Light Elements Across the Central American Arc, SE Guatemala

We have examined B-Be-Li systematics of volcanoes in southeastern Guatemala which represent a nearly continuous 120 km cross-arc transect of the active Central American Arc. Volcanoes behind the front are small monogenetic cinder cones erupting basaltic magmas that show compositional differences when compared to volcanic front lavas. Boron and Li abundances, B/Be (&<&20 behind the front versus 20-75 at the volcanic front) and Li/Yb (&<&5 behind the front versus 5-10 at the front) are generally higher in volcanic centers along the Guatemalan volcanic front, and are generally lower in behind the front volcanoes. All B/Be and Li/Yb ratios are elevated relative to MORB values, suggesting inputs from the slab both at the volcanic front and behind the front. No consistent B/Be or Li/Yb variation is observed with increasing distance from the front, as is seen in other arcs. B and Li systematics are similar to those of other fluid mobile elements in Guatemalan volcanoes. Be abundances, by contrast, are higher in behind the front volcanoes, and show systematics similar to those of Nb and other incompatible elements. Be/Nd ratios range from 0.03 - 0.08, similar to other calc-alkaline arc lavas. Be in Guatemalan lavas is thus controlled largely by changing extents of partial melting of a source with uniform Be contents. Some volcanoes at the front, from Pacaya to Fuego, record lower B concentrations and lower B/Be, similar to those of behind the front centers, suggesting overall lower slab inputs to the source regions in this section of the arc. Our data support the model of Walker et al (1995), in which melting processes abruptly change across the arc in Guatemala, from flux dominant melting at the front to decompression dominant melting behind the front. However, some level of slab input, as indicated by elevated B and Li ratios, is present behind the front. Li isotope variations are being examined in these lavas to try to identify the signatures in slab-modified mantle in both volcanic front and behind the front Guatemalan centers.