Instrument-specific calibration of STEM-EELS for determining iron oxidation states in bridgmanite and other deep-mantle minerals

Abstract

Iron (Fe), a multivalent element abundant in Earth’s crust, mantle, and core, occurs in multiple oxidation states that are closely linked to the redox conditions of Earth’s deep interior. Quantifying Fe oxidation states in deep-mantle minerals, such as garnet and bridgmanite, is therefore essential for constraining the redox evolution of Earth’s interior. Because deep-mantle minerals commonly occur at micro- to submicron scales, high spatial resolution analytical techniques are required for reliable determinations of Fe redox. Here, we optimize the workflow and establish an instrument-specific calibration curve for quantifying Fe3+/ΣFe using scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS). The calibration curve is based on a suite of independently characterized reference samples, including Al-bearing and Al-free bridgmanite, garnets, magnetite, and hematite, analyzed under the same STEM–EELS acquisition and data-processing conditions. The relationship between the Fe L3/L2 white-line intensity ratio, I(L3)/I(L2), and independently constrained Fe3+/ΣFe values can be described by a single calibration curve for these reference samples on our instrument. Importantly, none of the bridgmanite standards show a systematic offset relative to garnets and Fe oxide standards within the uncertainty of the present dataset. Interlaboratory comparisons further demonstrate that the previously proposed EELS “universal curve” cannot yield accurate Fe3+/ΣFe values across different instruments and analytical protocols, underscoring the need for instrument-specific calibration. We further apply the calibrated workflow to bridgmanite-bearing products synthesized using large-volume press and laser-heated diamond anvil cell techniques. These applications demonstrate that instrument-specific STEM–EELS calibration can provide quantitative, phase-specific Fe oxidation-state information from complex and fine-grained high-pressure experimental products. Our results provide a practical analytical framework for Fe redox analysis of bridgmanite and other deep-mantle minerals.