MAGMATIC WATER CONTENTS ACROSS SOUTHEASTERN GUATEMALA

Volcanism in Central America occurs both along and behind the volcanic front. The most significant area of behind-the-front (or BF) volcanism is in southeastern Guatemala where volcanoes stretch somewhat continuously 15-110 km behind the front. Most of these volcanoes are small monogenetic cinder cones constructed by the eruption of basaltic magmas. From the analysis of olivine-hosted melt inclusions, Walker et al. (2003) showed that BF basaltic magmas have pre-eruptive water contents ranging from <1 wt. % to 2.5 wt. %. The H₂O/Zr ratios of the melt inclusions display positive correlations with Sr/Zr and Li/Zr. Following the method outlined in Johnson et al. (in review) we have used linear regressions of these plots and whole-rock trace element data to estimate H₂O contents of BF magmas throughout southeastern Guatemala. These range from <1 wt. % to 3.3 wt. % H₂O. The maximum Cl contents of BF olivine-hosted melt inclusions also suggest that BF magmas have a maximum of about 3.5 wt. % H₂O (Wallace, 2005). Interestingly, peak H₂O contents do not occur closest to the front, but about 50-60 km behind the front within the Ipala graben, and near the “older” Retana and Suchitán polygenetic volcanoes. High water contents (> 2.5 wt. %) are also found in lavas erupted over 100 km behind the front.