MAGMATIC WATER CONTENT OF PACAYA VOLCANO, GUATEMALA, DETERMINED FROM A THERMODYNAMIC MODEL FOR THE PLAGIOCLASE-LIQUID HYGROMETER

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. Pacaya’s modern cone is accessible, and the bulk rock compositions have been well documented. Volcanic rocks from Pacaya are mostly basaltic to basaltic andesite and tend to be porphyritic, containing approximately 20-30% crystals. Data from olivine hosted melt inclusions suggest relatively low water contents for a subduction zone volcano, particularly compared to neighboring composite volcanoes along the Central American Volcanic Arc (Walker et al., 2003).

The goal of this research is to use plagioclase phenocrysts in basaltic lavas from Pacaya as a secondary method to verify the unusually low magmatic water contents as reported in previous work. Water contents were estimated using the plagioclase-liquid hygrometer (Lange et al., 2009). Plagioclase crystals were selected from thin sections made of bulk rocks from five different eruptive events. Phenocrysts were analyzed using the electron microprobe at Northern Illinois University. The plagioclase-liquid hygrometer from Lange et al. (2009) is a semi-empirical model based in part on thermodynamic data. Their model works for plagioclase compositions of An₉₃-An₃₇, and for liquid compositions of 46-74 wt. % SiO₂. This method requires an independent measurement of plagioclase crystallization temperature. The crystallization temperatures were estimated using the MELTS software package (Ghiorso and Sack, 1995).