Abstract
Subduction zones impose an important control on the geochemical cycling between the surficial and internal reservoirs of the Earth. Sulphur and carbon are transferred into Earth’s mantle by subduction of pelagic sediments and altered oceanic lithosphere. Release of oxidizing sulphate- and carbonate-bearing fluids modifies the redox state of the mantle and the chemical budget of subduction zones. Yet, the mechanisms of sulphur and carbon cycling within subduction zones are still unclear, in part because data are typically derived from arc volcanoes where fluid compositions are modified during transport through the mantle wedge. We determined the bulk rock elemental, and sulphur and carbon isotope compositions of exhumed ultramafic and metabasic rocks from Syros, Greece. Comparison of isotopic data with major and trace element compositions indicates seawater alteration and chemical exchange with sediment-derived fluids within the subduction zone channel. We show that small bodies of detached slab material are subject to metasomatic processes during exhumation, in contrast to large sequences of obducted ophiolitic sections that retain their seafloor alteration signatures. In particular, fluids circulating along the plate interface can cause sulphur mobilization during several stages of exhumation within high-pressure rocks. This takes place more pervasively in serpentinites compared to mafic rocks.
Highlights
Subduction of altered oceanic lithosphere returns water, C, S, B, and numerous other volatiles to Earth’s mantle (e.g.1–3)
Evidence for volatile transfer from slab to mantle wedge has been primarily inferred from arc-related tracers, such as 34S-enriched sulphur signatures in arc volcanoes that likely originate from subducted seawater sulphate[8], the oxidized state of arc magmas, and oxidizing, sulphate-bearing veins within porphyry deposits
These tracers retain an imprint from chemical interaction within the sub-arc mantle and do not provide direct evidence of sulphur and carbon cycling along the slab-mantle wedge interface
Summary
Evidence for volatile transfer from slab to mantle wedge has been primarily inferred from arc-related tracers, such as 34S-enriched sulphur signatures in arc volcanoes that likely originate from subducted seawater sulphate[8], the oxidized state of arc magmas, and oxidizing, sulphate-bearing veins within porphyry deposits (see references in[9]) These tracers retain an imprint from chemical interaction within the sub-arc mantle and do not provide direct evidence of sulphur and carbon cycling along the slab-mantle wedge interface. Due to the difficulty in detecting this sulphate, its impact during subduction metamorphism is often neglected and fails to capture the behaviour of the bulk sulphur species within both seafloor altered serpentinites and high-pressure equivalents To better understand both sulphur and carbon fluxes within subduction complexes, we investigated exhumed mafic and ultramafic samples from Syros, Greece. The presence of sulphur in both oxidized (sulphate) and reduced (sulphide) states in altered oceanic lithosphere makes it an ideal tool for tracking fluid sources and redox conditions, and allows us to infer mechanisms of element redistribution in the subduction channel mélange
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