Iron isotope fractionation in subduction-related high-pressure metabasites (Ile de Groix, France)
- 916 Downloads
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.
KeywordsFe isotopes Metabasites Subduction HP–LT metamorphism Blueschists Eclogites Greenschists Basaltic protoliths
This study benefited from constructive reviews of Oliver Nebel and Paolo Sossi. The editorial work of Othmar Müntener was greatly appreciated. Thanks to Stephanie Hayman for having read the English text. The research was supported by project P300P2_147749 of the Swiss National Science Foundation, and benefited from fundings from the Observatoire de la Terre et de l’Environnement en Lorraine (OTELo-CNRS) and the TelluS-SYSTER program from the Institut National des Sciences de l’Univers (INSU).
- Audren C, Triboulet C, Chauris L, Lefort JP, Vigneresse JL, Audrain J, Thiéblemont D, Goyallon J, Jégouzo P, Guennoc P, Augris C, Carn A (1993) Notice explicative de la feuille Ile de Groix à 1/25000, carte géologique. BRGM, OrléansGoogle Scholar
- El Korh A (2006) Métamorphisme HP–BT dans les métabasites de l’Ile de Groix, France: étude pétrologique et géochimique. Unpublished master thesis, University of Geneva, p 334Google Scholar
- El Korh A, Schmidt STh, Vennemann T, Ulianov A (2011) Trace element and O-isotope composition of polyphase metamorphic veins of the Ile de Groix (Armorican Massif, France): implication for fluid flow during HP subduction and exhumation processes. In: Dobrzhinetskaya L, Faryad W, Wallis S, Cuthbert S (eds) Ultrahigh pressure metamorphism: 25 years after discovery of coesite and diamond. Elsevier, Amsterdam, pp 243–291CrossRefGoogle Scholar
- Jenner FE, O’Neill HSC (2012) Analysis of 60 elements in 616 ocean floor basaltic glasses. Geochem Geophys Geosyst 13:2Google Scholar
- Kretz R (1983) Symbols for rock-forming minerals. Am Miner 68:277–279Google Scholar
- Pearce JA (1996) A user’s guide to basalt discrimination diagrams. In: Wyman DA (ed) Trace element geochemistry of volcanic rocks: applications for massive sulphide exploration, vol 12. Geological Association of Canada, Short Course Notes, St. John's, Newfoundland, pp 79–113Google Scholar
- Sun SS, McDonough WF (1989) Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders AD, Norry MJ (eds) Magmatism in ocean basins, vol 42. Geological Society of London Special Publication, pp 313–345Google Scholar
- Williams HM, Nielsen SG, Renac C, Griffin WL, O’Reilly SY, McCammon CA, Pearson N, Viljoen F, Alt JC, Halliday AN (2009) Fractionation of oxygen and iron isotopes by partial melting processes: implications for the interpretation of stable isotope signatures in mafic rocks. Earth Planet Sci Lett 283:156–166CrossRefGoogle Scholar