Details
Original language | English |
---|---|
Pages (from-to) | 340-346 |
Number of pages | 7 |
Journal | Journal of Volcanology and Geothermal Research |
Volume | 189 |
Issue number | 3-4 |
Early online date | 2 Dec 2009 |
Publication status | Published - 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
- Earth and Planetary Sciences(all)
- Geophysics
- Earth and Planetary Sciences(all)
- Geochemistry and Petrology
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In: Journal of Volcanology and Geothermal Research, Vol. 189, No. 3-4, 15.01.2010, p. 340-346.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
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
Y1 - 2010/1/15
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.
KW - clinopyroxene
KW - EPMA
KW - oxybarometer
KW - oxygen fugacity
KW - partition coefficient
KW - plagioclase
UR - http://www.scopus.com/inward/record.url?scp=73649099430&partnerID=8YFLogxK
U2 - 10.1016/j.jvolgeores.2009.11.023
DO - 10.1016/j.jvolgeores.2009.11.023
M3 - Article
AN - SCOPUS:73649099430
VL - 189
SP - 340
EP - 346
JO - Journal of Volcanology and Geothermal Research
JF - Journal of Volcanology and Geothermal Research
SN - 0377-0273
IS - 3-4
ER -