The ion microprobe was used to analyze trapped melt inclusions in phenocrysts from two rhyolitic eruptions for H2O, F and incompatible trace elements. Eleven melt inclusions from phenocrysts in air-fall tephra from Obsidian Dome near Long Valley, USA gave an average of 4.1±1.2 wt.% H2O and showed large variations in F and incompatible trace elements reflecting a complex history reported to involve mixing of at least two magma types. Eight inclusions in phenocrysts from the Taupo "ultraplinian" event, New Zealand gave 4.5 ± 0.8 wt.% H2O with small variations in the other elements analyzed. Measured water contents are similar to earlier estimates of the pre-eruptive water contents of these and similar rhyolites and major- and trace-element analyses of inclusions compare closely with bulk analyses suggesting that the analyses of inclusions represent the chemistry of the magma at the time of entrapment.
ASJC Scopus subject areas
- Geochemistry and Petrology