Release of oxidizing fluids in subduction zones recorded by iron isotope zonation in garnet

Abstract

Subduction zones are key regions of chemical and mass transfer between the Earth’s surface and mantle. During subduction, oxidized material is carried into the mantle and large amounts of water are released due to the breakdown of hydrous minerals such as lawsonite. Dehydration accompanied by the release of oxidizing species may play a key role in controlling redox changes in the subducting slab and overlying mantle wedge. Here we present measurements of oxygen fugacity, using garnet–epidote oxybarometry, together with analyses of the stable iron isotope composition of zoned garnets from Sifnos, Greece. We find that the garnet interiors grew under relatively oxidized conditions whereas garnet rims record more reduced conditions. Garnet δ56Fe increases from core to rim as the system becomes more reduced. Thermodynamic analysis shows that this change from relatively oxidized to more reduced conditions occurred during lawsonite dehydration. We conclude that the garnets maintain a record of progressive dehydration and that the residual mineral assemblages within the slab became more reduced during progressive subduction-zone dehydration. This is consistent with the hypothesis that lawsonite dehydration accompanied by the release of oxidizing species, such as sulfate, plays an important and measurable role in the global redox budget and contributes to sub-arc mantle oxidation in subduction zones.

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Fig. 1: Backscattered electron imagery, and Fe isotope and oxybarometry data, for an analysed garnet from sample 09DSF-23E.
Fig. 2: Fe isotope data, presented as δ56Fe values, plotted against the Δ log FMQ values for each zone (core, intermediate zones, rim).
Fig. 3: Pressure–temperature and mineralogical evolution of the Sifnos samples and model for the release of oxidizing fluids during subduction.

Data availability

The data generated or analysed during this study are included in this published article and its Supplementary Information files.

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Acknowledgements

E.F.B. acknowledges support from NSF grants EAR-0547999 for sample collection and OIA-1545903 for support during this project. A.R.G., B.D. and P.G.S. also acknowledge support from OIA-1545903. E.C.I. was supported as a postdoctoral research assistant on ERC starting grant PRISTINE: 637503 awarded to F. Moynier (IPG Paris).

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A.R.G. was responsible for the oxybarometry calculations, clean-lab preparation and analysis of Fe isotope compositions and wrote the manuscript. E.C.I. performed Fe isotope compositional analysis and contributed to method development. B.D. was responsible for PT modelling. P.G.S. performed clean-lab preparation, oxybarometry calculations and method development. E.F.B. and K.W.B. contributed by designing the project. All authors contributed to analysis and interpretation of data and editing of the manuscript.

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Correspondence to Ethan F. Baxter.

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Gerrits, A.R., Inglis, E.C., Dragovic, B. et al. Release of oxidizing fluids in subduction zones recorded by iron isotope zonation in garnet. Nat. Geosci. 12, 1029–1033 (2019). https://doi.org/10.1038/s41561-019-0471-y

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