Microanalysis of tephra by LA-ICP-MS - Strategies, advantages and limitations assessed using the Thorsmork ignimbrite (Southern Iceland)

Micron-scale analysis of vesicular volcanic glass can be problematic because thin vesicle walls and junctions limit the area available for analysis, subsurface vesicles limit the vertical thickness available, microcrysts at or below the surface may contaminate glass analyses and some glasses show compositional banding. In addition, distal tephra are very small (10-100 mu m) and material may be sparse. We have analysed the MPI-DING reference glasses and natural tephra samples (pumice, scoria and fiamme) from the Thorsmork ignimbrite (Southern Iceland) using laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS). Three different reduction strategies are used: averaging, uncertainty weighting and log-linear regression. We then assess the data quality achieved using the various strategies. Using our technique we show that the main limiting factor on data quality is precision, particularly for natural tephra analyses. At >20,000 cps, relative standard deviations (%RSDs) in the Thorsmork tephra are 5-10% - approximately twice those achieved in the MPI-DING glasses (3-5%) at the same conditions. Rhyolitic pumice and fiamme from the Thorsmork ignimbrite are compositionally homogenous. The proximal deposit also contains subordinate basalt scoria, therefore the deposit is bimodal. The Thorsmork rhyolite correlates with the North Atlantic Ash Zone 2 (NAAZ2) tephra described in a marine sediment core (Lacasse and Garbe-Schonberg, 2001, JVGR 170,113-147). (C) 2010 Elsevier B.V. All rights reserved.