Establishing apatite as a redox sensor in subducting slabs
Final Report Abstract
Plate tectonics drives the chemical exchange between Earth’s surface and interior. At subduction zones, the oceanic crust sinks into the Earth’s mantle. A key geochemical component of subduction is the recycling of multivalent elements, such as C and S, from the subducting plate (or slab) into the mantle and arc volcanoes. Multivalent elements can either be in their oxidized state (fewer electrons) or reduced state (more electrons), such as metallic iron, which is reduced, and rust, which is oxidized. Arc magmas and the subarc mantle are uniquely oxidized relative to their equivalents in other tectonic settings. The oxidized nature of these rocks and magmas influences their physical and chemical properties. For example, a widely accepted model of the formation of massive copper (± molybdenum ± silver ± gold) deposits requires the presence of oxidized magmas formed in subduction zone settings. Many studies of volcanic arc rocks suggest that arc magmas are oxidized by fluxes of sulfate bearing fluids from the slab. However, there is no consensus, and researchers propose both reduced and oxidized species in slab fluids. To address this knowledge gap, we measured the oxidation state of sulfur in samples of the mineral apatite from exhumed subduction zone rocks. Apatite is a unique indicator of the oxidation state of sulfur, as it can incorporate both reduced and oxidized sulfur into its crystal structure. Chemical reactions that release sulfur from the slab are likely coupled to iron, which is the most abundant multivalent element in the slab. Consequently, we also measured the oxidation state of iron in different minerals to constrain what reactions may release oxidized or reduced sulfur. We found that the deepest stage of subduction in our rocks is characterized by oxidized sulfur in apatite and an increase in the amount of reduced iron in other minerals. These observations are consistent with reactions that oxidized sulfur and reduce iron. The oxidized sulfur is dissolved into hydrous fluids and transported from the slab. Much of the sulfur is redeposited as sulfide minerals at the interface between the subducting plate and the overlying mantle. This process oxidizes iron and traps sulfur before it can reach the depth of arc magma generation. Physical transport of these oxidized sulfide-bearing rocks into the mantle through melting or buoyant rising is likely required to transport sulfur from the slab into arc volcanoes.
Publications
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Sulfur in the slab: A sulfur-isotopes and thermodynamic-modeling perspective from exhumed terranes. Copernicus GmbH.
Walters, Jesse; Cruz-Uribe, Alicia & Marschall, Horst
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The deep sulfur cycle: What do exhumed high-pressure rocks tell us? 14th International Eclogite Conference, Lyon
Walters, J.B., Cruz-Uribe, A.M. & Marschall, H.R.
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Apatite as a monitor for sulfur redox reactions during fluid-rock interaction in the subduction channel. Copernicus GmbH.
Walters, Jesse; Marschall, Horst; Grützner-Handke, Tobias; Klimm, Kevin; Konecke, Brian & Simon, Adam
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Oxidized slab fluids recorded by sulfur-in-apatite: A case study from Syros, Greece. Goldschmidt2023 abstracts. European Association of Geochemistry.
Marschall, Horst; Walters, Jesse; Grützner, Tobias; Klimm, Kevin; Konecke, Brian & Simon, Adam
