Tracking sulphur cycling in subduction zones by in situ trace element and δ34S signatures in sulphides

• Applicant: Professorin Dr. Esther Schwarzenbach
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 438104271

In this project we propose to study the in situ trace element and δ34S compositions of sulphide minerals in combination with new and previously published bulk rock trace element and δ34S data, and petrological observations to develop a new tracer for S sources and S cycling processes in subduction zones. The focus of this study will be to characterize the chemical signature of the incoming plate with emphasis on both mafic and ultramafic lithologies produced along fast- and slow-spreading ridges, and their representative, exhumed high-pressure equivalents from subduction zones (blueschists, eclogites, antigorite-serpentinites) as well as dehydration-related veins. Sample locations for unmetamorphosed ocean floor include well-characterized ODP (Ocean Drilling Program) sites, amongst others, rocks that formed along the East Pacific Rise and the Mid-Atlantic Ridge. In addition, peridotites from the Mirdita ophiolite in Albania, which preserves an up to 10 km thick section of Jurassic seafloor, will provide insight into the trace element and sulphur isotopic signatures of the upper mantle underlying fast-spreading oceanic crust. This sample selection will allow discrimination of S fluxes from different lithologies (mafic versus ultramafic), different hydrothermal systems (high-T versus low-T) and different tectonic settings (fast- versus slow-spreading ocean crust) by subduction of altered oceanic lithosphere.Target areas for subduction-related samples are the Raspas complex in Ecuador that comprises a complete section of subducted oceanic lithosphere including metasediments; the Voltri Massif in Italy where the focus will be on exhumed antigorite-serpentinites and partly dehydrated olivine-bearing serpentinites; and the Pouébo Eclogite Mélange in New Caledonia that includes dehydration-related veins formed by internally derived and externally derived fluids while the slab was subducting.Comparison between in situ and bulk rock trace element and δ34S signatures of oceanic lithosphere before and after subduction and exhumation will provide insight into the S fluxes into Earth’s interior and how these fluxes depend on tectonic setting and may have changed over time. In addition, subduction-related samples will provide insight into the effects of subduction metamorphism on S and trace element fractionation and S cycling processes in subduction zones. Using thermodynamic modelling we further aim to evaluate the impact of these processes on the redox budget of the mantle and provide new insights into ore forming processes within arc settings. Overall, this study can shed new light into the effective impact of subduction of altered oceanic lithosphere on the global sulphur cycle and the evolution of the budget and isotopic composition of the Earth’s internal and external sulphur reservoirs.

• DFG Programme: Research Grants
• International Connection: Albania, Sweden, Switzerland, USA
• Co-Investigator: Professor Dr. Timm John
• Cooperation Partners: Professor Dr. Jeffrey Alt; Dr. Kujtim Onuzi; Professor Dr. Martin Whitehouse, Ph.D.