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Iron isotope fractionation in subduction-related high-pressure metabasites (Ile de Groix, France)

  • El Korh, Afifé Centre de Recherches Pétrographiques et Géochimiques, CNRS-Université de LorraineVandœuvre-Lès-Nancy, France - Department of Geosciences, University of Fribourg, Switzerland
  • Luais, Béatrice Centre de Recherches Pétrographiques et Géochimiques, CNRS-Université de LorraineVandœuvre-Lès-Nancy, France
  • Deloule, Etienne Centre de Recherches Pétrographiques et Géochimiques, CNRS-Université de LorraineVandœuvre-Lès-Nancy, France
  • Cividini, Damien Centre de Recherches Pétrographiques et Géochimiques, CNRS-Université de LorraineVandœuvre-Lès-Nancy, France
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    01.06.2017
Published in:
  • Contributions to Mineralogy and Petrology. - 2017, vol. 172, no. 6, p. 41
English Characterisation of mass transfer during subduction is fundamental to understand the origin of compositional heterogeneities in the upper mantle. Fe isotopes were measured in high-pressure/low-temperature metabasites (blueschists, eclogites and retrograde greenschists) from the Ile de Groix (France), a Variscan high-pressure terrane, to determine if the subducted oceanic crust contributes to mantle Fe isotope heterogeneities. The metabasites have δ56Fe values of +0.16 to +0.33‰, which are heavier than typical values of MORB and OIB, indicating that their basaltic protolith derives from a heavy-Fe mantle source. The δ56Fe correlates well with Y/Nb and (La/Sm)PM ratios, which commonly fractionate during magmatic processes, highlighting variations in the magmatic protolith composition. In addition, the shift of δ56Fe by +0.06 to 0.10‰ compared to basalts may reflect hydrothermal alteration prior to subduction. The δ56Fe decrease from blueschists (+0.19 ± 0.03 to +0.33 ± 0.01‰) to eclogites (+0.16 ± 0.02 to +0.18 ± 0.03‰) reflects small variations in the protolith composition, rather than Fe fractionation during metamorphism: newly- formed Fe-rich minerals allowed preserving bulk rock Fe compositions during metamorphic reactions and hampered any Fe isotope fractionation. Greenschists have δ56Fe values (+0.17 ± 0.01 to +0.27 ± 0.02‰) similar to high-pressure rocks. Hence, metasomatism related to fluids derived from the subducted hydrothermally altered metabasites might only have a limited effect on mantle Fe isotope composition under subsolidus conditions, owing to the large stability of Fe-rich minerals and low mobility of Fe. Subsequent melting of the heavy-Fe metabasites at deeper levels is expected to generate mantle Fe isotope heterogeneities.
Faculty
Faculté des sciences et de médecine
Department
Département de Géosciences
Language
  • English
Classification
Mineralogy
License
License undefined
Identifiers
Persistent URL
https://folia.unifr.ch/unifr/documents/306039
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