Benjamin, E.R., T. Plank, E.H. Hauri, K.A. Kelley, J.A. Wade, G.E. Alvarado (2004) Water content of a hypothetically dry magma: the 1723 and 1963-65 eruptions of Irazu volcano, Costa Rica. Eos, Trans. AGU, 85(17) Jt. Assem. Suppl., Abstract V54B-02.
Despite the importance of water to arc magmas, few measurements exist because virtually all volcanic rocks are degassed. Most of our inferences regarding water cycling at subduction zones arise from "fluid-mobile" tracers, such as Ba, B and U. The volcanic front of the Central American Arc records dramatic variations in such tracers. For example, 10^Be and Ba/La reach global maxima in Nicaragua, and decrease both to the north (Guatemala) and south (Costa Rica). In Irazu Volcano, at the southeast terminus of the Central American Arc, Ba/La and 10^Be approach values of background mantle, with little to no inferred slab contribution. Thus, the expectation is that Nicaragua magmas should be wet, and Irazu magmas dry (i.e., < 1 wt% H2O). Indeed, Roggensack et al. (1997) have found very high water concentrations in mafic melt inclusions at Cerro Negro Volcano, Nicaragua (> 6 wt%).
We report here estimates of magmatic water in Irazu, based on samples of basaltic andesite tephra from two recent Strombolian eruptions (1723 and 1963-65). Water measured in olivine-hosted melt inclusions by ion microprobe range from 0.81 wt% to 3.29 wt% and correlate with Cl and S. CO2 is > 39 ppm in all samples and indicates that melts have not catastrophically degassed. Water contents > 3.0 wt% at Irazu are unexpected, and indicate an alternate water source other than subducted sediments, which are the main supply of recycled 10^Be and Ba to the arc.
REE have also been measured in clinopyroxene (cpx) by laser ablation ICP-MS, with the goal of developing a method for estimating magmatic water. Because plagioclase (plag) strongly fractionates Eu from the other REE, melts undergoing plag fractionation develop an Eu depletion that is recorded in co-crystallizing cpx. We use the magnitude of the Eu anomaly in cpx, together with its Mg#, to monitor the point of plag saturation along a liquid line of descent. The T (or liquid Mg#) of plag saturation is a strong function of water content, and may be used as an indirect measure of magmatic water.