The volatile content of magmas from Arenal volcano, Costa Rica

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 (AR-19) and ET6 (AR-16) 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.%). The new sulfur data reported here predict total sulfur output over the past ∼ 30 years from bulk basaltic andesite liquid that matches well spectroscopic estimates, and resolves the previously noted imbalance. MI from different ET3 and ET6 samples show different F/Cl, while most trace element ratios show a limited range similar to that of the host rocks. The high water content (4 wt.% H2O) of Arenal basaltic magma is somewhat surprising given the weak subduction signal recorded geochemically (e.g., low 10Be and B). The Arenal MI data contribute to a positive correlation between primary water contents and Ba/La in Central American volcanoes, although further testing is required given the small number of data points, and the expectation that water and trace elements should have different sources in the subduction zone.