Rhyolites of the Picabo volcanic field (10.4-6.6 Ma) of the Yellowstone hotspot in eastern Idaho are preserved as thick ignimbrites and lavas along the margins of the Snake River Plain. This study presents new O and Hf isotope data and U-Pb geochronology from individual zircons, O isotope data from major phenocrysts, whole rock Sr and Nd isotope data, whole rock geochemistry, and trace element and volatile analyses of quartz-hosted melt inclusions, which were used to characterize the evolution of rhyolite generation through the eruptive sequence. The chemical composition of the first eruption of the caldera complex, the Tuff of Arbon Valley, suggests magma generation through repeated magma injection into the crust, remelting, crystallization, mixing, and crustal assimilation. Subsequent eruptions have diverse and low δ18O signatures indicating rhyolite generation through the remelting of variably hydrothermally altered volcanics, followed by rapid batch assembly. This thesis includes co-authored material previously published.

The central segment of the Taupo Volcanic Zone (TVZ) is one of the world’s most productive areas of silicic volcanism and geothermal activity. Rhyolites largely predominate the eruptive output in the central TVZ, with only minor basalts, andesites and dacites. The rhyolites show diversity in composition, and form a compositional continuum between two end-member types (R1 and R2), as suggested in previous studies. In this thesis I present results from a quartz- (and rare plagioclase-) hosted melt inclusions study, focussing on the volatile concentration (i.e. H2O, Cl, F, CO2) and their relative distribution between R1 and R2 rhyolites. The main objective is to add further constraints on the magmatic systems with regard to their contribution to the hydrothermal systems in the central TVZ. A comparative study between R1 and R2 melt inclusions show distinct volatile, fluid-mobile, and highly incompatible element compositions. Differences in the bulk volatile concentration of the parental magmas (i.e. basalts intruding the lower crust) are suggested to be at the origin of these volatile disparities. Further analysis on the volatile exsolution of R1 and R2 melts lead to the observation that the two rhyolite types exsolve a volatile phase at different stages in their magmatic history. From Cl and H2O concentrations, it is suggested that R1 magmas exsolve a vapour phase first, whereas R2 rhyolites more likely exsolve a hydrosaline fluid phase. These results have considerable implications for the magmatic contribution into the hydrothermal systems in the central TVZ, as differences in the composition of the resulting volatile phase may be expected. The hydrothermal systems in the central TVZ are subdivided into two groups based on their gas and fluid chemistry; and the current model suggests that there are two distinct contributions: a typical ‘arc’ system, with geochemical affinity with andesitic fluids, located along the eastern margin of the TVZ, and a typical ‘rift’ system, with geochemical affinity with rhyolitic/basaltic fluids, located along the central and/or western region of the TVZ. The addition of the new data on the rhyolitic melt inclusions, leads to a re-evaluation of the magmatic contribution into the hydrothermal systems, with a particular focus on B and Cl. The results indicate a more diverse variety of contributions to the meteoric water in the hydrothermal systems, and also show that the east-west distribution of ‘arc’ and ‘rift’ fluids is not a viable model for the central TVZ. This work emphasises that melt inclusion data and their volatile degassing history cannot be underestimated when characterising and quantifying the magmatic component in hydrothermal fluids. The melt inclusion data also provide further insight into the pre-eruptive magmatic plumbing systems and are particularly important from a hazard perspective. Included in the thesis is a detailed petrological analysis of rhyolite melt inclusions across the central TVZ and an interpretation that large silicic magma systems (in the TVZ) are typically comprised of multiple batches of magma emplaced at some of the shallowest depths on Earth. Tectonic activity is suggested to play an important role in triggering large caldera-forming eruptions as the evacuation of one magma batch could cause a regional-scale readjustment that is sufficient enough to trigger and allow simultaneous eruption of an adjacent melt batch.

