Distribution of ferric iron in some upper-mantle assemblages

The distribution of ferric iron among the phases of uppermantle rocks, as a function of pressure (P), temperature (T) and bulk composition, has been studied using ⁵⁷Fe Mössbauer spectroscopy to determine the Fe³⁺ / ∑ 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 lherzplite 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.