The Monaro Volcanic Province (MVP) is an Oligocene-Eocene intraplate basaltic lava field situated in the Southern Highlands of New South Wales between the towns of Cooma and Bombala. The lava pile of the MVP consists of basal sub-alkali rocks (olivine tholeiite, transitional basalt) capped by a number of thick ankaramite lavas, above which lie less numerous alkali rocks including alkali olivine basalt, nepheline basanite and olivine nephelinite. Intercalated with the lava flows are massive and matrix-supported alkali and ankaramitic hyaloclastites, alkali pillow basalts, rare tuffs, bauxitic weathering profiles, lacustrine sediments and reworked late Cretaceous to early Tertiary river gravels. The lava pile is intruded through by numerous volcanic plugs and dykes and rare maars. Volcanic centres are principally concentrated in two NW-SE trending zones parallel to major crustal-scale fractures in the Palaeozoic basement. Centres almost always lie over the intersections of two or more conjugate strike-slip or transverse fractures. The stratigraphy, whole-rock geochemistry and Sr and Nd isotopic signatures of rocks from the MVP indicate magma-genesis initially from an asthenospheric source with EM1 characteristics, gradually becoming more lithospheric with DM source characteristics. The long-lived nature of the MVP rules out a mantle plume-type source for magmas. Instead, a diapiric source is envisaged. The MVP mantle xenolith suite appears to have equilibrated at slightly higher temperatures for given pressures than the Newer Volcanics Province suite suggesting the palaeogeotherm for the MVP was slightly hotter than the "South East Australian" geotherm. Large amounts of amphibole (pargasitic hornblende, pargasite, ferroan pargasite and kaersutite) occuring within the more silica-undersaturated rocks of the MVP, and rarely within Iherzolitic xenoliths, are interpreted to have formed as selvages on mantle veins in contact with peridotite beneath the MVP. Amphiboles were later sampled by magmas rising through the same conduits and were brought to the surface. MVP ankaramite lavas feature < 2cm clinopyroxene porphyrocrysts, the cores of which are shown to have crystallised at ca. 18 kb pressure or ca. 54 km depth. This defines the base of the local crust within the MVP region. Data from the MVP support a landscape evolution model based on the isostatic rise of the Southern Highlands due to voluminous magmatic underplating since the Cretaceous. Data further support limited denudation since the Early Tertiary based on a pulsatory but high palaeogeotherm.

Easter Island and Sala y Gomez are part of the Sala y Gomez Ridge, a broad band of high topography and scattered seamounts extending ESE from the East Pacific Rise. It has been proposed that the Sala y Gomez Ridge results from the movement of the Nazca Plate over a fixed melting spot in the mantle. To test this hypothesis volcanic rocks from Easter Island and Sala y Gomez were analyzed for their K-Ar ages and major element abundances. Subaerial Easter Island was constructed in three distinct episodes, occurring at 2.5 m.y., 0.9 m.y., and 0.4 m.y. ago. The youngest rocks on the island are the Roiho olivine basalts, and are probably less than 50,000 years old. Eruptive activity on Sala y Gomez was essentially contemporaneous with the early volcanism on Easter Island. No migration of volcanism with time is apparent along the Sala y Gomez Ridge, thus a major criterion of the melting spot hypothesis is not fulfilled. Volcanic rocks from Easter Island constitute a tholeiitic differentiation series; they are chemically similar to those from other islands situated near mid-ocean rise crests. The wide compositional spectrum is most likely the result of fractional crystallization from a basaltic parent liquid, though the data is ambiguous for the highly silicic differentiates. The youngest basalts possess more alkaline affinities which are probably not related to fractional crystallization from the earlier basalts. The alkaline nature of these rocks may be the result of a downward migration of the fusion zone with time, as the island moved eastward over a progressively thickening lithosphere. Volcanic rocks from Sala y Gomez belong to an alkali olivine basalt series. The fundamental chemical differences between the Easter Island and Sala y Gomez suites suggest that the two islands were not derived from a common source, as predicted by the melting spot hypothesis. The evidence does not support a melting spot origin for Easter Island, Sala y Gomez, and the Sala y Gomez Ridge. An alternative model involving diapiric intrusion and decompression melting of asthenosphere material along a major fracture in the Nazca Plate provides a better explanation for the data. Synchronous volcanism along the eastern extension of the Easter Island transform fault has given rise to the islands and seamounts on the Sala y Gomez Ridge. / Graduation date: 1975

Graduation date: 1999

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.

