The volatile content of magmas from Arenal volcano, Costa Rica Jennifer A. Wade*1, Terry Plank1, William G. Melson2, Gerardo J. Soto3, Erik Hauri4 1 Department of Earth Sciences, Boston University, Boston, MA 02215, USA 2 Division of Petrology and Volcanology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA 3 Consultant, Apdo. 360-2350 San Francisco de Dos Ríos, Costa Rica. 4 Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA * Corresponding author. Tel: 617-353-4085; Fax: 617-353-3290; Email:
[email protected] 1 Abstract We provide the first direct measurements of water in mafic melts from Arenal volcano, Costa Rica. Ion microprobe analyses of olivine-hosted melt inclusions (MI) from the prehistoric ET3 and ET6 tephra layers reveal high concentrations of volatile species: ~1-4 wt% H2O, 50-300 ppm CO2, and > 3000 ppm S and Cl. The MI with the highest water concentrations are the most mafic, and the dataset as a whole records a history of degassing coupled with fractionation and ascent from ~ 2 kbar to 0.2 kbar. Arenal MI form two groups based on their Al, CO2 and S contents. The ET3 high-Al MI were trapped at the highest pressure, are closest to equilibrium with their host olivines (~Fo79), which are closest to equilibrium with the bulk rock liquid. These MI are excellent candidates for Arenal parental liquids, and can generate most Arenal volcanic rock compositions by crystal accumulation (up to 30%), or crystal fractionation at a range of pressure (0.5 – 3 kb) and H2O contents (0.5-3.5 wt%).