Nanoscale characterisation of crystal zoning

• Application of low kV FEG-EPMA and NanoSIMS to zoned crystals
• NanoSIMS attained relative chemical profiles on a nanoscale spatial resolution.
• Quantitative nanoscale spatial resolution achieved by low kV FEG-EPMA

Advances in analytical techniques are fundamental to the enhanced understandings of many geological processes. Zoned volcanic crystals have been analysed by low (5) kV field emission gun electron probe micro-analyser (FEG-EPMA) and NanoSIMS to obtain sub-micrometre chemical profiles and compared to time-of-flight SIMS (TOF-SIMS) and high (15–20) kV EPMA profiles. Plagioclase and orthopyroxene crystals have been analysed by FEG-EPMA, at accelerating voltages of 5 kV providing a spatial resolution (step size) of ≤ 350 nm (the resolution of the lowest energy X-ray) for orthopyroxene crystals using a 30 nm beam and ca. 750 nm for plagioclase crystals which at low voltages are unstable and require a 500 nm defocused beam. Step sizes are comparable in size to interaction volumes. Analytical protocols are detailed that permit quantitative major and minor element compositions to be acquired at similar precision and accuracy as traditional EPMA analyses at 15–20 kV. NanoSIMS analysis of the same crystals provides a greater spatial resolution of up to 200 nm and allows the measurement of Li also. The NanoSIMS profiles, however, cannot currently be quantified. The ability to analyse crystals at sub-micrometre scales is demonstrated by the good agreement between NanoSIMS, FEG-EPMA, conventional EPMA and TOF-SIMS data. FEG-EPMA, NanoSIMS and TOF-SIMS techniques have broad applications within the earth sciences. In petrologic studies for example, these methods have the ability to analyse small crystals in experimental charges and provide chemical profiles of crystal zoning at a spatial resolution of ca. 200–300 nm. Such profiles are important in crystal forensics and diffusion chronometry studies. The implications for the latter application are that timescales of volcanic processes that occur in the days–years immediately prior to the eruption can now be studied.