Quantitative automated mineralogy to constrain metamorphic processes using ZEISS Mineralogic

Abstract

<p>The Scanning Electron Microscope (SEM) is the most prolific piece of analytical equipment in the Earth Sciences, therefore quantitative mineral chemistry obtained directly from the SEM has the potential to streamline many geological fields. Mineral chemistry provides direct constraints on geological processes that are used in a wide variety of Earth Science disciplines. As a result, major element analysis of rock forming minerals have been one of the major contributors to geochemistry for decades. Electron beam techniques have been the most widely used method of obtaining in situ major element chemistry, dominated by the quantitative Wavelength Dispersive Spectroscopy (WDS) employed by the Electron Probe Micro Analyser (EMPA). More rapid, and typically more qualitative Energy Dispersive Spectroscopy (EDS) major element measurements are often obtained on a standard SEM instrument.</p><p>The relative simplicity of the EDS technique saw the growth of automated mineralogy systems beginning in the 1980’s. The peaks of EDS spectra are characteristic of the major elements present, and therefore lookup tables can be used to match the spectra to known mineral compositions and provide a likely mineralogy in both grain mounts and mapped thin sections. The automated mineral analysis technique remained essentially unchanged for decades, with an experienced operator required for many of the analytical tasks, such as creating the files for matching spectra to known minerals, processing the data, and interpreting complex phases and solid solutions (e.g. Fe/Mg-bearing silicates).</p><p>The ZEISS Mineralogic automated quantitative mineralogy (AQM) takes a new approach, using EDS detectors, but following an analytical protocol more closely aligned with EPMA. A combination of matrix corrections, peak deconvolution, and standard calibration means that peak intensities are converted directly into wt% element directly at the time of analysis. The result is a data output that can be immediately interpreted, even for minerals not previously analysed, by both new and experienced users.</p><p>Here we demonstrate the use of the ZEISS Mineralogic system for mapping thin sections from high grade metamorphic rocks. The bulk chemistry of the entire thin section, as well as individual mineral compositions can be used to constrain P-T conditions directly from the SEM, without the need for an additional step of obtaining mineral chemistry from an EPMA. With quantitative analysis at every pixel, major element profiles can be obtained at any point in the thin section, and P-T can therefore be determined from any domain within the mapped section. This approach makes the use of P-T pseudosections possible with greater speed and flexibility than has previously been possible.</p>