Radiation-Induced Changes in Vanadium Speciation in Basaltic Glasses: Implications for Oxybarometry Measurements Using Vanadium K-edge X-ray Absorption Spectroscopy

Abstract

Abstract Magmatic oxygen fugacity (fO2) exerts a primary control on the discrete vanadium (V) valence states that will exist in quenched melts. Vanadium valence proxies for fO2, measured using X-ray absorption near-edge spectroscopy (XANES), can provide highly sensitive determinations of the redox conditions in basaltic melts. However, X-ray beam-induced changes in V speciation will introduce uncertainty in the calculated average V valence (V*) that must be properly evaluated to make meaningful interpretations of the igneous evolution of the system. The study presented here showed that beam-induced modifications in V speciation are observed in silicate glasses that are dependent on the radiation dose rate used during analysis. Changes in V speciation are observed to be most pronounced at the highest flux density tested, 9.25 × 1011 ph/s/µm2 (photons per second per square micrometer), with rapid changes occurring in the first 200 s of analysis. The high-dose rate conditions result in changes in calculated V* ~0.3 valence unit for the most oxidized glass analyzed (V* = 4.94), which can correspond to ~0.5 log unit reduction in calculated fO2. However, at flux densities ≤1.13 × 109 ph/s/μm2, measured changes in V* were found to be <0.03 for all standard glasses analyzed. The degree of reduction observed during analysis is also found to be progressively smaller as the initial V* of the glass decreases, such that magmatic glasses with V* values ≤3.7 show no statistically significant change in calculated valence during analysis at any flux density tested. For most terrestrial magmatic glasses, where V* is found to be <4, beam-induced changes in V* can be effectively minimized (<0.04), within analytical uncertainty of the XAFS analysis, by limiting flux densities to be ≤1 × 109 ph/s/μm2.