Issue 3: 1996

Abstract

Distribution of ferric iron in some upper-mantle assemblages

D. Canil and H. St. C. O'Neill Bayerisches Geoinstitut, Universitat Bayreuth, D-95440 Bayreuth, Germany, Present address: School of Earth and Ocean Sciences, University of Victoria, PO Box 1700, Victoria, B.C., V8W 2Y2, Canada and Present address: Research School of Earth Sciences, Australian National University, Canberra, A.C.T. 0200, Australia
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 Mossbauer spectroscopy to determine the Fe³⁺/[Sigma]Fe 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 lherzolite 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 f02, 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 f02 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.

Keywords: mantle; oxidation; partitioning; peridotite; thermo-barometry

Pages: 609 - 635

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