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

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.

Oxidation State of Arc Mantle Revealed by Partitioning of V, Sc, and Ti Between Mantle Minerals and Basaltic Melts

“…In Figure a, we compare the fO 2 values obtained from the μ‐XANES technique and mineral oxybarometers using the data reported in Davis and Cottrell (). The results exhibit good consistency between the μ‐XANES technique and mineral oxybarometers and demonstrate the accuracy of both the semi‐empirical equation (Ballhaus et al, ) and the thermodynamic formulation (Davis et al, ; Wood & Virgo, ). Because Davis and Cottrell () experiments were performed at 1 atm, the results in Figure a also confirm the consistency between mineral oxybarometers and gas‐mixing furnace fO 2 .…”

“…The former was developed based on the thermodynamic data presented in Bohlen et al () and Myers and Eugster () and requires accurate determination of the magnetite activity in spinel. To obtain accurate values for magnetite activity, a correction method was introduced by Wood and Virgo () and revisited recently by Davis et al (). However, magnetite activity in spinel, which strongly affects the calculated fO 2 , is model dependent (e.g., Davis & Cottrell, ; O'Neill & Wall, ), and so far, no universally accepted spinel model has been developed (Davis & Cottrell, ).…”

“…(a) μ‐XANES fO 2 versus olivine‐spinel oxybarometer fO 2 s (data from 1‐atm experiments of Davis & Cottrell, ). The magenta and cyan solid circles represent the results of olivine‐spinel formulations of Ballhaus et al () and Davis et al (), respectively. The results indicate accuracy of both of the mineral oxybarometers.…”

Variation in sub-arc mantle oxygen fugacity during partial melting recorded in refractory peridotite xenoliths from the West Bismarck Arc

“…This demonstrates the danger of deriving Fe 3+ in spinel by stoichiometry without using secondary standards as a check, since systematic differences between samples can erroneously be introduced (e.g. Davis et al 2017). Errors in Fe 3+ /∑Fe spinel are ±0.025 (Woodland et al 1992(Woodland et al , 1996.…”

Evolution of the Josephine Peridotite Shear Zones: 1. Compositional Variation and Shear Initiation

“…A linear background correction was used for each element and the ZAF procedure (Armstrong, ) applied for matrix correction. Spinel compositions were determined using the methods and spinel standards detailed in Davis et al (), which allows for the accurate determination of both Fe 2 O 3 and FeO concentrations. TiO 2 background measurements were incorrect during spinel analyses, but concentrations are estimated to be below 0.5 wt%.…”