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

Abstract

1during metamorphism: newly-formed Fe-rich minerals allowed preserving bulk rock Fe compositions during metamorphic reactions and hampered any Fe isotope fractionation. Greenschists have δ 56 Fe 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.Keywords Fe isotopes · Metabasites · Subduction · HP-LT metamorphism · Blueschists · Eclogites · Greenschists · Basaltic protoliths IntroductionHigh-pressure/low-temperature (HP-LT) rocks are remnants of ancient subduction zones. They provide information on geochemical processes and deep fluid-rock interactions occurring at present-day active convergent margins. Fluid infiltration during high-and lowtemperature hydrothermal alteration of the oceanic crust at mid-ocean ridges and on the seafloor is responsible for the hydration of basic rocks and serpentinisation of the lithospheric mantle. During subduction, the hydrothermally altered oceanic crust dehydrates continuously and releases large amounts of H 2 O at a relatively shallow level in the subduction zone (50-80 km) (Schmidt and Poli 1998;Rüpke et al. 2004). Deserpentinisation of the lithospheric mantle occurs at deeper levels (100-200 km), where fluids are expected to be the source of arc melting (Ulmer and Abstract 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 δ 56 Fe 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 δ 56 Fe 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 δ 56 Fe by +0.06 to 0.10‰ compared to basalts may reflect hydrothermal alteration prior to subduction. The δ 56 Fe 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

Uranium isotope fractionation during slab dehydration beneath the Izu arc

“…Isotope fractionation during fluid release from subducted slabs has also been proposed for stable isotope systems such as Fe (Debret et al, 2016;Inglis et al, 2017;Williams et al, 2018), Zn (Pons et al, 2016), and Mo (Freymuth et al, 2015). While additional examples exist where stable isotope compositions appear to be related to source inputs (El Korh et al, 2017;Nielsen et al, 2016), this suggests that stable isotope fractionation within slabs is common during subduction. It also highlights the use of stable isotope systems as tracers for processes acting within the subducted slabs and contrasts these with the more commonly used radiogenic isotope systems that exclusively trace input components.…”

Iron and zinc stable isotope evidence for open-system high-pressure dehydration of antigorite serpentinite in subduction zones

“…Furthermore, the effects of seafloor alteration (e.g., serpentinization) on Fe and to a lesser extent on Zn isotope systematics of mantle peridotites are comparatively minor (Craddock et al, 2013;Debret et al, 2018a), such that subduction-related fractionation processes should be relatively straightforward to identify. Recent studies have successfully applied Fe and Zn stable isotopes in subduction settings to identify processes associated with high-pressure metamorphism in metabasites, meta-serpentinites and metasedimentary rocks (e.g., Debret et al, 2016;Pons et al, 2016;Inglis et al, 2017;El Korh et al, 2017;Debret et al, 2018b;Turner et al, 2018;Gerrits et al, 2019;Huang et al, 2019;Chen et al, 2019;Debret et al, 2020). In these studies, Fe and Zn stable isotopic variations were attributed to redox reactions and associated metal mobility in sulfur, carbon and chlorine-bearing fluids during metamorphic processes.…”

Redistribution of Iron and Titanium in High‐Pressure Ultramafic Rocks

“…Iron is usually perceived to be immobile in subduction zone environments because fluids are considered to be of low salinity (El Korh et al, ; Manning, ). However, if Ti, also usually considered to have limited mobility, is in fact mobile, as discussed above, then transport of other “immobile” elements such as Fe 3+ may also be possible.…”