Pre-eruptive water content of rhyolitic magmas as determined by ion microprobe analyses of melt inclusions in phenocrysts

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.