Volatiles in melt inclusions from Mexican and Nicaraguan volcanoes: Implications for complex degassing processes.

摘要

The first section of this work examines melt inclusions in phenocrysts from Volcan Popocatepetl and Volcan de Colima within the Trans Mexican Volcanic Belt (TMVB). These inclusions are dacitic to rhyolitic. Trends in melt inclusion major element and water concentrations form the evolved extension of other Mexican volcanics including those presumably derived directly from primitive melts. Water concentrations in Popocatepetl and Colima melt inclusions are similar (0.3 to 3.4 wt. % H2O). Melt-vapor equilibration pressures calculated from dissolved H2O and CO2 (Popocatepetl) or H2O (Colima) in melt inclusions correspond to depths of entrapment of 12 km or less. Water and carbon dioxide concentrations correlate negatively with SiO2 and potassium. Normalized olivine-augite-quartz compositions are consistent with near cotectic crystallization under vapor-saturated conditions at pressures of 1.5 kb or less. Our results show that Popocatepetl and Colima magmas have undergone vapor-saturated crystallization during ascent in conjunction with varying degrees of mixing between degassed rhyo-dacitic and less degassed, mafic melts in the upper portions of the crust. These data suggest melt evolution occurred in conduits or inter-fingered dikes rather than a large stratified magma chamber.;Part II looks at the Masaya caldera in Nicaragua. This volcano has erupted frequently in recorded history, producing lava lakes and very high gas emissions. Melt inclusions from Masaya are basaltic, with low H2O (0.5 wt. %), low S (300 ppm) and high CO2 concentrations (up to ∼ 6000 ppm). Relationships between water, sulfur, Cl and F in combination with Masaya's high CO2 and Ba/Zr and Ba/Nb ratios suggest that Masaya has undergone a multi stage degassing process involving (1) shallow degassing, (2) recycling of magma into a deeper reservoir, and (3) fluxing of previously degassed magma with a nearly pure CO2 vapor. Trace element signatures of melt inclusions are consistent with contributions that have been variably metasomatized by fluids generated by dehydration of subducted sediments and/or altered oceanic crust.