Presentation Information
[R4P-06]Iron Redox State of Phlogopite in Deep Ultramafic Rocks: Correlation with microbes in deep subsurface
*Taro KIDO1,2, Mariko Kouduka1,2, Hiroki Suga3,2, Toshiaki Ina3, Takahiro Kawai3, Takanori Wakita3, Takuma Kaneko3, Tomoya Uruga2,3, Masaki Oura2, Yohey Suzuki1,2 (1. University of Tokyo, 2. RIKEN, 3. JASRI)
Keywords:
XANES,Pyroxenite,Peridotite,Microbe,ICDP
Deep subsurface environments are generally reducing because they are isolated from atmospheric oxygen (Beaver et al., 2024). In such environments, reduced chemical species such as hydrogen and methane produced by water–rock reactions can serve as electron donors that support deep subsurface microbial ecosystems (Onstott et al., 2019). It has also been estimated that approximately 80% of Earth’s microbial biomass is distributed in the subsurface (Bar-on et al., 2018). Although electron acceptors are also required for microbial metabolism, it remains poorly understood what types of oxidants occur in deep subsurface ultramafic rocks, which mineral phases host them, and how they are spatially related to microbial distributions within rocks. In this study, we investigated the Fe redox state of iron-bearing minerals in deep subsurface ultramafic rocks recovered by the International Continental Scientific Drilling Program (ICDP), with the aim of clarifying their spatial correlation with microbial cells in the rock interior.
We studied a drill core sample collected from a depth of 814 m in the Bushveld Igneous Complex, South Africa, which formed approximately 2 billion years ago (Allwright et al., 2025). The Fe redox state was investigated by Fe K-edge X-ray absorption near-edge structure (XANES) analysis using a hard X-ray microscope at BL36XU, SPring-8. Microbial signals in the rock were analyzed by N K-edge XANES using a soft X-ray microscope at BL13U, NanoTerasu, and their spatial relationships with Fe redox states were examined.
To minimize oxidation by atmospheric oxygen, the drill core was frozen and stored under vacuum with an oxygen absorber immediately after drilling. Analytical rock sections were prepared with a diamond wire saw in an Ar-filled glove box and mounted in a He-filled sample holder for Fe K-edge XANES measurements. Mineral identification was performed by powder X-ray diffraction (XRD), environmental scanning electron microscopy with energy-dispersive X-ray spectroscopy (ESEM-EDS), and comparison of Al K-edge XANES spectra obtained using a soft X-ray microscope at BL17SU, SPring-8.
The XRD pattern of the pyroxenite sample from 814 m depth showed a peak near 10 Å. EDS analysis of rock fragments also identified mineral phases enriched in Mg, Al, Si, K, and Fe, indicating the presence of phlogopite. Fe K-edge XANES spectra were obtained from phlogopite grains in rock sections prepared under non-atmospheric exposure conditions and compared with FeCl2 as a Fe(II) standard and nontronite (NAu-2) as a Fe(III) standard. The spectra showed that the rims of phlogopite grains were enriched in Fe(III), whereas the surrounding pyroxene exhibited a more Fe(II)-rich signature. In addition, N K-edge XANES spectra similar to those of cultured Escherichia coli cells were detected from Fe(III)-rich phlogopite rims, whereas similar spectra were not detected from Fe(II)-rich pyroxene.
