Abstract
The sulfur cycle at convergent margins remains poorly constrained yet is fundamentally important for understanding the redox state of Earth's reservoirs and the formation of ore deposits. In this study we investigate the sulfur isotope composition of high temperature volcanic gases emitted from the Nicaraguan (average of +4.8 ± 1.3‰) and Costa Rican (average of +2.3 ± 1.3‰) arc segments contributing to emissions from the Southern Central American Volcanic Arc (SCAVA; average of +3.8 ± 1.7‰). Along-arc variations in geochemical tracers at SCAVA are widely accepted to reflect variations in subduction parameters and deep fluid sources and correlations between these parameters and gas S isotope compositions are observed. These correlations suggest that gas emissions are sourced from a mixture of mantle S with δ34S ~ 0‰ and isotopically heavy slab-derived sulfur with δ34S ≥ ~ +8‰. We employ Monte Carlo mass balance modeling to constrain S inputs to the subduction zone and relative contributions from mantle and slab to arc sulfur emissions. The models indicate that bulk subduction input in Nicaragua has a S isotope composition of +1.4 ± 0.5‰ compared to −0.2 ± 0.4‰ in Costa Rica, requiring preferential release of isotopically heavy oxidized S from the slab to explain the relatively high δ34S observed in arc outputs. We show that the flux of S from the slab is sufficient to oxidize the entire mantle wedge within the lifetime of the arc, indicating that S is a primary oxidizing agent in subduction zones. Furthermore, the preferential removal of heavy S from the slab requires retention of isotopically light S in the residual slab. Subduction-scale fractionation of S isotopes is fundamentally important in explaining why Earth's bulk surface reservoirs are isotopically positive.
Highlights
Sulfur plays a fundamental role in the redox state of Earth’s reser voirs and has played a key role in the evolution of life
In this study we investigate S isotope compositions of high temper ature volcanic gases emitted at the Southern Central America Volcanic Arc (SCAVA)
20% to 40% of the subducted S is released from the slab and the heavy isotopic composition of this relatively oxidized sfltuaitde.sHugogweestvsetrh, atthesuSlf6u+r/i∑s dSovmailnuaenitnlythreelebauslekdsiunbaducted slab is low and in the range of 0.09 to 0.12
Summary
Sulfur plays a fundamental role in the redox state of Earth’s reser voirs and has played a key role in the evolution of life. Large explosive volcanic eruptions can cause global climate cooling due to the injection of sulfur into the stratosphere Sigurdsson, 1990), and understanding the degassing mechanisms of sulfurous gases is crucial for volcanic eruption forecasting using gas monitoring The subduction process provides the primary mechanism for formation of continental crust and modification of the Earth’s deep mantle and is fundamental to understanding global geochemical cycles. Volatiles released from metamorphic reactions driven by increasing T and P in the slab cause melting in the mantle wedge, forming arc magmas. These magmas are more oxidized than those formed at mid ocean ridges and intraplate settings These magmas are more oxidized than those formed at mid ocean ridges and intraplate settings (e.g. Kelley and Cottrell, 2009), typically carrying more water, sulfur, and metals
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