USGS1992
Rare melt inclusions ~ 100 μ m 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 H 2 O, OH groups, and CO 2 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 H 2 O 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 H 2 O concentrations believed to represent magmatic H 2 O contents (wt%): Llao Rock, 5.3, 5.3; Cleetwood,3.8,4.7; climactic 3.9 ± 0.2 (1 σ, n = 6). Ratios of OH/molecular H 2 O indicate closure temperatures of 200–500 °C that reflect syn- or posteruptive cooling. CO 2 concentrations are ≤25 ppm. H 2 O and CO 2 concentrations indicate saturation pressures of 1.0–1.8 kbar or depths ≥5 km. Six inclusions from climactic pumice average 400 ± 40 (1 σ ) 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 H 2 O. Location of melt inclusions near boundaries of patchy zones, which are mantled by oscillatory-zoned overgrowths, suggests that their H 2 O concentrations represent magmatic values significantly before eruption. Although mean H 2 O concentrations of analyzed melt inclusions decrease for the three successive eruptions, it is not certain that this indicates a corresponding change in magmatic H 2 O content during the interval between eruptions.