xvi, 167 p. ; ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / Volatiles, particularly H 2 O, play an important role in subduction zone magmatism, from instigating melting of the mantle wedge to influencing the explosivity of eruptions at the surface. To better understand both small-scale eruptive processes and large-scale melt generation processes, concentrations of H 2 O, CO 2 , Cl and S were measured in olivine-hosted melt inclusions from nine monogenetic volcanoes across the Michoacan-Guanajuato Volcanic Field (MGVF) in central Mexico. Melt inclusions, tiny blebs of melt trapped within crystals during growth, record pre-eruptive melt compositions and dissolved volatile contents. Analyses of olivine-hosted melt inclusions from the long-lived (15 years) eruption of Volcan Jorullo illustrate the complexities of cinder cone eruptions. The later-erupted melt inclusions record decreases in crystallization depths, increases in magma storage time, and shallow assimilation of granitic bedrock, suggesting significant evolution of the magma plumbing system over time. Because melt inclusions are trapped at variable depths during magma crystallization, they record progressive degassing of melts during ascent and eruption. Degassing of basaltic melts is variable due to differences in solubility of the volatile components. Estimated volatile solubilities based on variations in melt inclusion data for the MGVF suggest that Cl and S have high solubility, with little to no degassing of these species during ascent and eruption, whereas H 2 O and CO 2 show evidence of substantial degassing. Furthermore, increases in concentrations of incompatible elements in melt inclusions correlate with extents of degassing, suggesting that degassing during ascent drives melt crystallization in many cinder cone eruptions. The volatile contents of mafic arc magmas as revealed by melt inclusions reflect the influx of H 2 O-rich components from the subducted slab to the mantle wedge. Across-arc patterns in volatile and incompatible trace element concentrations for MGVF magmas show that the flux of H 2 O-rich subduction components remains high for large distances across the arc. These data, combined with oxygen isotope analyses of olivine phenocrysts and 2-D thermo-mechanical models of the subduction zone, suggest a complex origin for the H 2 O-rich subduction components, involving dehydration of subducted sediment and storage of volatiles in hydrous minerals in the mantle wedge. This dissertation includes co-authored materials both previously published and submitted for publication. / Adviser: Paul J. Wallace

The role and behavior of fluids in hydrothermal and magmatic environments have been studied. Experimental studies have been carried out to determine fluid properties, in natural environments and in both synthetic and natural fluid and melt inclusions. One of these studies dealt with the effect of composition on the critical P-T-X properties of aqueous salt solutions approximated by the H₂O-NaCl-KCl-CaCl₂ system. The results indicate a systematic variation in critical properties as a function of composition over the range of P-T-X studied. A technique for analyzing individual H₂O-CO₂ inclusions using Raman spectroscopy has also been developed. The resulting empirical equation relating Raman intensities and composition is valid for compositions ≤50mol% CO₂. The technique has been applied to H₂O-CO₂ inclusions from the Butte, MT Porphyry Cu-Mo deposit and the results agree with compositions estimated from microthermometric and petrographic observations. The aim of another study was to study water loss from melt inclusions during laboratory heating. Melt inclusions had lost insignificant amounts of water when held at experimental conditions (800°C, 1 kbar) for ≤24 hours. However, significant water loss was observed for longer duration experiments. Ocelli, which are globular bodies of felsic minerals are interpreted as products of magmatic melt immiscibility. As such, the carbonate aggregates in Cretaceous lamprophyres from Hungary with similar petrographic characteristics have also generally been interpreted to be products of magmatic immiscibility. Petrographic and geochemic studies have shown three three distinct genetic groups for these aggregates, none of which were consistent with a magmatic origin. / Ph. D.