Microbeam methods for the analysis of glass in fine-grained tephra deposits: a SMART perspective on current and future trends

Abstract

Correlation of tephra deposits frequently relies on the analysis of glass shards separated from their host. Small shards from distal deposits (marine-, ice- or lake-cores, peat bogs) are difficult to analyse. Here current methods for glass shard analysis from (particularly) marine tephra deposits are reviewed. These methods apply equally to other distal deposits, where linking repositories of climatic information is central to many research programmes. Electron probe microanalysis is used widely to determine the major element compositions of volcanic glass. However, electron beam irradiation causes permanent damage to glass, especially hydrated or silica- and alkali-rich compositions. Recent developments have shown that reliable data is obtained with beam diameters >3 µm on basaltic or moderately hydrated rhyolitic glass, provided low electron beam current densities are used. For robust correlation of tephra deposits from different environments, it is recommended that analytical data are normalized to an anhydrous basis. Glass trace element analysis is commonly performed by laser ablation inductively coupled plasma mass spectrometry, which determines approx. 30 elements at <1 ppm from ablation craters 10 µm diameter, and although element fractionation occurs, it can be corrected. Recent developments in both methods should facilitate analysis of smaller, more distal material, expanding the geographical range over which reliable tephra correlations can be achieved.

Tephra without Borders: Far-Reaching Clues into Past Explosive Eruptions

“…With recent advances in microanalytical techniques, the composition of micronsized individual ash particles can be characterized. Electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) permit the determination of major, minor and trace-element abundances and lead (Pb) isotope data (Tomlinson et al, 2010;Kuehn et al, 2011;Hayward, 2012;Pearce et al, 2014;Kimura et al, 2015;Maruyama et al, in press). These techniques allow characterization of glass, minerals, and melt inclusions in minerals to support correlations between widely dispersed locations as well as to the source areas (e.g., Lowe, 2011;Matsu'ura et al, 2011;Smith et al, 2011c).…”

“…(4) Inter-laboratory comparison exercises have long been undertaken between electron microprobe laboratories (e.g., Kuehn et al, 2011) to provide a robust assessment of the data quality. Some efforts have been also made to introduce standardized protocol for laser-ablation trace element analyses (e.g., Tomlinson et al, 2010;Pearce et al, 2014). However, in contrast to microprobe analysis, the analytical conditions and equipment used for trace element analysis of single glass shards vary considerably.…”

The RESET project: constructing a European tephra lattice for refined synchronisation of environmental and archaeological events during the last c. 100 ka

“…As alluded to above, complications arise when highly evolved magmas, particularly those from a single volcano, are compositionally similar (e.g. Tomlinson et al, 2010;Pearce et al, 2014). Trace elements show greater variability than major elements because they are more strongly affected by differences in source composition and by sub-volcanic magmatic processes, such as fractional crystallisation and assimilation.…”

Marine tephrochronology: an introduction to tracing time in the ocean

“…In particular, the advances in analytical methods (e.g. Hall & Hayward 2014;Pearce et al 2014) together with the development of cryptotephra extraction techniques for marine sediments (e.g. Bourne et al 2010;Abbott et al 2013Abbott et al , 2014 highlight exciting possibilities for this technique.…”

“…These authors outline an innovative and inexpensive method for the preparation of tephra grains as small as 10-50 mm and highlight the importance of obtaining high-quality polished surfaces as an essential pre-requisite for the collection of high-quality geochemical data. Pearce et al (2014) expand on these analytical challenges, pointing out that, under appropriate analytical conditions, reliable electron microprobe data are now being obtained with beam diameters as small as 3 mm on basaltic or moderately hydrated rhyolitic glass. These authors make a recommendation that analytical data are normalized to an anhydrous basis in order to facilitate robust correlations from different environments.…”