Fe isotope fractionation in high-pressure Variscan rocks

High-pressure terrains are witnesses of palaeo-plate suture zones in collision belts. They provide evidence for a complex metamorphic history and various stages of fluid–rock interactions during subduction and exhumation of ancient oceanic lithosphere. Devolatilisation reactions during subduction zone metamorphism release large amounts of fluids and volatiles, which may generate significant mass transfer from the subducted crust to the overlying mantle wedge. This large-scale mantle metasomatism results in compositional and isotopic heterogeneities. In particular, dehydration-redox reactions taking place in the subducting crust are responsible for a transfer of redox-sensitive and/or mobile elements from the oceanic lithosphere to the mantle wedge. These processes are responsible for the distinctive geochemistry and redox state of arc magmas, which are shown to be oxidised and enriched in large ion lithophile elements (LILE: Cs, Rb, Ba), Li, B and light rare earth elements (LREE) compared to mid-ocean ridge basalts.

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Non-traditional Fe stable isotopes were recently employed as tracers of mass transfer processes, because of the strong sensitivity of Fe isotope fractionation to changes in redox conditions during magmatic and hydrothermal processes. The project employs a multi-disciplinary approach, combining petrology, bulk rock and mineral geochemistry, and stable isotope geochemistry (Fe, O), with the aim to characterise geochemical cycling during fluid-rock interactions related to subduction zone metamorphism in Variscan high-pressure rocks (eclogites, paragneiss) from two European Palaeozoic basements:

1) the Münchberg massif (Saxothuringian zone, Bohemian Massif, Germany) (SNF project; PhD research of Johannes Pohlner);

2) the Alps (Moldanubian zone, Adula Nappe, Switzerland).

 

Trace element and isotopic data will allow tracing metamorphic processes during subduction zone metamorphism, metasomatic mass transfer and deep mantle hydration in palaeo-subduction zones, with inferred consequences for the Fe isotopic composition of igneous rocks and bulk Earth mantle. The data will bring information on the source, composition, temperature and redox conditions of fluids derived from hydrothermally altered basic and metasedimentary rocks, and will further our understanding on metasomatic processes occurring in modern subducted crust.

Fe isotope compositions will be compared with contemporaneous rocks from other localities of the Moldanubian zone (Armorican Massif, French Massif Central) in the Variscan belt. The new geochemical data on basic and ultrabasic magmatic precursors will allow a better characterisation on the pre-Variscan magmatic event that took place during the Cambrian-Ordovician on the northern margin of the Gondwana (magmatic source(s), geodynamical setting).

Duration: 2018–2021

Funded by: Swiss National Science Foundation (SNF), University of Fribourg

Collaborators: PD Dr. Afifé El Korh, Prof. Bernard Grobéty, Johannes Pohlner (PhD student)

Research partners: Dr. Massimo Chiaradia (University of Geneva), Prof. Jörg Hermann (University of Bern), Prof. Reiner Klemd (University of Erlangen-Nürnberg, Germany), Prof. Torsten Vennemann (University of Lausanne)

Contact:

PD Dr. Afifé El Korh
Earth Sciences
Department of Geosciences
University of Fribourg
Chemin du Musée 6

1700 Fribourg
 +41 26 300 89 45
  Email