Distribution of Ferric Iron in some Upper-Mantle Assemblages
Identifieur interne : 000E45 ( Main/Exploration ); précédent : 000E44; suivant : 000E46Distribution of Ferric Iron in some Upper-Mantle Assemblages
Auteurs : Dante Canil [Allemagne] ; Hugh St. C. O'Neill [Allemagne]Source :
- Journal of Petrology [ 0022-3530 ] ; 1996-06.
Abstract
The distribution of ferric iron among the phases of upper-mantle rocks, as a function of pressure (P), temperature (T) and bulk composition, has been studied using 57Fe Mössbauer spectroscopy to determine the Fe3+/ΣFe ratios of mineral separates from 35 peridotite and pyroxenite samples. The whole-rock Fe3+ complement of a peridotite is typically shared approximately evenly among the major anhydrous phases (spinel and/or garnet, orthopyroxene and clinopyroxene), with the important exception of olivine, which contains negligible Fe3+. Whole-rock Fe3+ contents are independent of the T and P of equilibration of the rock, but show a well-defined simple inverse correlation with the degree of depletion in a basaltic component. Fe3+ in spinel and in both pyroxenes from the spinel Iherzolite facies shows a positive correlation with temperature, presumably owing to the decrease in the modal abundance of spinel. In garnet peridotites, the Fe3+ in garnet increases markedly with increasing T and P, whereas that in clinopyroxene remains approximately constant. The complex nature of the partitioning of Fe3+ between mantle phases results in complicated patterns of the activities of the Fe3+ -bearing components, and thus in calculated equilibrium fO2, which show little correlation with whole-rock Fe3+ or degree of depletion. Whether Fe3+ is taken into account or ignored in calculating mineral formulae for geothermobarometry can have major effects on the resulting calculated T and P. For Fe-Mg exchange geothermometers, large errors must occur when applied to samples more oxidized or reduced than the experimental calibrations, whose fO2 conditions are largely unknown. Two-pyroxene thermometry is more immune to this problem, and probably provides the most reliable P—T estimates. Accordingly, the convergence of P—T values derived for a given garnet peridotite assemblage may not necessarily be indicative of mineral equilibrium. The prospects for the calculation of accurate Fe3+ contents from electron microprobe analyses by assuming stoichiometry are good for spinel, uncertain for garnet, and distinctly poor for pyroxenes.
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DOI: 10.1093/petrology/37.3.609
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The whole-rock Fe3+ complement of a peridotite is typically shared approximately evenly among the major anhydrous phases (spinel and/or garnet, orthopyroxene and clinopyroxene), with the important exception of olivine, which contains negligible Fe3+. Whole-rock Fe3+ contents are independent of the T and P of equilibration of the rock, but show a well-defined simple inverse correlation with the degree of depletion in a basaltic component. Fe3+ in spinel and in both pyroxenes from the spinel Iherzolite facies shows a positive correlation with temperature, presumably owing to the decrease in the modal abundance of spinel. In garnet peridotites, the Fe3+ in garnet increases markedly with increasing T and P, whereas that in clinopyroxene remains approximately constant. The complex nature of the partitioning of Fe3+ between mantle phases results in complicated patterns of the activities of the Fe3+ -bearing components, and thus in calculated equilibrium fO2, which show little correlation with whole-rock Fe3+ or degree of depletion. Whether Fe3+ is taken into account or ignored in calculating mineral formulae for geothermobarometry can have major effects on the resulting calculated T and P. For Fe-Mg exchange geothermometers, large errors must occur when applied to samples more oxidized or reduced than the experimental calibrations, whose fO2 conditions are largely unknown. Two-pyroxene thermometry is more immune to this problem, and probably provides the most reliable P—T estimates. Accordingly, the convergence of P—T values derived for a given garnet peridotite assemblage may not necessarily be indicative of mineral equilibrium. The prospects for the calculation of accurate Fe3+ contents from electron microprobe analyses by assuming stoichiometry are good for spinel, uncertain for garnet, and distinctly poor for pyroxenes.</div></front></TEI><affiliations><list><country><li>Allemagne</li></country></list><tree><country name="Allemagne"><noRegion><name sortKey="Canil, Dante" sort="Canil, Dante" uniqKey="Canil D" first="Dante" last="Canil">Dante Canil</name></noRegion><name sortKey="O Neill, Hugh St C" sort="O Neill, Hugh St C" uniqKey="O Neill H" first="Hugh St. C." last="O'Neill">Hugh St. C. O'Neill</name></country></tree></affiliations></record>Pour manipuler ce document sous Unix (Dilib)
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