A new method to estimate the oxidation state of basaltic series from microprobe analyses

Research output: Contribution to journalArticleResearchpeer review

Authors

  • Lydéric France
  • Benoit Ildefonse
  • Juergen Koepke
  • Florent Bech

Research Organisations

External Research Organisations

  • Centre national de la recherche scientifique (CNRS)
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Details

Original languageEnglish
Pages (from-to)340-346
Number of pages7
JournalJournal of Volcanology and Geothermal Research
Volume189
Issue number3-4
Early online date2 Dec 2009
Publication statusPublished - 15 Jan 2010

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 ({increment}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.

Keywords

    clinopyroxene, EPMA, oxybarometer, oxygen fugacity, partition coefficient, plagioclase

ASJC Scopus subject areas

Cite this

A new method to estimate the oxidation state of basaltic series from microprobe analyses. / France, Lydéric; Ildefonse, Benoit; Koepke, Juergen et al.
In: Journal of Volcanology and Geothermal Research, Vol. 189, No. 3-4, 15.01.2010, p. 340-346.

Research output: Contribution to journalArticleResearchpeer review

France L, Ildefonse B, Koepke J, Bech F. A new method to estimate the oxidation state of basaltic series from microprobe analyses. Journal of Volcanology and Geothermal Research. 2010 Jan 15;189(3-4):340-346. Epub 2009 Dec 2. doi: 10.1016/j.jvolgeores.2009.11.023
France, Lydéric ; Ildefonse, Benoit ; Koepke, Juergen et al. / A new method to estimate the oxidation state of basaltic series from microprobe analyses. In: Journal of Volcanology and Geothermal Research. 2010 ; Vol. 189, No. 3-4. pp. 340-346.
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T1 - A new method to estimate the oxidation state of basaltic series from microprobe analyses

AU - France, Lydéric

AU - Ildefonse, Benoit

AU - Koepke, Juergen

AU - Bech, Florent

PY - 2010/1/15

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N2 - 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 ({increment}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.

AB - 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 ({increment}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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