The first section of this work examines melt inclusions in phenocrysts from Volcán Popocatépetl and Volcán 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 Popocatépetl and Colima melt inclusions are similar (0.3 to 3.4 weight percent Hsub2O). Melt-vapor equilibration pressures calculated from dissolved Hsub2O and COsub2 (Popocatépetl) or Hsub2O (Colima) in melt inclusions correspond to depths of entrapment of 12 km or less. Water and carbon dioxide concentrations correlate negatively with SiOsub2 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 Popocatépetl 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 Hsub2O (below 0.5 wt. %), low S (less than 300 ppm) and high COsub2 concentrations (up to approximately 6000 ppm). Relationships between water, sulfur, Cl and F in combination with Masaya's high COsub2 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 COsub2 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.

The Toba caldera located in north Sumatra, Indonesia, is the source of the largest volcanic eruption in the Quaternary (Rose & Chesner 1987). Its enormous volume of 2800 km3 has been a matter of debate for decades and it is still unclear where and how the Toba magma was assembled. This study documents oxygen isotope data for a suite of whole rocks and minerals erupted as part of the Young Toba Tuff (YTT), some 74 ka ago (cf. Chesner et al. 1991). Oxygen isotope data has been obtained in-situ from quartz crystals (SIMS), whole rocks (conventional), as well as quartz, feldspar, amphibole and biotite (laser fluorination). In combination with cathodoluminescence (CL) imaging on the quartzes, the data are used to test the relative roles of shallow magmatic processes such as crystal fractionation, magma-crust interaction and crystal recycling within the Toba magmatic system. In addition, thermobarometric calculations have been performed on plagioclase and amphibole phenocrysts from the YTT to help unravel the magma storage and plumbing system that gave rise to the YTT. The combined evidence will be used to derive a model for shallow magma evolution and storage at Toba some 74 ka ago. The CL images of quartz crystals exhibit defined patterns of magmatic zoning, which broadly coincide with fluctuations in δ18O values in the quartz crystals, allowing correlation of textural and compositional data. Measured δ18Oquartz values range from 6.7 ‰ to 9.4 ‰, independent of position on crystal core or rim. Values for δ18Omagma have been calculated from quartz phenocrysts (assuming Δquartz-magma is 0.7 ‰ at magmatic temperatures). The lowest magma value is 6.0 ‰, apparently reflecting a primitive isotopic signal (Taylor & Sheppard 1986). The maximum calculated magma value is 8.7 ‰, indicating a significant crustal component and thus multiple sources to the Toba magmatic system. Several crystals, however, show internal zoning with gradually lower values towards the rims, pointing to a late-stage low-δ18O input, most probably from the shallow volcanic edifice. The crystals therefore record a complex and heterogeneous origin of the YTT magma, comprising a primitive and an evolved magmatic component topped up with several substantial crustal contributions to finally assemble the massive volume of the 74 ka Toba eruption.