The Fe(III) observed in phlogopite may have formed through the following processes: (1) oxidation by atmospheric exposure, (2) hydrothermal alteration, or (3) dehydrogenation. In this study, atmospheric oxidation is unlikely to be the primary cause because sample handling and analysis were designed to minimize atmospheric exposure. In addition, the Fe(III)-bearing regions were localized within phlogopite grains, and Fe(III) was particularly enriched in the grain center, which is difficult to explain by atmospheric oxidation or hydrothermal alteration. Therefore, the Fe(III) in phlogopite may have formed through dehydrogenation during magma cooling. In trioctahedral mica, heating can oxidize Fe(II) to Fe(III) and generate hydrogen through the following dehydrogenation-reaction (Zema et al., 2010):[Fe2+ + OH-]phlogopite → [Fe3+ + O2-]phlogopite + 1/2H2The magmatic cooling stage of the Bushveld Igneous Complex is estimated that cooling process lasting several tens of millions of years after emplacement of the intrusion (Scotes et al., 2021). Thus, the Fe(III) observed in phlogopite in this study may have been preserved as an oxidative mineral phase formed in association with igneous activity approximately 2 billion years ago. The spatial correlation between Fe(III)-rich phlogopite rims and microbial signals suggests that phlogopite in deep subsurface ultramafic rocks retained Fe(III) as a potential electron acceptor and may have contributed to the formation of localized microbial habitats within the rock interior.
We studied a drill core sample collected from a depth of 814 m in the Bushveld Igneous Complex, South Africa, which formed approximately 2 billion years ago (Allwright et al., 2025). The Fe redox state was investigated by Fe K-edge X-ray absorption near-edge structure (XANES) analysis using a hard X-ray microscope at BL36XU, SPring-8. Microbial signals in the rock were analyzed by N K-edge XANES using a soft X-ray microscope at BL13U, NanoTerasu, and their spatial relationships with Fe redox states were examined.
To minimize oxidation by atmospheric oxygen, the drill core was frozen and stored under vacuum with an oxygen absorber immediately after drilling. Analytical rock sections were prepared with a diamond wire saw in an Ar-filled glove box and mounted in a He-filled sample holder for Fe K-edge XANES measurements. Mineral identification was performed by powder X-ray diffraction (XRD), environmental scanning electron microscopy with energy-dispersive X-ray spectroscopy (ESEM-EDS), and comparison of Al K-edge XANES spectra obtained using a soft X-ray microscope at BL17SU, SPring-8.
The XRD pattern of the pyroxenite sample from 814 m depth showed a peak near 10 Å. EDS analysis of rock fragments also identified mineral phases enriched in Mg, Al, Si, K, and Fe, indicating the presence of phlogopite. Fe K-edge XANES spectra were obtained from phlogopite grains in rock sections prepared under non-atmospheric exposure conditions and compared with FeCl2 as a Fe(II) standard and nontronite (NAu-2) as a Fe(III) standard. The spectra showed that the rims of phlogopite grains were enriched in Fe(III), whereas the surrounding pyroxene exhibited a more Fe(II)-rich signature. In addition, N K-edge XANES spectra similar to those of cultured Escherichia coli cells were detected from Fe(III)-rich phlogopite rims, whereas similar spectra were not detected from Fe(II)-rich pyroxene.
The Fe(III) observed in phlogopite may have formed through the following processes: (1) oxidation by atmospheric exposure, (2) hydrothermal alteration, or (3) dehydrogenation. In this study, atmospheric oxidation is unlikely to be the primary cause because sample handling and analysis were designed to minimize atmospheric exposure. In addition, the Fe(III)-bearing regions were localized within phlogopite grains, and Fe(III) was particularly enriched in the grain center, which is difficult to explain by atmospheric oxidation or hydrothermal alteration. Therefore, the Fe(III) in phlogopite may have formed through dehydrogenation during magma cooling. In trioctahedral mica, heating can oxidize Fe(II) to Fe(III) and generate hydrogen through the following dehydrogenation-reaction (Zema et al., 2010):[Fe2+ + OH-]phlogopite → [Fe3+ + O2-]phlogopite + 1/2H2The magmatic cooling stage of the Bushveld Igneous Complex is estimated that cooling process lasting several tens of millions of years after emplacement of the intrusion (Scotes et al., 2021). Thus, the Fe(III) observed in phlogopite in this study may have been preserved as an oxidative mineral phase formed in association with igneous activity approximately 2 billion years ago. The spatial correlation between Fe(III)-rich phlogopite rims and microbial signals suggests that phlogopite in deep subsurface ultramafic rocks retained Fe(III) as a potential electron acceptor and may have contributed to the formation of localized microbial habitats within the rock interior.