High spatial resolution analysis of the iron oxidation state in silicate glasses using the electron probe

Abstract

The iron oxidation state in silicate melts is important for understanding their physical properties, although it is most often used to estimate the oxygen fugacity of magmatic systems. Often high spatial resolution analyses are required, yet the available techniques, such as μXANES and μMössbauer, require synchrotron access. The flank method is an electron probe technique with the potential to measure Fe oxidation state at high spatial resolution but requires careful method development to reduce errors related to sample damage, especially for hydrous glasses. The intensity ratios derived from measurements on the flanks of FeLα and FeLβ X-rays (FeLβ f /FeLα f ) over a time interval (time-dependent ratio flank method) can be extrapolated to their initial values at the onset of analysis. We have developed and calibrated this new method using silicate glasses with a wide range of compositions (43-78 wt% SiO 2 , 0-10 wt% H 2 O, and 2-18 wt% FeO T , which is all Fe reported as FeO), including 68 glasses with known Fe oxidation state. The Fe oxidation state (Fe 2+ /Fe T ) of hydrous (0-4 wt% H 2 O) basaltic (43-56 wt% SiO 2 ) and peralkaline (70-76 wt% SiO 2 ) glasses with FeO T > 5 wt% can be quantified with a precision of ±0.03 (10 wt% FeO T and 0.5 Fe 2+ /Fe T ) and accuracy of ±0.1. We find basaltic and peralkaline glasses each require a different calibration curve and analysis at different spatial resolutions (~20 and ~60 μm diameter regions, respectively). A further 49 synthetic glasses were used to investigate the compositional controls on redox changes during electron beam irradiation, where we found that the direction of redox change is sensitive to glass composition. Anhydrous alkali-poor glasses become reduced during analysis, while hydrous and/or alkali-rich glasses become oxidized by the formation of magnetite nanolites identified using Raman spectroscopy. The rate of reduction is controlled by the initial oxidation state, whereas the rate of oxidation is controlled by SiO 2 , Fe, and H 2 O content.

Effect of redox on Fe–Mg–Mn exchange between olivine and melt and an oxybarometer for basalts

“…A particular challenge with using Eq. (1) is the need to know the Fe 3+ content of the silicate melt, which is not readily measurable by conventional electron microprobe techniques (see review by Hughes et al 2018). Olivine contains negligible Fe 3+ (less then a few thousand ppm and not more than a few percent of the total Fe, Fe T ; Ejima et al 2018) so that where Fe 3+ in the melt is low, i.e., in relatively reduced systems, Kd Fe 2+ −Mg can be used with confidence assuming that Fe 2+ = Fe T .…”

“…4), probably due to the smaller spot size of µXANES (i.e., greater flux density) compared to the SMS simulation. The mechanism of oxidation is not clear, but may involve the formation of tiny magnetite nanolites (Di Genova et al 2017;Hughes et al 2018), as suggested by Raman spectra of some µXANES spots and the darkening of the glass after irradiation. This mechanism may operate in tandem with the alkali migration proposed above.…”

How to Measure the Oxidation State of Multivalent Elements in Minerals, Glasses, and Melts?

“…The peaks of the L series emitted from the metals of first order transition are the result of electronic transitions between the 3d orbitals partially filled and the 2p core levels (L 2,3 ). Different methods are based on the analysis of the Fe L α,β emission spectra of minerals and glasses to measure their Fe 3+ /Fe TOT (Höfer et al, 1994, Fialin et al, 2001, 2004, Zhang, Almeev, et al, 2018Hughes et al, 2018Hughes et al, , 2020.…”

“…Examples of such unwanted effects are self-absorption or induced beam damage (i.e., see for sulfur Metrich et al, 2002Metrich et al, , 2003Metrich et al, , 2010andfor iron Goncalves et al, 2013, andCottrell et al, 2018;Hughes et al 2018Hughes et al , 2020. Finally, the accuracy of the experiment will depend on the standard used and on how the sample is prepared.…”

Low analytical totals in EPMA of hydrous silicate glass due to sub-surface charging: Obtaining accurate volatiles by difference

“…Silicate glass can become unstable during EPMA due to the diffusive migration of mobile elements (e.g., Na), sometimes referred to as 'beam damage' (e.g., Nielsen and Sigurdsson, 1981). Glass composition controls the severity of beam damage and hydrous glass is more susceptible than anhydrous glass (e.g., Hughes et al, 2018;Humphreys et al, 2006;Zhang et al, 2018). Element migration can also occur in hydrous minerals, such as amphibole and apatite, causing errors in VBD estimates of these minerals (e.g., Stock et al, 2015).…”