No abstract

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+ /SFe ratios. Wood and Virgo (1989) /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 O2 . Because there is a logarithmic relationship between the activity of magnetite and logf O2 , the uncertainty in f O2 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 O2 . We also document and correct errors that appear in the literature when formulating f O2 that, combined, could yield errors in absolute f O2 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 O2 from spinel peridotite mineral compositions.