Trace Element Analysis and Classification of Lunar Plagioclase and Olivine by Electron Microprobe Methods

Much of the Moon's surface has been saturated with impacts over the past ~4.5 Ga. Thus, most lunar samples are polymict, containing mineral-, glass-, and lithic-clasts from a variety of precursor lithologies whose provenances vary widely. One of the goals when studying lunar breccias is to identify the clast lithologies and determine likely provenances. Understanding the components of lunar breccias helps to interpret local and regional geology and the Moon's cratering history. Identification of lithic clasts using texture, mineral assemblage, and mineral compositions is usually straightforward and a general provenance is likewise apparent (e.g., mare, ferroan anorthosite, KREEP, etc.). Dealing with individual mineral clasts is more difficult, however, as major-element compositions by themselves may not be diagnostic. In these cases, concentrations of trace elements within individual mineral clasts can help identify their parent lithology. For example, Sr concentrations correlate with Na concentrations in lunar plagioclase, and different rock suites define different trends. Similarly, Ni and Co concentrations in olivine from different rock suites vary considerably (e.g., ferroan anorthosites, norites, dunites, troctolites, various basalt groups, etc.; Shearer and Papike; GCA 69:3445). Typically, Sr concentrations in plagioclase and Ni and Co concentrations in olivine are determined by SIMS analyses; however, it is possible to determine selected trace-elements using the electron microprobe, which is readily available for most investigators. Detection limits approaching 10s of ppm are achievable by simultaneous measurement with multiple spectrometers at high probe current and extended count times, with attention to background selection. As an example, Sr measurement using 4 spectrometers on a JEOL JXA-8200 electron microprobe, at 15 keV accelerating voltage, 200 nA probe current, and 150 seconds count time produced analytical uncertainties of ~11 ppm. Thus, Sr concentrations in lunar plagioclase (typically >200 ppm) and Ni and Co concentrations in lunar olivine (typically >80 ppm) are measurable.