CONSTRAINING PRE-ERUPTIVE MAGMATIC CONDITIONS FOR GUATEMALAN DOME-FORMING ERUPTIONS

Cerro Quemado and Almolonga are two lava domes located in northwestern Guatemala, approximately 10 km from the city of Quetzaltenango. Both domes reside near Santa María, which produced a massive eruption of dacite and basaltic andesite in 1902 and subsequently created the Santiaguito dome that has been erupting periodically since 1922. Cerro Quemado’s most recent eruption in 1818 produced a dacitic lava flow that contains phenocrysts of plagioclase, olivine and biotite. Cerro Quemado lavas also contain mafic inclusions that hint at mixing between a felsic and mafic magma end-member. The Almolonga lava flow is andesitic to dacitic and is composed of plagioclase, amphibole and two-pyroxene. The Almolonga lava does not contain any mafic inclusions.

The goal of this research was to use the mineral assemblages in the Cerro Quemado and Almolonga samples and apply various thermobarometers and hygrometers to delineate the temperature, pressure, and water content in the magmas prior to eruption. Phenocryst data was analyzed using the electron microprobe at the University of Wisconsin-Madison. Plagioclase and olivine compositions in Cerro Quemado samples indicate magmatic temperatures between 1000-1160°C. These mafic end-member temperatures coincide with the pre-eruptive temperatures (1020 °C) from the 1902 Santa María basaltic andesite eruption (Singer et al., 2013). Pressure estimates from plagioclase crystallization exceed 300 MPa and match well with estimates for magma storage beneath the Santiaguito dome (Scott et al., 2012). Plagioclase hygrometers indicate water contents between 1.8-3.0 wt.%, similar to the 1.4-6.9 wt.% for the Santa María dacite (Singer et al., 2013). Almolonga samples using amphibole thermobarometry indicates pre-eruptive temperatures of 807-933°C, pressures of 120-300 MPa, and water contents of 3.9-6.1 wt.%, conditions similar to those proposed by Andrews (2014) for the Santa María dacite erupted in 1902.