Submicron Quantitative Analysis by Field-emission Gun EPMA at low kV (Invited)

Electron microprobes with a field-emission gun source have superior imaging capabilities compared to probes with a traditional W-filament and even LaB6 source. Fine-scale zoning and complex mineral intergrowths can be chemically identified qualitatively below 100 nm scale, depending on compositional contrast, mean atomic number, sample preparation and many other factors. Despite the small beam size (<50 nm for most routine operating conditions) on the sample surface, the activation volume from which X-rays are generated is dominantly controlled by acceleration potential and the sample material. Quantitative chemical spatial resolution, however, is mainly needed at grain boundaries, where accurate minor element acquisition can be strongly distorted by secondary fluorescence. Using the JEOL JXA-8500F probe at UH and Probe for EPMA software, I explored the analytical conditions needed to acquire accurate minor element concentrations at a submicron scale near such grain boundaries. The Ca content of low-Ca orthopyroxene (opx) is a useful and sensitive thermometer to help reconstruct the cooling history of mantle peridotite bodies exposed on land and on the ocean floor. Coarse Ca-rich clinopyroxenes (cpx) typically have abundant fine submicron to a few micron wide opx exsolution lamellae. With decreasing temperature, the equilibrium opx lamellae incorporate lower Ca concentrations. I obtained Ca contents of cpx-hosted opx lamellae from an extremely well preserved ocean floor lherzolite (Gakkel Ridge, Arctic Ocean). This undeformed sample was previously studied in great detail by conventional EPMA, SIMS and NanoSIMS, the latter technique yielding a CaO content in the opx lamellae of 0.45 wt% [1]. Quantitative Ca profiles across a 1.5 micron wide opx lamella were acquired at 15, 10 and 7 keV, at a constant probe current (15 nA). Fe concentrations were specified. At 15 keV, the minimum CaO content in the central part of the opx lamella was 0.7 wt%. At 10 keV, this value was slightly lower (0.6%). At 7 keV, which is below the critical excitation energy of Fe-ka (7.1 keV), the central part of the lamella remained constant at 0.51 wt% CaO, as close as 0.6 micron from the grain boundary. Three-micron wide lamellae, previously analyzed by NanoSIMS yielded slightly lower concentrations (0.46 wt%), confirming the accuracy of this low-kV approach. Reference: [1] Hellebrand et al. Contrib. Mineral. Petrol. 150, 486-504.