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

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 liminted 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 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 FeLα and FeLβ. These two end-members, along with the other eight samples, can be employed to quantify the relationship between Fe Lβ/Lα 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.