CHANGES IN VOLATILE INPUT FROM THE SUBDUCTING COCOS PLATE ACROSS THE NORTHERN PART OF THE CENTRAL AMERICAN SUBDUCTION ZONE: AN IMPORTANT ROLE FOR SERPENTINE?

Most subduction zones are characterized by a segmented band of large polygenetic volcanoes called the volcanic front. Olivine-hosted melt inclusions have demonstrated that most primitive basaltic magmas feeding the Central American volcanic front (VF) are characterized by high (>2 wt.%) water contents. This reflects an origin tied to dehydration of the subducting Cocos plate. In southeastern Guatemala, the pre-eruptive water contents of basaltic magmas erupted behind the volcanic front are lower (most are ~ 2 wt. %). Recent seismological evidence suggests that the major influx of VF-forming, slab-derived water in southeastern Guatemala occurs between slab depths of 85-105 km. Moreover, the transition to lessened hydrous inputs and the eruption of behind-the-front magmas takes place abruptly at slab depths > 105 km. Accompanying the changing pre-eruptive water contents of erupted basaltic magmas across southeastern Guatemala are dramatic reductions in B, Cs, Cl concentrations and B/Nb, B/Be and B/La ratios. These across-arc geochemical changes are consistent with greatly diminished slab fluid inputs behind the volcanic front. These across-arc changes are also quite selective and suggest that the major influx of VF-forming, slab-derived water may be due to dehydration of a specific hydrous phase: serpentine. A model for the for the formation of the VF in southeastern Guatemala involving the dehydration of down-dragged forearc serpentinite (Iwamori, 1998; Hattori and Guillot, 2003; Savov et al., 2007) fits well with the observed slab depths and geochemical variations.