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Interplay of crystal fractionation, sulfide saturation and oxygen fugacity on the iron isotope composition of arc lavas : an example from the Marianas.

Williams, H.M. and Prytulak, J. and Woodhead, J.D. and Kelley, K.A. and Brounce, M. and Plank, T. (2018) 'Interplay of crystal fractionation, sulfide saturation and oxygen fugacity on the iron isotope composition of arc lavas : an example from the Marianas.', Geochimica et cosmochimica acta., 226 . pp. 224-243.

Abstract

Subduction zone systems are central to a multitude of processes from the evolution of the continental crust to the concentration of metals into economically viable deposits. The interplay between oxygen fugacity, sulfur saturation, fluid exsolution and fractionating mineral assemblages that gives rise to typical arc magma chemical signatures is, however, still poorly understood and novel geochemical approaches are required to make further progress. Here we examine a well-characterized suite of arc lavas from the Marianas (W. Pacific) for their stable Fe isotope composition. In agreement with previous work and mass balance considerations, contributions from sediments and/or fluids are shown to have negligible effect on Fe isotopes. Instead, we focus on disentangling processes occurring during basalt through dacite differentiation using a sample suite from the island of Anatahan. Anatahan whole rock Fe isotope compositions (δ57Fe) range from −0.05 ± 0.05 to 0.17 ± 0.03 (2 S.D.)‰. A fractionation model is constructed, where three distinct stages of differentiation are required to satisfy the combined major and trace element and isotopic observations. In particular, the sequestration of isotopically heavy Fe into magnetite and isotopically light Fe into sulfide melts yields important constraints. The data require that lavas are first undersaturated with respect to crystalline or molten sulfide, followed by the crystallisation of magnetite, which then triggers late sulfide saturation. The model demonstrates that the final stage of removal of liquid or crystalline sulfide can effectively sequester Cu (and presumably other chalcophiles) and that late stage exsolution of magmatic fluids or brines may not be required to do this, although these processes are not mutually exclusive. Finally, the new Fe isotope data are combined with previous Tl-Mo-V stable isotope determinations on the same samples. Importantly, the multi-valent transition metal stable isotope systems of Fe and V are decoupled by sulfide saturation, thus providing a potential tool to constrain its somewhat intractable timing. The observed decoupling of notionally redox-sensitive tracers underlines the caution required in the application of transition metal isotopes as direct redox proxies.

Item Type:Article
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Available under License - Creative Commons Attribution.
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1016/j.gca.2018.02.008
Publisher statement:© 2018 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Date accepted:02 February 2018
Date deposited:19 April 2018
Date of first online publication:13 February 2018
Date first made open access:19 April 2018

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