Developments in the analysis of volcanic glass shards by laser ablation ICP-MS: Quantitative and single internal standard-multi-element methods

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been applied to the analysis of fine grained (125-250 μm) volcanic glass shards separated from tephra deposits. Our earlier calibration strategies used Ce as an internal standard (which required the use of another trace element analytical technique to determine Ce) followed by off-line data correction by comparison with well characterised glass separates. We have progressed from these earlier studies to use a minor isotope of a major element, routinely determined by EPMA in tephrochronological studies, as an internal standard for LA-ICP-MS analysis. Internal standards used during this study include ²⁹Si, ³⁰Si, or ⁴⁴Ca for fully quantitative determinations, or ⁵⁷Fe for a single internal standard-multi-element calibration (so called 'semi-quantitative' analysis). No subsequent off-line data correction is required. The differences between each technique are minimal and choice of technique depends largely upon instrument sensitivity. As little as 80 μg of material is consumed during a single determination which takes approximately two minutes, and detection limits are below 1 part per million. Accuracy is generally 15% when compared with other analytical techniques, and precision may vary between 5 and 20%, the worse 'precisions' reflecting within-sample variability. Rare earth and other trace element data can be used to correlate or distinguish individual tephra deposits with confidence, and specific examples are illustrated with Quaternary tephra deposits from Alaska and India and Cretaceous tephra deposits from Banks Island in Arctic Canada.