Determination of Fe3+/Fe using the electron microprobe: A calibration for amphiboles

Abstract Iron is a common constituent in minerals from the Earth’s crust and upper mantle and often occurs in minerals as mixtures of two valence states, Fe3+ or Fe2+. Quantification of the values of Fe3+/FeTotal, where FeTotal = Fe3++Fe2+, in minerals may be necessary to accurately apply certain mineral equilibria to determine equilibrium values of important variables such as temperature (T), pressure (P), and oxygen fugacity (ƒO₂). Most useful would be an analytical technique that permits determination of values of Fe3+/FeTotal within a single mineral grain that is contained within a standard petrographic thin section, and the excellent spatial resolution and relative accessibility of the electron microprobe (EMP) have resulted in various attempts to use this instrument to determine values of Fe3+/FeTotal. These efforts have typically involved quantifying characteristics of the FeLα and/or FeLβ peaks. In this paper, we employ the method of Fialin et al. (2001), who have shown that the location of the FeLα peak changes as a function of Fe content and values of Fe3+/ FeTotal, to determine values of Fe3+/FeTotal in amphiboles. We have characterized the FeLα peak in several amphiboles with known values of Fe3+/FeTotal using the electron microprobe at Texas A&M University. Initial analyses employed a beam current of 20 nA in an effort to avoid Fe-oxidation due to electron beam generated H-loss (Wagner et al. 2008). Subsequent analyses were conducted at 100 nA, and the results are consistent with the 20 nA data only when relatively short duration analytical times were used. The position of the FeLα peak was determined for three suites of amphiboles that have been experimentally treated such that grains in any one of these mineral suites are chemically identical except for differences in the values of Fe3+/FeTotal. A linear relation between the FeLα peak location and value of Fe3+/FeTotal was observed for each of these three amphibole suites. These three lines differ from one another in both their slope and intercept and these differences vary as a function of Fe content. Thus, these amphiboles served as the basis for the derivation of a relation between Fe content and FeLα peak location, both measured with the EMP, and the value of Fe3+/FeTotal as originally determined with 57Fe Mössbauer spectroscopy. The relation between the relative peak position (RPP = hematite standard FeLα peak position - amphibole FeLα peak position), Fe content, and Fe3+/FeTotal is Fe3+/FeTotal = RPP - RPP(0)/RPP(1) - RPP(0), where RPP(0) = -1.37 × FeO2 + 19.59 × FeO - 3.85, RPP(1) = -1.25 × FeO2 + 21.39 × FeO + 13.05, and FeO refers to the wt%FeO. This relation reproduces the measured values of Fe3+/FeTotal to within ±0.07 and, therefore, should permit determination values of Fe3+/FeTotal in amphiboles with Fe contents from 7 to 13 wt% FeO with similar precision. The amphiboles that were used in this study were kaersutites, Ti-bearing pargasites, and pargasitic hornblendes. The calibration presented here should, at the very least, be applicable to amphiboles with similar compositions, and although further verification is necessary, this calibration may be useful for determining values of Fe3+/FeTotal in amphiboles with distinctly different compositions and may even be more universally applicable.