Estimating the iron oxidation state of serpentinite using X-ray absorption fine structure spectroscopy

Abstract

Iron can exist in serpentinite in various oxidation states. Changes in the oxidation state of Fe play a crucial role in geological processes, including supporting diverse biological systems on the ocean floor and the redox states of aqueous fluids released in subduction zones. Synchrotron X-ray absorption near-edge structure (XANES) spectroscopy has been used to measure the oxidation state of Fe in serpentine minerals; however, a general scheme for estimating the oxidation state of Fe in serpentinite composed of several Fe-bearing minerals has not been established. We have evaluated the accuracy of using XANES spectra to estimate the Fe3+/ΣFe ratio (molar ratio of Fe3+ to total Fe) of physical olivine–magnetite mixtures and serpentinites. Two approaches were tested: (1) linear combination fitting (LCF) and (2) pre-edge peak analysis of XANES spectra. Estimating Fe3+/ΣFe using analysis of the pre-edge peaks was not always suitable when magnetite was included. Using LCF, XANES spectra are well fitted by the optimal combination of Fe-bearing minerals, even for mixtures of magnetite and Fe-bearing silicate minerals (i.e., serpentine and olivine). The Fe3+/ΣFe ratios of the serpentinites were estimated with a root mean squared error (RMSE) of 0.14 with LCF. These results demonstrate that LCF of XANES spectra is useful for estimating the oxidation state of Fe in serpentinites.