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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 29 Jun 2018

Research article | 29 Jun 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Geoscientific Instrumentation, Methods and Data Systems (GI).

Artifacts from manganese reduction in rock samples prepared by focused ion beam (FIB) slicing for X-ray microspectroscopic analysis

Dorothea S. Macholdt1, Christopher Pöhlker1, Jan-David Förster1, Maren Müller2, Bettina Weber1, Michael Kappl2, A. L. David Kilcoyne3, Markus Weigand4, Klaus Peter Jochum1, and Meinrat O. Andreae1,5 Dorothea S. Macholdt et al.
  • 1Max Planck Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany
  • 2Physics of Interfaces Department, Max Planck Institute for Polymer Research, Mainz, Germany
  • 3Lawrence Berkeley National Laboratory, Berkeley, CA, United States
  • 4Modern Magnetic Systems Department, Max Planck Institute for Intelligent Systems, Stuttgart, Germany
  • 5Geology and Geophysics Department, King Saud University, Riyadh, Saudi Arabia

Abstract. Manganese (Mn)-rich natural rock coatings, so-called rock varnishes, are discussed controversially regarding their genesis. Biogenic and abiogenic mechanisms, as well as a combination of both, have been proposed to be responsible for the Mn oxidation and deposition process. We conducted scanning transmission X-ray microscopy - near edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS) measurements to examine the abundance and spatial distribution of the different oxidation states of Mn within these nano- to micrometer thick crusts. Such microanalytical measurements of thin and hard rock crusts require sample preparation with minimal contamination risk. Focused ion beam (FIB) slicing, a well-established technique in geosciences, was used in this study to obtain 100–200nm thin slices of the samples for X-ray transmission spectroscopy. However, even though this preparation is suitable to investigate element distributions and structures in rock samples, we observed that, using standard parameters, modifications of the Mn oxidation states occur in the surfaces of the FIB slices. Based on our results, the preparation technique likely causes the reduction of Mn4+ to Mn2+/3+. We draw attention to this issue, since FIB slicing, SEM imaging, and other preparation and visualization techniques operating in the keV range are well-established in geosciences, but researchers are often unaware of the potential for reduction of Mn and possibly other elements in the samples’ surface layers.

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