Redox transfer at subduction zones: insights from Fe isotopes in the Mariana forearc

Abstract

Subduction zones are active sites of chemical exchange between the Earth's surface and deep interior and play a fundamental role in regulating planet habitability. However, the mechanisms by which redox sensitive elements (e.g., iron, carbon and sulfur) are cycled during subduction remains unclear. Here we use Fe stable isotopes (δ 56 Fe), which are sensitive to redox-related processes, to examine forearc serpentinite clasts recovered from deep sea drilling of mud volcanoes formed above the Mariana subduction zone in the Western Pacific. We show that serpentinisation of the forearc by slab-derived fluids produces dramatic δ 56 Fe variation. Unexpected negative correlations between serpentinite bulk δ 56 Fe, fluid-mobile element concentrations (e.g., B, As) and Fe 3+ /ΣFe suggest a concomitant oxidation of the mantle wedge through the transfer of isotopically light iron by slab-derived fluids. This process must reflect the transfer of either sulfate-or carbonate-bearing fluids that preferentially complex isotopically light Fe.

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

“…However, the lack of significant Fe isotope variation in the Cerro del Almirez samples is surprising as isotopically light Fe is also expected to be mobile in carbonate-bearing fluids (Fujii et al, 2014). Indeed, previous studies working in forearc mantle wedge or shallow slab/mantle interface areas have shown that large Fe isotope variations in ultramafic rocks (down to -0.26 ‰) may be attributed to carbonate-bearing metasomatism (Inglis et al, 2017;Debret et al, 2018bDebret et al, , 2020 and the dissolution of Ca-(i.e., calcites, aragonite) and/or Fe-(i.e., siderite and ankerite) bearing carbonates in metamorphic fluids at low temperature (i.e., below 400 °C; Facq et al, 2014;Milesi et al, 2015). Both Ca-(down to -0.92 ‰; Craddock and Dauphas, 2011) and Fe-(down to -3.9 ‰; Belshaw et al, 2000;Johnson et al, 2008) bearing seafloor carbonates are known to concentrate isotopically light Fe relative to silicate and oxide phases.…”

“…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.…”

High-pressure synthesis and storage of solid organic compounds in active subduction zones

“…1A). Three types of serpentinized forearc peridotite clasts, namely, lizardite, antigorite, and blue serpentine-bearing clasts, were identified on the basis of their petrographic and geochemical features (35)(36)(37). These clasts record various stages of serpentinization from deep forearc mantle hydration by slab-derived fluids to shallow fluid-rock interaction during clast exhumation nearby the seafloor (Fig.…”

Heavy iron in large gem diamonds traces deep subduction of serpentinized ocean floor

“…However, although this shallow fluid loss apparently strengthens the attribution of a high  56 Fe signature to serpentinite, it is likely not relevant for CLIPPIR diamonds. Fluid evolution in this process removes carbon, hindering later diamond growth, and produces variably oxidizing conditions (32,33) that would be difficult to reconcile with the reduced nature of the metallic inclusions studied here. Such shallow devolatilization typically affects hotter slabs, whereas CLIPPIR diamonds are thought to be associated with the deep subduction of cooler slabs, whose partially serpentinized interiors remain hydrated to great depths, reaching the mantle transition zone (5-7) with their carbon budget intact.…”