QUANTITATIVE X-RAY COMPOSITIONAL MAPPING BY ELECTRON PROBE MICROANALYSIS OF COMPLEX LUNAR SAMPLES

Analysis of the lunar sample collection is crucial for understanding geologic processes on the Moon. Owing to impact processes, many samples are highly brecciated, complicating interpretations of their origins. Phases of interest in these breccias are typically fine-grained, chemically zoned clasts. Complex lithic clasts are ideal candidates for quantitative X-ray compositional mapping [1,2,3]. The result of this technique is quantitative maps of a selected region, where each pixel contains a full wavelength-dispersive spectroscopy (WDS) chemical analysis [1].

Using Probe for EPMA (Probe Software), maps 512×512 – 1024×1024 pixel size were collected using a 1 – 10 μm step size (with a beam size equal to the step size), 20 – 50 msec count time, and 50 nA beam current (methods of [1]). A standard WDS calibration was done using the mean atomic number method, and a Φ(ρz) correction, relative to a set of standards, was calculated at each pixel using CalcImage. The data is filtered to eliminate pixels with anomalous analytical totals, such as analyses that occurred on cracks or boundaries between grains. Using the data generated it is possible to obtain a bulk composition for any region of the quantitative map. In addition to the visual data products, all numerical data is output for processing in a spreadsheet, e.g., Microsoft Excel.

Quantitative mapping extends the analytical capabilities of the electron microprobe beyond conventional WDS spot analysis. For example, quantitative maps of lunar meteorite NWA 3170 reveal the full range of zoning in pyroxene by providing the wt.% of mapped elements at each pixel. Spot analyses (n=31) of the pyroxene indicate that composition ranges from Wo₁₄En₄₆Fs₄₀ to Wo₃₆En₅Fs₅₉, whereas quantitative compositional maps yielded over 4000 data points and indicate that the pyroxene has a more varied composition than captured by spot analyses, including compositions with moderate Mg and high-Ca concentrations, which were not included in spot analysis. This method is also applied to obtaining the bulk composition of fine grained clasts, such as a eutectoid intergrowth in NWA 2727, as an alternative to defocused beam WDS analysis.

[1] P. K. Carpenter et al., LPSC 44 (2013) Abstract 1827.

[2] S. N. North-Valencia et al., M&M 2014, Abstract 714.

[3] P. K. Carpenter et al., LPSC 48 (2017), Abstract 2607.