Wade, J.A., T. Plank, E.H. Hauri, W.G. Melson, G.J. Soto (2004) High water contents in basaltic melt inclusions from Arenal volcano, Costa Rica. Eos. Trans. AGU, 85(47) Fall Meet. Suppl., Abstract V23D-05.
Despite the importance of water to arc magma genesis, fractionation and eruption, few quantitative constraints exist on the water content of Arenal magmas. Early estimates, by electron microprobe sum deficit, suggested up to 4 wt% H2O in olivine-hosted basaltic andesite melt inclusions (MI) from pre-historic ET-6 tephra (Melson, 1982), and up to 7 wt% H2O in plagioclase and orthopyroxene-hosted dacitic MI from 1968 lapilli (Anderson, 1979). These high water contents are consistent with abundant hornblende phenocrysts in Arenal volcanics, but inconsistent with geochemical tracers such as 10^Be and Ba/La that suggest a low flux of recycled material (and presumably water) from the subduction zone.
In order to test these ideas, and provide the first direct measurements of water in mafic Arenal magmas, we have studied olivine-hosted MI from the prehistoric (900 yBP; Soto et al., 1998) ET3 tephra layer. MI range from andesitic (SiO2) to basaltic compositions (~ 50% SiO2), the latter of which are similar to the most primitive whole rocks analyzed from Arenal. SIMS analyses yield up to 4 wt% H2O in the basaltic MI, and water declines systematically (to 1-2 wt%) with increasing silica content. Water also correlates strongly with sulfur (up to 2500 ppm S) and CO2 (up to 300 ppm). H2O and CO2 in the MI define a closed-system degassing path that begins at 2 kb. Chlorine (> 2000 ppm) and fluorine (> 400 ppm) show less variation, as expected from their higher solubilities in these melts. The high sulfur contents (> 2000 ppm on average for basaltic MI) would provide more than enough "petrologic" sulfur to balance recent (1982, 1995, and 1996; Williams-Jones et al., 2001) COSPEC measurements.
Although host olivines are quite evolved (< Fo76), the high CO2 and sulfur contents indicate that their inclusions are not highly degassed. The high water contents (> 4 wt%) found here for Arenal basaltic MI support the semi-quantitative data from earlier studies, but are somewhat unexpected given predictions from slab tracers. Arenal water contents (4%) approach those of the 1995 eruption of Cerro Negro in Nicaragua (4-5 wt% in basaltic MI; Roggensack et al., 1997), despite the fact that the latter has Ba/La of > 100, while Arenal has Ba/La < 50. Thus, Ba/La, which has commonly been used to infer the proportion of slab-fluids and water-fluxed melting in Central American magmas, seems to be a poor water proxy. Even more surprising are the very high Cl contents of Arenal MI, which exceed those in Cerro Negro MI by more than a factor of two. The high Cl and high Cl/K2O (0.3-0.5) of Arenal, are also inconsistent with an enriched, Galapagos-type mantle (Cl/K2O < 0.1), as has been proposed for this part of Costa Rica. Thus, contrary to previous inferences, Arenal receives a major flux of H2O and Cl from the subduction zone.
References:
Anderson, A.T. (1979) Journal of Geology; Melson, William G. (1982) Boletin de Volcanologia; Roggensack et al. (1997) Science; Soto et al. (1998) OSIVAM; Williams-Jones et al. (2001) Journal of Volc. and Geoth. Res.