A new method to estimate the oxidation state of basaltic series from microprobe analyses |
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| Authors: | Lydéric France Benoit Ildefonse Juergen Koepke Florent Bech |
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| Institution: | 1. Università degli Studi di Catania, Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Sezione di Scienze della Terra, Corso Italia 57, I-95129 Catania, Italy;2. Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Piazza Roma 2, I-95129, Catania, Italy;1. Departamento de Mineralogía y Petrología, Universidad de Granada, Campus de Fuentenueva 18002 Granada, Spain;2. Department of Geology and Natural Resources, Institute of Geosciences, University of Campinas – UNICAMP, 13083-970 Campinas, SP, Brazil;3. UA Petrología Experimental, CSIC-Universidad de Huelva, Facultad de Ciencias Experimentales, 21071 Huelva, Spain;4. IGG Russian Academy of Science, Ekaterinburg, Russia;1. Istituto di Geoscienze e Georisorse, Consiglio Nazionale delle Ricerche, Via Moruzzi, 1, I-56124 Pisa, Italy;2. Dipartimento di Scienze della Terra, Università degli Studi di Pisa, Via Santa Maria, 53, I-56126 Pisa, Italy;3. Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via Giorgio La Pira, 4, I-50121 Firenze, Italy;4. Jeoloji Mühendisli?i, Munzur Üniversitesi, Bölümü 62000, Merkez, Tunceli, Turkey;5. Istituto di Geologia Ambientale e Geoingegneria, Consiglio Nazionale delle Ricerche, Via Salaria km 29,300, I-00015 Monterotondo, Italy;1. Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italy;2. Dipartimento di Fisica e Science della Terra, Università di Ferrara, Via Saragat 1, 44122 Ferrara, Italy;3. Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Catania, Via A. Longo 19, 95125 Catania, Italy;4. Dipartimento di Ingegneria & Geologia, Università G. d’Annunzio, Via Dei Vestini 30, 66013 Chieti, Italy |
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| Abstract: | The oxygen fugacity and therefore the iron redox state of a melt is known to have a strong influence on the liquid line of descent of magmas and thus on the composition of the coexisting melts and crystals. We present a new method to estimate this critical parameter from electron probe microanalyses of two of the most common minerals of basaltic series, plagioclase and clinopyroxene. This method is not based on stoichiometric calculations, but on the different partitioning behaviour of Fe3+ and Fe2+ between both minerals and a melt phase: plagioclase can incorporate more Fe3+ than Fe2+, while clinopyroxene can incorporate more Fe2+ than Fe3+. For example, the effect of oxidizing a partly molten basaltic system (Fe3+ is stabilized with respect to Fe2+) results in an increase of FeOtotal in plagioclase, but a decrease in the associated clinopyroxene. We propose an equation, based on published partition coefficients, that allows estimating the redox state of a melt from these considerations. An application to a set of experimental and natural data attests the validity of the proposed model. The associated error can be calculated and is on average < 1 log unit of the prevailing oxygen fugacity.In order to reduce the number of different variables influencing the Fe2+/Fe3+ mineral/melt equilibrium, our model is restricted to basaltic series with SiO2 < 60% that have crystallized at intermediate to low pressure (< 0.5 GPa) and under relatively oxidizing conditions (?FMQ > 0; where FMQ is the fayalite–magnetite–quartz oxygen buffer equilibrium), but it may be parameterized for other conditions. A spreadsheet is provided to assist the use of equations, and to perform the error propagation analysis. |
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