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High resolution imaging of crystal zoning reveals events occurring in the months and days before an eruptions
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