Quantitative mapping of elements in a lava sample provides insights into the kinetics of explosively-erupting magmatic systems
As magmas ascend from depth towards the surface they undergo decompression and cooling, the former being responsible for the release of dissolved volatiles in the form of volcanic gases; all three factors induce crystals to form. The speed at which magma travels towards the surface affects the rate of crystallisation, and as such, the study of the textures of explosively erupted lavas can reveal quantitative information about magmatic ascent rates and crystallisation history.
Muir and co-workers from the University of Bristol present a novel method of analysing lava microtextures. They harness a powerful new technique (EDS element mapping) whereby a rock sample is bombarded with electrons. Sample interaction with electrons produces a variety of emissions, some of which are X-rays with wavelengths characteristic to the elemental composition of the target. The combination of different elements translates into an energy spectrum which is analysed to determine the abundance of specific elements. This is repeated many thousands of times over a small area to build up a ‘map’ for each element, where the intensity of colour is proportional to the elemental concentration. Individual mineral phases can be identified and isolated; calculations are then performed to obtain the relative proportions, sizes and distributions of glass, minerals and bubbles.
The technique of EDS element mapping is applied to lavas from Mt St Helens erupted bewteen 1980 – 1986. The observed trends in microtextures are similar to those previously published, with groundmass crystallinity displaying a sharp increase after the catastrophic eruption in the summer of 1980, before increasing more gradually during the next dome-building phase of activity.
EDS element mapping presents significant advantages over the previous method of manually extracting data from greyscale backscattered electron images through faster data processing, reduction in operator time and accurate identification of all textural components. The authors also highlight the potential for coupling of this technique with developing technologies, such as field emission gun (FEG) sources, which would radically reduce acquisition time and enable better spatial resolution at small crystal sizes.
Muir, DD, Blundy, JD & Rust, AC 2012, ‘Multiphase petrography of volcanic rocks using element maps: a method applied to Mount St. Helens, 1980–2005′ Bulletin of Volcanology. http://dx.doi.org/10.1007/s00445-012-0586-0