Improving the reliability of microprobe-based analyses of andesitic glasses for tephra correlation

Andesite tephras have not been used in tephrostratigraphic studies to the same extent as rhyolitic units because they are seen as being chemically more complex than the latter. They are particularly difficult to 'fingerprint' owing to apparent heterogeneity in glass analyses from single andesitic layers. Using case study tephras from two andesite volcanoes in New Zealand, Mts Taranaki and Ruapehu, glass chemical heterogeneity is shown to be due predominantly to (1) differing particle types generated during closed-and open-vent phases of single eruption sequences, resulting in a broad range of glass compositions, and (2) contaminated glass microprobe analyses by various proportions of microlite phase(s). A simple evaluation procedure using least-squares mixing calculations is presented to classify glass data sets for hybrid analyses and to estimate the proportions of the main contaminant microlite phase. By employing particle morphology studies as well as the glass-analysis evaluation procedure, variations in andesitic glass compositions can be significantly reduced. Hence this approach shows promise for enabling the use of some andesitic tephras for tephrostratigraphic correlation. This may facilitate the addition of a new degree of resolution in tephrostratigraphic records.11 page(s