Volatile degassing and plinian eruption dynamics of the mafic Fontana eruption, Nicaragua
Fontana Tephra has been generated by a large basaltic-andesitic plinian eruption in the Late-Pleistocene in the Masaya area, west-central Nicaragua, at the Central American subduction zone. The eruption evolved through several plinian fallout phases, alternating with phreatomagmatically-affected plinian fallout activity. The minimum estimate of the total erupted tephra volume amounts to 1.4 to 1.8 km3. Comparisons of tephra dispersal parameters with theoretical modelling results suggest eruption column heights between 24 and 30 km. The eruption was powered by exsolution of magmatic water. The gas chemistry of melt inclusions and matrix glasses as determined by SIMS, supported by petrographic observations, indicate a retarded degassing process in disequilibrium with the pressure release during magma ascent in the conduit, associated with a substantial volatile supersaturation. This sudden vesiculation process, combined with the high temperature and rapid ascent of the magma is considered to be responsible for the unusual plinian eruptive style of the mafic Fontana magma. High magmatic bromine and iodine contents, positively correlating with high Ba-La ratios, indicate strong sediment recycling through the subduction zone.