New natural garnet reference materials for determining the oxidation state of iron in garnet using the electron microprobe flank method

Abstract

The oxidation state of iron (e.g., Fe³⁺/ΣFe) in minerals is a direct proxy for the oxygen fugacity of magma and fluid, which plays a key role in the formation of various types of ore deposits. Although many techniques have been developed to determine the Fe³⁺/ΣFe ratio in minerals, the electron microprobe flank method is particularly notable for its easy accessibility and high efficiency. However, the application of this method is limited by a shortage of suitable calibration standards. In this study, we collected a series of natural, euhedral garnet grains and gem-quality garnet fragments, which were carefully crushed and separated under a binocular microscope. Following a detailed examination of their major element compositions and Mössbauer spectroscopy measurements for their Fe³⁺/ΣFe ratios, we report ten new garnet samples (three belonging to the andradite–grossular series and seven to the almandine–pyrope–grossular series) that can be used as reference materials to calibrate the Fe³⁺/ΣFe ratio of garnet using the flank method. The andradite–grossular samples are highly enriched in Fe³⁺, exhibiting Fe³⁺/ΣFe ratios ranging from 0.89 ± 0.03 to 1.00 ± 0.03, while the almandine–pyrope–grossular samples contain minimal Fe³⁺ with Fe³⁺/ΣFe ratios ranging from 0.01 ± 0.02 to 0.03 ± 0.01. One andradite sample (And1902) and one almandine sample (Ald1906) were identified as ideal for determining the flank positions for Fe L_α and Fe L_β. These two end-members, along with the other eight samples, can be employed to quantify the relationship between Fe L_β/L_α at flank positions and the Fe²⁺ or ΣFe content. The results indicate that the Fe²⁺ contents and Fe³⁺/ΣFe ratios of the ten garnet samples align with those obtained through Mössbauer spectroscopy, with an uncertainty of ±1 wt% for Fe²⁺ and ±0.05 for Fe³⁺/ΣFe, respectively. Consequently, these well-characterized natural garnet samples can serve as reliable reference materials when synthetic garnet standards are unavailable.