A major acquisition under the CRITMAG grant was a FEG-EPMA (Field Emission Gun Electron Probe MicroAnalyser), which was purchased in 2012 for around €1 million. The instrument is housed in the microbeam laboratories at the School of Earth Sciences at the University of Bristol.
What is FEG-EPMA?
Electron probe microanalysis (EPMA) is a non-destructive analytical technique that is used to quantify the chemical composition of solid materials on sub-millimetre scale. A polished sample (usually a thin section or polished stub) is inserted into a vacuum before being bombarded with a focussed beam of electrons. Target spots on the sample surface are ~ 1 – 40 µm in diameter. The electrons interact with uppermost portion of the sample under the target, generating X-rays of wavelengths and energies specific to the elements present. Using this method, it is possible to detect every element in the periodic table with atomic numbers higher than boron (i.e. >5). EPMA can also be carried out over a relatively large area (1 mm) to create ‘maps’ of the distribution of individual elements.
Most microprobes generate electrons via heating of a tungsten or lanthanum hexaboride thermionic filament. FEG-EPMA differs from traditional EPMA in that the source of electrons is provided by a Field Emission Gun (FEG) electron source, whereby a large negative electric field of several kilovolts is applied to a sharply pointed tungsten tip of ~ 100 nm diameter. Unlike thermionic filaments, this cathode is not heated, the electrons being emitted due to the concentration of the huge electric charge over such a small area. The resulting beam of electrons is narrower, lower signal to noise ration and more stable, lending microprobes with FEG sources the highest possible spatial resolution that can be achieved by EPMA.
The CRITMAG FEG-EPMA
The FEG-EPMA purchased under the CRITMAG grant is a JEOL JXA8530F Hyperprobe and is equipped with 5 wavelength dispersive spectrometers and 1 energy dispersive spectrometer. It has a quantitative resolution of <100 nm, depending on the material being analysed.
What is FEG-EPMA used for?
In igneous petrology, EPMA is used to quantify the chemical composition of mineral phases and glasses in volcanic and plutonic rocks, and in experimental run products. It is particularly valuable in the study of chemical zoning in minerals, which is often invisible using light microscopy.
The increased spatial resolution available from FEG-EPMA has two main advantages. Firstly, it allows quantitative analysis of mineral phases <1 µm diameter, common features of experimental charges and rapidly quenched natural samples, where compositional data was previously unobtainable. Secondly, it reveals previously unrecorded subtleties in the chemical zoning of volcanic phenocrysts; when applied to diffusion chronometry, the ability to map elements on nanometre scale translates to a temporal resolution of months and weeks (e.g., Saunders et al., 2014 – read summary).
The use of FEG-EPMA to interrogate the geologic record is very much an area of active research. Globally, there are only a small number of such instruments tailored specifically for the analysis of geologic samples, and thus the related research conducted by CRITMAG-funded researchers at the University of Bristol is at the forefront of scientific discovery.
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