DISTRIBUTION OF FERRIC IRON IN SOME UPPER-MANTLE ASSEMBLAGES

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 Fe-57 Mossbauer spectroscopy to determine the Fe3+/Sigma Fe ratios of mineral separates from 35 peridotite and pyrox enite samples. The whole-rock Fe3+ complement of a peridotite is typic ally shared approximately evenly among the major anhydrous phases (spi nel and/or garnet, orthopyroxene and clinopyroxene), with the importan t 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 o f depletion in a Basaltic component. Fe3+ in spinel and in both pyroxe nes from the spinel lherzolite facies shows a positive correlation wit h temperature, presumably owing to the decrease in the modal abundance of spinel. In garnet peridotites, the Fe3+ in garnet increases marked ly with increasing T and P, whereas that in clinopyroxene remains appr oximately constant. The complex nature of the partitioning of Fe3+ bet ween mantle phases results in complicated patterns of the activities o f the Fe3+-bearing components, and thus in calculated equilibrium f(O2 ), which show little correlation with whole-rock Fe3+ or degree of dep letion. Whether Fe3+ is taken into account or ignored in calculating m ineral formulae for geothermobarometry can have major effects on the r esulting calculated T and P. For Fe-Mg exchange geothermometers, large errors muse occur when applied to samples more oxidized or reduced th an the experimental calibrations, whose f(O2) conditions are largely u nknown. Two-pyroxene thermometry is more immune to this problem and pr obably provides the most reliable P-T estimates Accordingly, the conve rgence of P-T values derived for a given garnet peridotite assemblage may not necessarily be indicative of mineral equilibrium. The prospect s for the calculation of accurate Fe3+ contents from electron micropro be analyses by assuming stoichiometry are good for spinel, uncertain f or garnet, and distinctly poor for pyroxenes.