Testing the reliability of the JEOL FEGSEM 6500F electron microprobe for quantitative major element analysis of glass shards from rhyolitic tephra

Abstract

Davies, S. M. 2010 (January): Testing the reliability of the JEOL FEGSEM 6500F electron microprobe for quantitative major element analysis of glass shards from rhyolitic tephra.Electronprobe microanalysis is now widely adopted in tephra studies as a technique for determining the major element geochemistry of individual glass shards. Accurate geochemical characterization is crucial for enabling robust tephra-based correlations; such information may also be used to link the tephra to a specific source and often to a particular eruption. In this article, we present major element analyses for rhyolitic natural glass standards analysed on three different microprobes and the new JEOL FEGSEM 6500F microprobe at Queen's University Belfast. Despite the scatter in some elements, good comparability is demonstrated among data yielded from this new system, the previous Belfast JEOL-733 Superprobe, the JEOL-8200 Superprobe (Copenhagen) and the existing long-established microprobe facility in Edinburgh. Importantly, our results show that major elements analysed using different microprobes and variable operating conditions allow two high-silica glasses to be discriminated accurately.

Geochemical signatures of tephras from Quaternary Antarctic Peninsula volcanoes

“…15B) at identical microprobe conditions, thus contra-indicating analytical problems as source of the trends observed in the Deception Island data. Moreover, studies on inter-microprobe differences of rhyolitic glass analyses found good comparability applying different sets of analytical parameters (Coulter et al, 2010), including also a set of parameters similar to ours (see section 5: methodology).…”

Tephrochronology and its application: A review

“…However, in other cases, geochemical differences can be subtle, as noted previously, and hence high levels of precision and accuracy are required for successful correlation and identification. Further, small differences between results may be produced by different laboratories (as can occur with all analytical systems) but these have been, and continue to be, addressed by use of appropriate standards and agreed protocols (Froggatt, 1992;Turney et al, 2004) and by interlaboratory comparison exercises (Hunt and Hill, 1996;Potts et al, 2002;Coulter et al, 2010). re-analysed samples of potential and likely correlatives from reference collections at the same time they analysed the unknown tephras to help eliminate possible small, day-to-day differences in probe calibration.…”

“…The X-rays produced are of particular energies and wavelengths relating directly to individual elements, and their intensities are a measure of specific element abundance (Shane, 2000;Reed, 2005;Coulter et al, 2010). Typically, around 10 to 13 elements -Si, Al, Ti, Fe, Mn, Mg, Ca, Na, K, and P, and also S, F, and Cl -are able to be assayed (depending in part on glass composition) (Hayward, submitted for publication).…”

“…Being non-crystalline, glass strictly is a mineraloid rather than a mineral. Because of its unique properties, glass analysis by EPMA involves some different procedures from those used to analyse most minerals (phenocrysts or free crystals), and care and understanding at each step are therefore required to obtain accurate and robust data (Froggatt, 1983(Froggatt, , 1992Hunt and Hill, 1993;Shane, 2000;Westgate et al, 2008;Coulter et al, 2010; Hayward, submitted for publication).…”

“…Two types of analysis are available for the detection and recording of X-ray signals: energy-dispersive spectrometry (EDS) and wavelength-dispersive spectrometry (WDS). Both EDS and WDS systems are widely used in tephra studies and the use of one or the other involves compromises (see Coulter et al, 2010, for discussion). Usually a defocussed beam (e.g., 10-20 m in diameter), or a rastered beam extending over an area of 5 x 5 µm for example, and moderate beam current, are deployed to minimise mobilisation (instability) and hence loss of Na in particular (also K may be underestimated, and Si and Al overestimated) (Froggatt, 1983;Hill, 1996, 2001;Suzuki, 1996;Davies et al, 2008;Tryon et al, 2008;Coulter et al, 2010).…”