Towards a protocol for the trace element analysis of glass from rhyolitic shards in tephra deposits by laser ablation ICP-MS

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is used widely to analyse single volcanic glass shards from Quaternary tephra deposits for stratigraphic correlation. As yet, no generally accepted protocol for these analyses exists and published methods report significant differences in crater size and calibration strategies. Using Ca as the internal standard' (Ca-I.S.), and not Si-I.S., for the analysis of rhyolitic glass can cause significant problems, because of ablation of (i) Ca-rich phenocrysts, notably ubiquitous' plagioclase, but also calcic-amphibole, apatite and allanite or (ii) mounting epoxy resin which can contribute to the Ca internal standard signal. Using Ca-I.S. can cause underreporting of many elements which concentrate in Ca-rich phenocrysts, making their ablation difficult to recognize, but when using Si-I.S., no such underreporting occurs. Additionally, larger ablation craters (50 mu m diameter) potentially incorporate even small volumes of phenocryst material, whereas smaller craters (