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A sulfide perspective on iron isotope fractionation during ocean island basalt petrogenesis

TitleA sulfide perspective on iron isotope fractionation during ocean island basalt petrogenesis
Publication TypeJournal Article
Year of Publication2019
AuthorsPeters B.J, Shahar A., Carlson R.W, Day JMD, Mock T.D
JournalGeochimica Et Cosmochimica Acta
Date Published2019/01
Type of ArticleArticle
ISBN Number0016-7037
Accession NumberWOS:000453771600004
Keywordscrust; differentiation; Fe; ferric iron; fractional crystallization; fractionation; Geochemistry & Geophysics; Highly siderophile elements; indian-ocean; Intraplate magmatism; iron isotopes; isotope; Magma redox state; magmatic; Mantle geochemistry; mantle source; Ocean island basalt; oxygen fugacity; Recycled; Redox state; reunion; source regions; Sulfide segregation

Iron isotopic compositions are demonstrably powerful tracers of foundational planetary processes, including crust and core formation. In many volcanic environments, however, geochemical vestiges of these processes are obscured by the effects of magmatic differentiation on Fe isotopic compositions. Recent decades have witnessed continued refinement of observational and experimental approaches to Fe isotope fractionation during silicate differentiation. In contrast, the influence of sulfide fractionation on Fe isotopic compositions in terrestrial environments is known only from theoretical approaches and limited experimental data for relatively siliceous magmatic systems. One reason for this may be that sulfide fractionation is difficult to definitively trace using traditional major and minor element variation patterns. We utilize well-characterized lavas and cumulate xenoliths from Piton de la Fournaise and Piton des Neiges, Reunion Island, that have previously been examined for their highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) contents to investigate the effect of sulfide fractionation on Fe isotopes. The Fe isotopic compositions of the basalts range from delta Fe-56 values of 0.04 to 0.15 parts per thousand (average: 0.10 parts per thousand) and the compositions of the cumulate xenoliths range from delta Fe-56 values of -0.07 to 0.08 parts per thousand (average: 0 parts per thousand). In the absence of metal, HSE preferentially partition into sulfide phases, making them important tracers of sulfide segregation during magmatic differentiation. We find that commonly-observed co-variations between Fe isotopic compositions and major element oxide abundances are relatively underdeveloped for Reunion lavas. The correlation between Fe isotopic composition and MgO, for example, has a similar statistical significance to the correlation between Fe isotopic composition and Pd/Ir ratios, suggesting an important role of sulfides during Fe isotopic fractionation. After accounting for sulfide segregation, we determine that the parental magma Fe isotopic composition calculated for Piton de la Fournaise would be overestimated by 0.04 parts per thousand (within propagated error, 0.01-0.06 parts per thousand) when considering silicate differentiation alone. An analogous calculation for Kilauea Iki basalts, for which there is available Fe isotopic and HSE data, yields a somewhat smaller difference of 0.02 parts per thousand (0-0.03 parts per thousand). These differences may partially explain Fe isotopic compositions in other settings that could not previously be reconciled with a dominantly peridotitic and/or chondritic mantle source. This discovery may warrant discussion of the apparent decoupling between Fe and radiogenic isotopes in ocean island basalts, where the latter shows significant global variations and the former may show little or none. Our work highlights the need for additional constraints on the behavior of Fe isotopes during crustal recycling processes and reinforces the notion that consideration must be given to the effect of magmatic differentiation on Fe isotopic compositions. (C) 2018 Elsevier Ltd. All rights reserved.

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