Development of new techniques enables quantification of major and trace element zoning in lava phenocysts to nanometre scale
Whilst the long-term evolution of a magmatic system may occur over many thousands of years, changes immediately preceding volcanic eruptions may occur on the timescale of months, days and minutes. This kind of resolution is not obtainable using classical radiometric dating, being is constrained by the half-lives of elements. Within the last decade, petrologists have increasingly turned to the technique of diffusion chronometry; here, the blurring in chemical zoning within lava phenocrysts is used to estimate the duration between the last perturbation of the magmatic system and the eruption. The better the resolution of the electron microscope image, the smaller the timescale that can be calculated from a crystal.
Saunders and co-workers interrogated plagioclase and orthopyroxene crystals from the 1980 eruption of Mt St Helens using the FEG-EPMA at the University of Bristol, a high-resolution electron microprobe purchased as part of the CRITMAG grant. The FEG-EPMA has a beam size of ~ 30 nm, two orders of magnitude smaller than is common in conventional microprobe analysis. This allows collection of both detailed back scattered electron (BSE) images and major element composition. The latter has a quantitative resolution of 750 nm (0.00075 mm), making FEG-EPMA ideally suited for application to diffusion chronometry. The authors also look at two other methods of microanalysis, NanoSIMS and TOF-SIMS, which cannot image samples but have the advantage of being able to measure a large range of major and trace elements at ≥50 nm resolution.
Using one, or a combination of the three techniques described in the paper, the authors demonstrate is it possible to obtain chemical profiles of zoned minerals with nanoscale precision. Such detail facilitates characterisation of events that occurred in the months and days before historic eruptions.
Saunders K, Buse B, Kilburn MR, Kearns S, & Blundy J (2014) ‘Nanoscale characterisation of crystal zoning’, Chemical Geology, 364, 20-32. http://dx.doi.org/10.1016/j.chemgeo.2013.11.019
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.