Abstract

Rare melt inclusions ~ 100 μm in diameter trapped near the boundaries of corroded patchy zones in plagioclase phenocrysts from Plinian pumice of three Holocene eruptions were analyzed by IR spectroscopy for molecular H2O, OH groups, and CO2 and by electron microprobe for Cl and F. The three rhyodacitic eruptions, each of which began with a Plinian phase, occurred over ~200 yr. The Llao Rock and Cleetwood eruptions ended with degassed lava flows and the subsequent climactic eruption with voluminous ignimbrite. Groundmass glass and melt inclusion compositions (anhydrous) are similar. Inclusions with total H2O concentrations of 1.2–3.4 wt% are adjacent to fractures or are hourglass inclusions, suggesting partial degassing caused by depressurization. Melt inclusions in phenocrysts in climactic ignimbrite may have vesiculated for the same reason. Intact inclusions in Plinian pumices have total H2O concentrations believed to represent magmatic H2O contents (wt%): Llao Rock, 5.3, 5.3; Cleetwood,3.8,4.7; climactic 3.9 ± 0.2 (1σ, n = 6). Ratios of OH/molecular H2O indicate closure temperatures of 200–500 °C that reflect syn- or posteruptive cooling. CO2 concentrations are ≤25 ppm. H2O and CO2 concentrations indicate saturation pressures of 1.0–1.8 kbar or depths ≥5 km. Six inclusions from climactic pumice average 400 ± 40 (1σ) ppm F, 1880 ± 70 ppm Cl. F and Cl concentrations of the other samples are similar and not well correlated with each other or with total H2O.

Location of melt inclusions near boundaries of patchy zones, which are mantled by oscillatory-zoned overgrowths, suggests that their H2O concentrations represent magmatic values significantly before eruption. Although mean H2O concentrations of analyzed melt inclusions decrease for the three successive eruptions, it is not certain that this indicates a corresponding change in magmatic H2O content during the interval between eruptions.

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