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Ferric iron in mantle-derived garnets

Implications for thermobarometry and for the oxidation state of the mantle

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Abstract

The oxidation state of a mantle assemblage may be defined by heterogeneous reactions between oxygen and iron-bearing minerals. In spinel lherzolites, the presence of Fe3+ in spinel allows use of the assemblage olivine-orthopyroxene-spinel to define f O 2 at fixed T and P. As a first step towards establishing an analogous reaction for garnet lherzolites, garnets from mantle-derived xenoliths from South Africa and the USSR have been analyzed with 57Fe Mössbauer spectroscopy at 298 and 77K to determine Fe3+/Fe2+ and the coordination state of iron. Garnets from South African alkremites (pyrope+Mg-spinel) and eclogites, as well as garnet-spinel and low-temperature garnet lherzolites from both South Afica and the USSR, have Fe3+/ΣFe<0.07. In contrast, garnets from high-temperature garnet lherzolites from within the Kaapvaal craton of South Africa have Fe3+/ΣFe>0.10. Ferric iron is octahedrally coordinated, and ferrous iron is present in the dodecahedral site in all samples. The occurrence of significant Fe3+ in these garnets necessitates caution in the use of geothermometers and geobarometers that are applied to mantle samples. For example, the presence of ∼12% of the Fe as Fe3+ in garnets can increase temperatures calculated from existing Fe/Mg geothermometers by>200°C. The concomitant increase in pressures calculated from geobarometers that use the Al content in orthopyroxene coexisting with garnet are ∼10–15 kbar. Results of calculations based on heterogeneous equilibria between garnet, olivine, and pyroxene are consistent with the derivation of the peridotite samples from source regions that are relatively oxidized, between the f O 2 of the FMQ (quartz-fayalite-magnetite) buffer and that of the WM buffer. No samples yield values of f O 2 as reduced as IW (iron-wüstite buffer).

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Author notes

  1. Robert W. Luth

    Present address: Department of Geology, University of Alberta, T6G 2EH, Edmonton, Canada

Authors and Affiliations

  1. Bayerisches Geoinstitut, Universität Bayreuth, Postfach 101251, D-8580, Bayreuth, Federal Republic of Germany

    Robert W. Luth

  2. Geophysical Laboratory, Carnegie Institution of Washington, 2801 Upton Street, NW, 20008, Washington, D.C., USA

    David Virgo & Francis R. Boyd

  3. Department of Geological Sciences, Northwestern University, 60208, Evanston, IL, USA

    Bernard J. Wood

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  1. Robert W. Luth
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  3. Francis R. Boyd
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Luth, R.W., Virgo, D., Boyd, F.R. et al. Ferric iron in mantle-derived garnets. Contr. Mineral. and Petrol. 104, 56–72 (1990). https://doi.org/10.1007/BF00310646

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