Accessible Requires Authentication Published by De Gruyter December 7, 2020

Quantitative microscale Fe redox imaging by multiple energy X-ray fluorescence mapping at the Fe K pre-edge peak

Eric T. Ellison ORCID logo, Lisa E. Mayhew, Hannah M. Miller and Alexis S. Templeton
From the journal American Mineralogist

Abstract

Fe oxidation/reduction reactions play a fundamental role in a wide variety of geological processes. In natural materials, Fe redox state commonly varies across small spatial scales at reaction interfaces, yet the approaches available for quantitatively mapping the Fe redox state at the microscale are limited. We have designed an optimized synchrotron-based X‑ray spectroscopic approach that allows microscale quantitative mapping of Fe valence state by extending the Fe XANES pre-edge technique. An area of interest is mapped at nine energies between 7109–7118 eV and at 7200 eV, allowing reconstruction, baseline subtraction, and integration of the pre-edge feature to determine Fe(III)/ΣFe with 2 μm spatial resolution. By combining the Fe redox mapping approach with hyperspectral Raman mineralogy mapping, the Fe oxidation state distributions of the major mineral phases are revealed. In this work, the method is applied to a partially serpentinized peridotite with various Fe-bearing secondary mineral phases to trace the Fe transformations and redox changes that occurred during its alteration. Analysis with the Fe redox mapping technique revealed that the peridotite contained relict olivine with abundant Fe(II), while serpentine, pyroaurite, and another hydroxide phase are secondary mineral reservoirs of Fe(III). Although serpentine is not Fe-rich, it contained approximately 74% ± 14% Fe(III)/ΣFe. These analytical results are integral to interpreting the sequence of alteration reactions; serpentinization of primary olivine formed Fe(II)-rich brucite and oxidized serpentine, which could have contributed to H2 production during serpentinization. Subsequent weathering by oxidizing, CO2-bearing fluids led to the partial carbonation and oxidation of brucite, forming pyroaurite and a hydroxide phase containing dominantly Fe(III). This Fe redox imaging approach is applicable to standard petrographic thin sections or grain mounts and can be applied to various geologic and biogeochemical systems.

Funding source: Russian Science Foundation

Award Identifier / Grant number: DE-AC02-76SF00515

Funding statement: Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Raman and EPMA analyses were performed at the Raman Microspectroscopy Laboratory and the Electron Microprobe Laboratory, respectively, at the Department of Geological Sciences, University of Colorado-Boulder. This work was funded by the Rock Powered Life NASA Astrobiology Institute (Cooperative Agreement NNA15BB02A).

Acknowledgments

We thank Manuel Muñoz and Franck Bourdelle for providing mineral specimens with known Fe valence and coordination. We thank SSRL staff scientists Sam Webb and Courtney Krest for their assistance with the synchrotron XRF and XAS analyses. We thank Aaron Bell for assistance with EPMA analyses. We thank Sebastian Kopf for assistance with R code. We appreciate the suggestions from Mathew Marcus, M. Rita Cicconi, and three anonymous reviewers, which greatly improved the manuscript.

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Received: 2019-11-08
Accepted: 2020-04-24
Published Online: 2020-12-07
Published in Print: 2020-12-16

© 2020 Walter de Gruyter GmbH, Berlin/Boston