High spatial resolution electron probe microanalysis of tephras and melt inclusions without beam-induced chemical modification

Wavelength dispersive analysis of tephra grains and melt inclusions by EPMA has been carried out using a focused beam of 3 mu m diameter without detected loss of sodium or potassium in standard glasses, including anhydrous basalts, a slightly hydrated rhyolite and a sodium-rich intermediate composition. The ability to make analyses without chemical modification is strongly dependent upon current density at the analysis site. Analysis with narrow beams requires extremely low beam currents that are normally associated with energy dispersive analysis. Experiments indicate that a value of 0.1 nA/mu m(2) must not be exceeded, at least for moderately hydrated samples, if sodium loss is to be avoided. High resolution analysis without beam-induced analytical artefacts enables fully quantitative analysis of very distal and/or highly vesicular tephras and very small melt inclusions without the need to use post-analysis corrections. This development has enabled high quality analyses from crypto-tephra layers that were previously impossible to analyse, and has removed the potential for sampling bias within mixed tephra layers by making ( in most cases) tephra grains accessible for analysis. The use of focused beams also increases the level of automation, and hence the cost-effectiveness of data collection. The current data suggest limits to the applicability of the beam conditions reported, and that they may lead to alkali loss in compositions most prone to beam-induced modification such as significantly hydrated and/ or sodic tephras.