Under this funding, the PIs will carry out a pilot program of Ar/Ar dating to determine whether volcanic rock samples can be found of appropriate age and chemistry along a well-characterized across-arc transect in SE Guatemala. If so, it may then be possible to determine (1) the relative contributions of flux and decompression processes to partial melting (2) how these processes vary across the arc and (3) how these processes have changed in time, associated with changing geodynamic boundary conditions. Southeastern Guatemala provides access to an across-arc transect with recent volcanism occurring nearly continuously for ~125 km transverse to the subduction zone. Sorting out the timing of fluid addition and its influence on partial melting is the main goal of the this study. U-series isotopic analyses will eventually be used to constrain the timing and character of slab fluid additions to the source of arc magmas and their relationship to partial melting processes. A U-Th-Ra-Pa disequilibrium study will eventually be done on young volcanic rocks from the volcanic front to over 100 km behind the front. 40Ar/39Ar dating of the young front and behind-the-front lavas will be carried out as an integral prelude to U-series.
