 S and is subducted into the deep mantle, which could cause a long-term increase in the δ 34 S of Earth surface reservoirs.

The intrinsic nature of antigorite breakdown at 3 GPa: Experimental constraints on redox conditions of serpentinite dehydration in subduction zones

“…For example, Philippot and Selverstone (1992) report the presence of anhydrite (CaSO4) and hematite in fluid inclusions within omphacites in eclogites from the Alps, but anhydrite coexisting with pyrite seems to be common to many inclusions worldwide (Frezzotti and Ferrando, 2015). Still, the picture is probably more complicated as evidences for reduced fluids also exist in the form of CH4 inclusions (Song et al, 2009), precipitation of graphite (Malvoisin et al, 2011;Galvez et al, 2013) or sulfide veins (Li et al, 2020). We tested the effect of our experimental strategy on the phase equilibria by varying : i) furnace material (lanthanum chromite versus graphite), ii) capsule material, iii) heating duration and iv) presence of an olivine trap.…”

Dehydration at subduction zones and the geochemistry of slab fluids

“…Aqueous fluids at shallow depths (<25 km) can be directly sampled from ocean drilling or mud volcanoes (Manning, 2004). Information about greater depths comes from high-pressure (HP) and ultrahigh-pressure (UHP) rocks (and the veins they contain) sparsely exhumed from oceanic subduction zones (Gao et al, 2007;Li et al, 2013Li et al, , 2016Li et al, , 2020Su et al, 2019). More common continental HP and UHP rocks especially with fluid inclusions and multiphase solid inclusions can supplement composition information of deep fluids (Frezzotti and Ferrando, 2015;Zheng and Hermann, 2014).…”

“…Alteration by seawater only produces minor sulfate at the topmost oceanic crust (Alt and Shanks, 2011;Tomkins and Evans, 2015), and for the most part S in slabs is mainly stored in sulfides. Unlike carbon and nitrogen, the primary reservoir of sulfur is the oceanic crust with 71% of the total subducted sulfur, whereas sediments and the lithospheric mantle only account for 23% and 6%, respectively (Li et al, 2020). Anhydrite is representative of sulfate minerals in the slab.…”

“…Despite discrepancies in details, it is generally accepted that sulfate dissolution and sulfide dissolution result in two major episodes of sulfur release by slab (Tomkins and Evans, 2015;Walters et al, 2019). Based on the observation that sulfur in HP and UHP rocks in Southwest Tianshan is predominantly in sulfides, Li et al (2020) ). Sulfur released by slab at 30-230 km only accounts for 6.4% of total subducted sulfur.…”

“…Li et al (2016) inferred the oxygen fugacity of sub-arc fluid to be below QFM based on low oxygen fugacity for HP veins in eclogite. In a petrological and sulfur isotope study of sulfide-bearing HP metasedimentary rocks, altered oceanic crust, serpentinite and veins, Li et al (2020) concluded that sulfur in slab fluids was mainly S 2− during prograde metamorphism. Tao et al (2018) calculated the oxygen fugacity of carbonated eclogite to be QFM−2 at 2.5 GPa.…”