THE 2012 ERUPTION OF COPAHUE VOLCANO, ARGENTINA

Copahue Volcano, located on the Chile-Argentina border at 37.8 ^oS on the western rim of the large, ~2.5Ma Caviahue caldera, erupted on December 22, 2012. The initial phreatomagmatic event deposited mm-sized hydrothermally altered material, abundant sulfur, and crushed pumice in a thin lobe up to > 100 km south of the volcano. The following violent Strombolian eruption with a plume height of 1.5 km deposited disc-like pumice clasts (10 cm diameter) with a chilled rim. The eruption then transitioned during the day into quieter Strombolian activity with production of scoria and ejection of almost vesicle-free cinders. Bulk rock analysis of the pumice, scoria, and cinders for major and trace elements was performed by WD-XRF at Wesleyan University. These high-K basaltic andesites have 54-55% SiO₂, 4.5% MgO, and 1.9% K₂0. Matrix glasses in the pumice and scoria samples have 56-57% SiO₂ and up to 2.7% K₂O. The cinder glasses are more evolved with 56-60% SiO₂ and up to 4.3% K₂O as a result of continued crystallization after eruption. Matrix glasses have low volatiles, with 20-30 ppm S, 100-190 ppm Cl, and (by difference) 0.5-1.5% H₂O. The identical bulk rock composition of pumice, scoria and cinders suggests a pre-eruptive magma column with a highly volatile-enriched top. The rocks have abundant plagioclase with lesser amounts of olivine, pyroxene, and magnetite. Most plagioclase is euhedral with An 50-60%, whereas some crystals have clean rims with heavily corroded cores and higher Ca contents (An=70-87 %). The augite is Al-poor with Mg# of 0.7, and orthopyroxene is a hypersthene with up to 2.2% CaO. The bulk rock composition of the 2012 Copahue magma is identical to that of the 2000 Copahue eruption and has comparable mineralogy, indicating that no magmatic evolution took place over the last decade in the underlying magma reservoir. Copahue is positioned behind the main active Andes arc and has similar MgO but higher Fe, Ti, Ba and K compared to the main arc lavas. The backarc volcanics just east of Copahue have higher Nb, Ti, and Ba. We will present estimates of magmatic temperatures and pre-eruptive volatile concentrations based on ongoing microprobe work and IR absorption spectroscopy of glass inclusions, as well as a detailed comparison with main Andes arc rocks and local back arc basalts.