SYSTEMATIC CHANGES IN U-SERIES ISOTOPES ALONG THE GUATEMALAN VOLCANIC FRONT

Guatemalan lavas exhibit systematic along-arc variations in U-series isotope data. Lavas from the northwestern part of the volcanic front have (²³⁰Th/²³⁸U) < 1. In contrast, lavas erupted in central Guatemala, from Fuego volcano to cinder cones immediately east of Pacaya volcano, have (²³⁰Th/²³⁸U) > 1. Excesses in (²³⁸U) then reappear in lavas from southeastern Guatemala, including a lava erupted in the Ipala Graben over 50 km behind the front. All of the (²³⁰Th/²³²Th) ratios of Guatemalan lavas are similar to values found in Costa Rica at the opposite end of the volcanic chain. A dacitic lava erupted in 1818 from Cerro Quemado, in the northwestern part of Guatemala, has only a slight (²³⁸U) excess, suggesting it suffered prolonged differentiation and/or crustal contamination. A subset of the mafic lavas has elevated slab signatures. The (²³⁰Th/²³⁸U) ratios of the mafic lavas show positive correlations with Th/Nb and U/La. The data suggest that two particular processes may be crucial in explaining the observed geographic variations in (²³⁰Th/²³⁸U) along the Guatemalan volcanic front: sediment melting in the subducting Cocos plate and decompression melting in the overlying mantle wedge.