Microanalysis of tephra by LA-ICP-MS — Strategies, advantages and limitations assessed using the Thorsmörk 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 μm) and material may be sparse. We have analysed the MPI-DING reference glasses and natural tephra samples (pumice, scoria and fiamme) from the Thorsmörk 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 Thorsmörk 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 Thorsmörk ignimbrite are compositionally homogenous. The proximal deposit also contains subordinate basalt scoria, therefore the deposit is bimodal. The Thorsmörk rhyolite correlates with the North Atlantic Ash Zone 2 (NAAZ2) tephra described in a marine sediment core (Lacasse and Garbe-Schönberg, 2001, JVGR 170, 113–147).

Research Highlights
► Data reduction by averaging is the most appropriate for natural tephra analysis.
► Signal intensity is the limiting factor for tephra analysis, dictating the minimum spot size.
► The Thorsmörk ignimbrite comprises homogenous rhyolitic pumice and subordinate basaltic scoria.
► The rhyolitic Thorsmörk shows a good statistical correlation with North Atlantic Ash Zone 2.