EGU2020-1077, updated on 13 Jan 2021
https://doi.org/10.5194/egusphere-egu2020-1077
EGU General Assembly 2020
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

New constraints on the Sulfur isotope signature of the sub-continental lithospheric mantle wedge: in situ δ34S analyses of pentlandite from the exhumed orogenic garnet-bearing peridotite of the Ulten Zone, Eastern Italian Alps

Giulia Consuma1, Roberto Braga1, Marco L. Fiorentini2, Laure Martin3, Peter Tropper4, and Sonja Aulbach5
Giulia Consuma et al.
  • 1Department of Biological, Geological, and Environmental Sciences, University of Bologna, Bologna, Italy (giulia.consuma@unibo.it)
  • 2Centre for Exploration Targeting, ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS), University of Western Australia, Perth, Australia
  • 3Centre for Microscopy, Characterisation, and Analysis, ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS), University of Western Australia, Perth, Australia
  • 4Institute of Mineralogy and Petrology, University of Innsbruck, Innsbruck, Austria
  • 5Goethe University, Institut für Geowissenschaften, Facheinheit Mineralogie, Frankfurt am Main, Germany

Orogenic peridotites associated with high-grade felsic rocks record mass exchange between crust and mantle reservoirs at convergent plate margins. In this geodynamic framework, fluids released by submerging slabs can mobilize redox-sensitive elements such as carbon (C) and sulfur (S) and percolate the mantle wedge, eventually forming hydrate minerals associated with carbonate and sulfide phases at appropriate T, P and f O2 conditions. The introduction of sulfur into the sub-continental lithospheric mantle (SCLM) wedge and its mobilization at grain-scale can be investigated by means of in situ δ34S analyses of mantle wedge sulfides, which may have inherited the composition of the fluid sources. To date, the impact of the S transfer through the SCLM wedge is poorly known and limited in situ S isotope values of sulfides from mantle wedge peridotite are available in literature. Our study focuses on the Ulten Zone (UZ) orogenic-garnet peridotites, which provide an ideal case to investigate the S mobilization through the SCLM wedge and the effects of crustal fluids on the sulfide δ34S signature, especially during the exhumation stage. We therefore integrate a well-constrained paragenesis with mineral chemistry and in situ S isotope signature of sulfides. The UZ peridotites were involved in a collisional setting during the Variscan orogenesis, recording HP-eclogite-facies conditions and exhumation after their incorporation in a mélange with the associated garnet-kyanite gneisses. A suite of coarse to fine-grained peridotites was investigated in order to cover all the metasomatic stages preserved in these rocks, considering the grade of serpentinization and the occurrence of carbonates. Microstructural observations and major element compositions indicate that pentlandite (± chalcopyrite ± chalcocite ± sphalerite) is the ubiquitous primary sulfide, which is commonly replaced by secondary heazlewoodite and millerite in medium to highly serpentinized peridotite. Pentlandite occurs in different textural positions related to several metasomatic stages: (i) polycrystalline aggregates (pentlandite + Cl-apatite + phlogopite + ilmenite + calcite-brucite intergrowths) included in spinel (in garnet); (ii) interstitial in matrix; (iii) in carbonate and serpentine veins. Overall, the S isotope signature of pentlandite exhibits a relatively narrow range between -1.62 and +3.76 ‰. The relatively low S isotope values require a mantle-like source for the metasomatizing fluids enriched in sulfur, with possible contamination with fluids of other different sources. These new results show that sulfur was introduced into the lithospheric mantle and mobilized by influxes of late metasomatic fluids, in part related to the serpentinization, and provide additional constraints on the S isotope composition of the SCLM wedge.

How to cite: Consuma, G., Braga, R., Fiorentini, M. L., Martin, L., Tropper, P., and Aulbach, S.: New constraints on the Sulfur isotope signature of the sub-continental lithospheric mantle wedge: in situ δ34S analyses of pentlandite from the exhumed orogenic garnet-bearing peridotite of the Ulten Zone, Eastern Italian Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1077, https://doi.org/10.5194/egusphere-egu2020-1077, 2019

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