Estimation of deep redox conditions using crystalline rock cores that rarely contained redox sensitive Fe minerals via various analytical methods

This study aimed to evaluate the feasibility of various analytical methods for recognizing the characteristics of redox processes recorded in crystalline rock cores that rarely contain redox-sensitive Fe minerals (for example, pyrite, goethite, and biotite: Fe as Fe₂O₃ < 2 wt%). Nine rock core samples were extracted at a deep borehole and analyzed using a suite of analytical techniques, including X-ray fluorescence (XRF), X-ray diffraction, Fourier transform infrared spectroscopy, optical microscopy, micro-XRF (μ-XRF), Mössbauer spectroscopy, X-ray absorption near edge structure (XANES), and inductively coupled plasma spectrometry. Compared to the samples from greater depths (607, 894, 897, and 959 m), those from shallower depths (22, 171, 271, and 337 m) possessed oxidizing characteristics: (1) widespread distribution of ferric (oxy)hydroxides, (2) ferric iron in mineral structures, and (3) a high Fe(III)/Fe(II) ratio. Although the matrices of the samples barely contained Fe-bearing minerals, the presence of secondary ferric (oxy)hydroxides was a clear indication of the oxidizing conditions. Optical microscopy and μ-XRF analysis were effective in identifying the distribution of ferric (oxy)hydroxides, whereas Mössbauer and Fe K-edge XANES spectroscopy were useful for identifying their phases. Unlike previous work, the Ce anomaly or Ce(IV)/Ce(III) ratio in the rock matrices was not as a reliable indication of oxidation, compared to presence of ferric (oxy)hydroxides, likely because of the low Ce concentration. The results suggest that the deep redox conditions of crystalline rocks with low Fe-bearing mineral content can still be estimated by the presence of ferric (oxy)hydroxides with the help of other auxiliary techniques used in this study.