Revisiting the electron microprobe method of spinel-olivine-orthopyroxene oxybarometry applied to spinel peridotites

Natural peridotite samples containing olivine, orthopyroxene, and spinel can be used to assess the oxygen fugacity f O 2 of the upper mantle. The calculation requires accurate and precise quantification of spinel Fe 3+ /ΣFe ratios. Wood and Virgo (1989) presented a correction procedure for electron microprobe (EPMA) measurements of spinel Fe 3+ /ΣFe ratios that relies on a reported correlation between the difference in Fe 3+ /ΣFe ratio by Mossbauer spectroscopy and by electron microprobe (ΔFe 3+ /ΣFe Moss-EPMA ) and the Cr# [Cr/(Al+Cr)] of spinel. This procedure has not been universally adopted, in part, because of debate as to the necessity and effectiveness of the correction. We have performed a series of replicate EPMA analyses of several spinels, previously characterized by Mossbauer spectroscopy, to test the accuracy and precision of the Wood and Virgo correction. While we do not consistently observe a correlation between Cr# and ΔFe 3+ /ΣFe Moss-EPMA in measurements of the correction standards, we nonetheless find that accuracy of Fe 3+ /ΣFe ratios determined for spinel samples treated as unknowns improves when the correction is applied. Uncorrected measurements have a mean ΔFe 3+ /ΣFe Moss-EPMA = 0.031 and corrected measurements have a mean ΔFe 3+ /ΣFe Moss-EPMA = −0.004. We explain how the reliance of the correction on a global correlation between Cr# and MgO concentration in peridotitic spinels improves the accuracy of Fe 3+ /ΣFe ratios despite the absence of a correlation between ΔFe 3+ /ΣFe Moss-EPMA and Cr# in some analytical sessions. Precision of corrected Fe 3+ /ΣFe ratios depends on the total concentration of Fe, and varies from ±0.012 to ±0.032 (1σ) in the samples analyzed; precision of uncorrected analyses is poorer by approximately a factor of two. We also present an examination of the uncertainties in the calculation contributed by the other variables used to derive f O 2 . Because there is a logarithmic relationship between the activity of magnetite and log f O 2 , the uncertainty in f O 2 relative to the QFM buffer contributed by the electron microprobe analysis of spinel is asymmetrical and larger at low ferric Fe concentrations (+0.3/−0.4 log units, 1σ, at Fe 3+ /ΣFe = 0.10) than at higher ferric Fe concentrations (±0.1 log units, 1σ, at Fe 3+ /ΣFe = 0.40). Electron microprobe analysis of olivine and orthopyroxene together contribute another ±0.1 to ±0.2 log units of uncertainty (1σ). Uncertainty in the temperature and pressure of equilibration introduce additional errors on the order of tenths of log units to the calculation of relative f O 2 . We also document and correct errors that appear in the literature when formulating f O 2 that, combined, could yield errors in absolute f O 2 of greater than 0.75 log units—even with perfectly accurate Fe 3+ /ΣFe ratios. Finally, we propose a strategy for calculating the activity of magnetite in spinel that preserves information gained during analysis about the ferric iron content of the spinel. This study demonstrates the superior accuracy and precision of corrected EPMA measurements of spinel Fe 3+ /ΣFe ratios compared to uncorrected measurements. It also provides an objective method for quantifying uncertainties in the calculation of f O 2 from spinel peridotite mineral compositions.