Magmatic water content of Pacaya Volcano, Guatemala, determined from a thermodynamic model for the plagioclase-liquid hygrometer
Knudson, Christine A.
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Pacaya Volcano is an active composite volcano located in southern Guatemala, about 30 km from the nation's capital, Guatemala City. This volcano has variable eruptive styles ranging from non-explosive to moderately explosive. Volcanic rocks from Pacaya are mostly basaltic to basaltic andesite and tend to be porphyritic, containing approximately 25-35% crystals, dominantly plagioclase. Previous data from olivine-hosted melt inclusions in Pacaya's tephras suggest relatively low magmatic water contents for a subduction zone volcano, particularly compared to neighboring composite volcanoes along the Central American Volcanic Arc.The goal of this research was to use plagioclase phenocrysts in basaltic andesite lavas from Pacaya as a secondary method to verify the unusually low magmatic water contents reported previously. Water contents were estimated using a plagioclase-liquid hygrometer. Plagioclase crystals were selected from thin sections made of lavas from five different eruptive events. Phenocrysts were analyzed using the electron microprobe at Northern Illinois University. Plagioclase crystallization temperatures were obtained using a geothermometer. Water contents were calculated to be ≤ 3.6 wt. % H2O for lavas erupted from Pacaya, and are consistent with olivine-hosted melt inclusion data, but low for subduction zone volcanoes.