Ferric Iron Concentrations in Silicate Glasses: a Mössbauer and XAS Study

Friday, 19 December 2014
Melinda Darby Dyar, Mount Holyoke College, South Hadley, MA, United States and Molly C McCanta, Tufts University, Medford, MA, United States
Fe2+/Fe3+ ratios of volcanic materials on a planet reflect oxidation of the magma source region and the additional effects of magma interaction with the near surface environment. Synchrotron X-ray absorption spectroscopy (XAS) enables measurement of Fe2+/Fe3+ ratios in situ at micro-scales on standard thin sections. Because melt composition and structure affect Fe3+/ΣFe, XAS measurements of unknowns require calibration standards with like compositions. To address this need, a suite of 33 synthetic glass standards covering a wide range of geologically-relevant compositions was generated under four varying fO2conditions.

Runs were conducted in a 1-atm gas mixing furnace using the Pt (fO2 ≥ QFM) or Re (fO2 < QFM) wire-loop technique. Air, MH (CO2), QFM, and IW were used to equilibrate each composition, with fO2 values fixed by gas mixing or use of a solid buffer. Samples were then prepared for Mössbauer and XAS analyses, with Fe2+/Fe3+values from the former used to calibrate the latter using a multivariate model. XAS data were acquired both at NSLS beamline x26a and at the GSECARS beamline (13-ID) at APS.

Over the compositional range from andesite (7.1-11.8 wt.% FeO and 53.6-55.9 wt. SiO2) to basaltic andesite (4.6-7.9 wt.% FeO and 58.8-62.4 wt. SiO2) to basalt (6.8-11.0 wt.% FeO and 47.9-50.7 wt. SiO2), Fe3+ contents of glasses equilibrated at each buffer are comparable, as expected. Those samples equilibrated in air are ~89±2% Fe3+, in CO2, ~77±3 Fe3+, and in QFM and IW, ~11±4% Fe3+. Predictive models (e.g., Kilinc et al., 1983) suggest that the increased SiO2 concentrations in dacites and rhyolites result in increased Fe3+ at higher fO2s (Figure 1). Our data support this prediction; e.g., dacites with 67.1-71.6 wt.% SiO2 at IW are 20±10%Fe3+. Once synthesized and characterized, these equilibrated glasses will form the basis for multivariate analysis of XAS spectra that will provide a broad calibration for measuring Fe3+ in silicate glasses at any synchrotron facility.