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

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 delta Fe-56 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 delta Fe-56 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 delta Fe-56 by +0.06 to 0.10% compared to basalts may reflect hydrothermal alteration prior to subduction. The delta Fe-56 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 delta Fe-56 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 sub-solidus 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.