No description available.

Thesis (Ph. D.)--University of Oregon, 2003. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 190-205). Also available for download via the World Wide Web; free to University of Oregon users.

Thesis (M.A.)--University of Texas at Austin, 1990. / Vita. Includes bibliographical references (leaves 96-101).

Sixteen percent of New Zealand’s power comes from geothermal sources which are primarily located within the Taupo Volcanic Zone (TVZ). The TVZ hosts twenty three geothermal fields, seven of which are currently utilised for power generation. Ngatamariki Geothermal Field is the latest geothermal power generation site in New Zealand, located approximately 15 km north of Taupo. This was the location of interest in this project, with testing performed on a range of materials to ascertain the physical properties and microstructure of reservoir rocks. The effect of burial diagenesis on the physical properties was also investigated. Samples of reservoir rocks were taken from the Tahorakuri Formation and Ngatamariki Intrusive Complex from a range of wells and depths (1354-3284 mbgl). The samples were divided into four broad lithologies: volcaniclastic lithic tuff, primary tuff, welded ignimbrite and tonalite. From the supplied samples twenty one small cylinders (~40-50mm x 20-25mm) were prepared and subjected to the following analyses: dual weight porosity, triple weight porosity, dry density, ultrasonic velocity (saturated and dry) and permeability (over a range of confining pressures). Thin sections impregnated with an epoxy fluorescent dye were created from offcuts of each cylinder and were analysed using polarised light microscopy and quantitative fluorescent light microstructural microscopy. The variety of physical testing allowed characterisation of the physical properties of reservoir rocks within the Ngatamariki Geothermal Field. Special attention was given to the petrological and mineralogical fabrics and their relation to porosity and matrix permeability. It was found that the pore structures (microfractures or vesicles) had a large influence on the physical properties. Microfractured samples were associated with low porosity and permeability, while the vesicular samples were associated with high porosity and permeability. The microfractured samples showed progressively lower permeability with increased confining pressure whereas samples with a vesicular microstructure showed little response to increased confining pressure. An overall trend of decreasing porosity and permeability with increasing density and sonic velocity was observed with depth, however large fluctuations with depth indicate this trend may be uncertain. The large variations correlate with changes in lithology suggest that the lithology is the primary control of the physical properties with burial diagenesis being a subsidiary factor. This project has established a relationship between the microstructure and permeability, with vesicular samples showing high permeability and little response increased confining pressure. The effects of burial diagenesis on the physical properties are subsidiary to the observed variations in lithology. The implications of these results suggest deep drilling in the Tahorakuri Formation may reveal unexploited porosity and permeability at depth.

White Island is a highly active volcano with an acidic, S-rich hydrothermal system in the Bay of Plenty, North Island, New Zealand. In this acidic environment a series of subsurface Fe-rich layers are ubiquitous in the crater sediments at shallow depth and are capable of modifying the flow and gas flux dynamics in the system. The mineralogy of the subsurface Fe-rich layer(s) and the processes leading to their formation are unknown. Here the mineralogy and formation of the subsurface Fe-rich layers in relation to the surface and subsurface environment(s) within the Main Crater at White Island are assessed. Based on geochemical analyses, subsurface Fe-rich crusts are composed of a mix of jarosite and goethite, cementing crater fill sediments into cohesive layers. Saturation index (SI) and Eh/pH assessments identify that fluids evolved at White Island are undersaturated with respect to the mineral phases present in the Fe-rich subsurface layers. Formation of the Fe-rich subsurface layers is most likely related to the transition between atmospheric gases and/or meteoric water mixing with hydrothermal fluids. This transition zone creates an environment conducive to forming jarosite and goethite forming in the same layer. Additionally, subsurface sediments including the Fe-rich layers show a consistent organic carbon isotopic signature of -23 ‰. Microscopic investigations confirm diatoms and microbes are present in the subsurface Fe-rich layers. The full extent of microbial activity in relation to the Fe-rich layers at White Island still requires further investigation. Based on chemical extractions for isotopic analyses, Fe-rich layers are shown to preserve δ¹³C signatures indicative of microbial life. Interface zones such as those identified in the hydrothermal environment at White Island can create metal-rich deposits and habitable/preservative microbial environments as well as affecting the macroscopic dynamics of volcanic and epithermal systems.

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