Isotopic Compositions of Sulfides in Exhumed High‐Pressure Terranes: Implications for Sulfur Cycling in Subduction Zones

Abstract

Subduction is a key component of Earth's long‐term sulfur cycle; however, the mechanisms that drive sulfur from subducting slabs remain elusive. Isotopes are a sensitive indicator of the speciation of sulfur in fluids, sulfide dissolution‐precipitation reactions, and inferring fluid sources. To investigate these processes, we report δ34S values determined by secondary ion mass spectroscopy in sulfides from a global suite of exhumed high‐pressure rocks. Sulfides are classified into two petrogenetic groups: (1) metamorphic, which represent closed‐system (re)crystallization from protolith‐inherited sulfur, and (2) metasomatic, which formed during open system processes, such as an influx of oxidized sulfur. The δ34S values for metamorphic sulfides tend to reflect their precursor compositions: −4.3 ‰ to +13.5 ‰ for metabasic rocks, and −32.4 ‰ to −11.0 ‰ for metasediments. Metasomatic sulfides exhibit a range of δ34S from −21.7 ‰ to +13.9 ‰. We suggest that sluggish sulfur self‐diffusion prevents isotopic fractionation during sulfide breakdown and that slab fluids inherit the isotopic composition of their source. We estimate a composition of −11 ‰ to +8 ‰ for slab fluids, a significantly smaller range than observed for metasomatic sulfides. Large fractionations during metasomatic sulfide precipitation from sulfate‐bearing fluids, and an evolving fluid composition during reactive transport may account for the entire ~36 ‰ range of metasomatic sulfide compositions. Thus, we suggest that sulfates are likely the dominant sulfur species in slab‐derived fluids.

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

“…4a; Supplementary Table 1), and previous experimental results 11 . If slab fluids are dominated by sulfate as some recent studies propose 13 , several predictions follow. First, the oxidizing fluid will produce a redox gradient in the immediate wallrock, but this is not recorded in the selvages we examined.…”

“…3) are similar to their protoliths, the young marine sedimentary rocks (Phanerozoic) that mostly have δ 34 S values of −24 to −8‰ due to the presence of biogenically produced sulfide 19,44 . It is suggested that sulfide in sediments retains its δ 34 S characteristics during subduction metamorphism 13 and that metasediments may act as a negative δ 34 S reservior in subducting slabs. The vein pyrite core with negative δ 34 S (−8‰) in sample L1013 (Fig.…”

“…Experimental results suggest that slab-derived aqueous fluids are an effective agent for transporting sulfur from the slab to the mantle wedge 11,12 . In addition, some studies predict that sulfates are likely the dominant sulfur species in slab-derived fluids 9,13 . On the other hand, sulfate is relatively rare in high-pressure (HP) rocks [13][14][15][16][17][18] , and experimental studies have proposed that reduced sulfur species are dominant in slab fluids 11 .…”

“…In addition, some studies predict that sulfates are likely the dominant sulfur species in slab-derived fluids 9,13 . On the other hand, sulfate is relatively rare in high-pressure (HP) rocks [13][14][15][16][17][18] , and experimental studies have proposed that reduced sulfur species are dominant in slab fluids 11 . Furthermore, in situ measurements of the δ 34 S compositions of sulfides from HP eclogites and serpentinites reveal significant isotopic heterogeneity and complicated sulfur behavior during slab metamorphism and metasomatism [13][14][15][16] .…”

“…Sulfur is transported into the subduction zone by sediments, variably altered oceanic crust (AOC), and hydrated slab mantle (serpentinites) [19][20][21][22] . Sulfides are commonly observed in exhumed fragments of the oceanic lithosphere such as eclogites, blueschists, HP-metapelites, and serpentinites, as well as related HP veins [13][14][15][16][17][18]23 . Such vein systems represent fossilized pathways for channelized flow of dehydration-related slab fluids and, thus, directly record fluid geochemical signatures 17,24,25 .…”