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

• Published in: Nature communications Vol. 11; no. 1; p. 514
• Main Authors: Li, Ji-Lei, Schwarzenbach, Esther M., John, Timm, Ague, Jay J., Huang, Fang, Gao, Jun, Klemd, Reiner, Whitehouse, Martin J., Wang, Xin-Shui
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 24-01-2020; Nature Publishing Group; Nature Portfolio
• Subjects: 140/58; 147/28; 704/2151/209; 704/2151/431; Carbon dioxide; Chemical cycles; Dehydration; Den föränderliga jorden; Earth mantle; Earth surface; Fluids; Fugacity; Humanities and Social Sciences; Isotope composition; Lava; Lithosphere; multidisciplinary; Oceanic crust; Organic chemistry; Pressure; Recycling; Rocks; Science; Science (multidisciplinary); Speciation; Subduction (geology); Sulfates; Sulfur; Sulfur cycle; The changing Earth; Volatile compounds; Volatiles
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-019-14110-4

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.
Sulfur is one of the key volatiles in Earth’s chemical cycles; however, sulfur speciation, isotopic composition, and flux during the subduction cycle remain unclear. Here, the authors provide direct constraints on subduction zone sulfur recycling from high-pressure rocks and explore implications for arc magmatism.