Revised Mossbauer Calibration for Fe3+/FeT of XANES Basalt Standards: Implications for MORB

Abstract:

Among techniques for determining Fe³⁺/Fe^T of natural glasses, XANES affords high precision, spatial resolution, and sample throughput and consequently has become widely used. However, because XANES determinations depend on standardization against materials of known Fe³⁺/Fe^T, they are only as accurate as the methods used for calibration. In many cases, calibration is performed with Mossbauer spectroscopy. Accurate determination of Fe³⁺/Fe^T by Mossbauer spectroscopy is the subject of a long-standing controversy, in part owing to debate as to the influence of recoilless fraction on the area ratios of room temperature (RT) Mossbauer absorption doublets associated with paramagnetic Fe²⁺ and Fe³⁺ in silicate glasses. Recoilless fraction effects for glasses are comparatively subtle, and so characterization efforts have not always produced statistically resolvable effects, in part because glasses produce broadened line shapes that degrade analytical precision, but both theoretical considerations of bond strengths and abundant evidence from minerals demonstrate that RT Mossbauer analyses will overestimate Fe³⁺/Fe^Tof Fe-bearing silicates.

Cottrell & Kelley (2011) used the basalt XANES calibration of Cottrell et al. (2009) to show that the average Fe3+/FeT of MORB glasses is close to 0.16, but this calibration depends chiefly on RT Mossbauer spectra of a suite of standard glasses. New cryogenic (10 K) Mossbauer spectra of these same glasses suggests a correction factor, C, of 1.1, where [Fe³⁺/Fe^T(corrected)]=[Fe³⁺/Fe^T (RT)]/([Fe³⁺/Fe^T (RT)]+C(1-[Fe³⁺/Fe^T(RT)]). If this correction is applied to the XANES data, the median of MORB glasses is ~0.15. Because recoilless fractions are not exactly unity even at 10 K, this correction represents a minimum; however the 10 K data are in good agreement with Fe³⁺/Fe^T predicted for the Mossbauer standards by the wet-chemistry-based model of Kress and Carmichael (1991) based on the synthesis f_O2. The precision of XANES for the determination of Fe³⁺/FeT is ±0.005; the absolute accuracy is tied to the accuracy of the technique used to calibrate. For Mossbauer, accuracy is likely closer to ±0.02