A continuum of rhyolite compositions has been observed throughout the Taupo Volcanic Zone (TVZ) over the past 550 kyr. reflecting changes in the ƒH2O, ƒO₂, and P-T conditions in a lower crustal 'hot-zone' (10-30 km) where these evolved melts are generated by crystal fractionation of successively intruded basaltic magmas. The rhyolite compositional continuum is bound by two distinct end-member types: R1 is characterized by hydrous minerals (hornblende ± biotite), low FeO*/MgO (calc-alkaline series), low MREE, Y, and Zr, and high Sr; and R2 is characterized by anhydrous minerals (orthopyroxene ± clinopyroxene), high FeO*/MgO (tholeiitic series), high MREE, Y, and Zr, and low Sr. Slab-derived aqueous fluid components (Ba, Cl) correlate well with oxygen fugacity, and other well defined characteristics of silicic magmas in the Taupo Volcanic Zone (TVZ) between a cold-wet-oxidizing magma type (R1: amphibole ± biotite; high Sr, low Zr and FeO*/MgO, depleted MREE) and a hot-dry-reducing magma type (R2: orthopyroxene ± clinopyroxene; low Sr, high Zr, and FeO*/MgO, less depleted MREE). Oxygen fugacity was obtained from analysis of Fe-Ti oxides and ranges between -0.039 to +2.054 log units (ΔQFM; where QFM = quartz + fayalite + magnetite buffer) and is positively correlated with the bulk-rock Ba/La ratio, indicating that slab-derived fluid is the oxidizing agent in the rhyolites. Chlorine contents in hornblende also correlate with the bulk-rock Ba/La ratio. Hence, high fluid-flux typically correlates with the R1 and low fluid-flux with R2 rhyolite magma types. A geochemical evolution and distribution can be tracked in time and space throughout the central region of the TVZ from 550 ka to present and has revealed two distinct magmatic cycles that vary in length. The first cycle included widespread R1 type magmatism across the central TVZ beginning ca. 550 ka and was directly associated with previously unreported dome-building and ignimbrite-forming volcanism, and led to a voluminous (>3000 km³) ignimbrite 'flare-up' between ca. 340 and 240 ka. These magmas also display the highest K₂O and Pb isotopic compositions compared to those erupted more recently, and is consistent with a peak in slab-derived sediment input. The second cycle began roughly 180 ka, erupting ca. 800 km³ of magma, and continues to the present. The duration, rate, and composition of melt production within these cycles appears to be governed by the flux of fluid/sediment released from the subducting slab, while the distribution of melts may be governed more by extension along the central rift axis. The Matahina Ignimbrite (~160 km³ rhyolite magma; 330 ka) was deposited during a caldera-forming eruption from the Okataina Volcanic Centre, TVZ. The outflow sheet is distributed primarily from the northeast to southeast and consists of a basal plinian fall member and three ash-flow members. Pumice clasts are separated into three groups defined by differences in bulk geochemistry and mineral contents: high CaO, MgO, Fe₂O₃T, TiO₂, and low Al₂O₃, +hornblende (A2), low CaO, MgO, Fe2O3T, TiO2, ±hornblende (A1), and a subset to A1, which has high-K, +biotite (B). Two types of crystal-rich mafic clasts were also deposited during the final stages of the eruption. The distinct A and B rhyolite magma types are petrogenetically related to corresponding type A and B andesitic magma by up to 50% crystal fractionation under varying ƒO₂-ƒH₂O conditions. Further variations in the low- to high-silica rhyolites can be accounted for by up to 25% crystal fractionation, again under distinct ƒO₂-ƒH₂O conditions. Reconstruction of the P-T-ƒO₂-ƒ’H₂O conditions of the andesite to rhyolite magmas are consistent with the existence of a compositional and thermal gradient prior to the eruption. Magma mingling/mixing between the basalt to andesite and main compositionally zoned rhyolitic magma occurred during caldera-collapse, modifying the least-evolved rhyolite at the bottom of the reservoir and effectively destroying the pre-eruptive gradients. A detailed examination of the diverse range of calcic-amphibole compositions from the ca. 330 ka Matahina eruption (ca. 160 km³ rhyolitic magma) of the Okataina Volcanic Complex, Taupo Volcanic Zone, including crystal-rich basalt to dacite pumice from post-collapse deposits, reveals several pre- and syn-eruption magmatic processes. (1) Amphibole phenocrysts in the basaltic-andesite and andesite crystallized at the highest pressures and temperatures (P: up to 0.6±0.06 GPa and T: up to 950°C), equivalent to mid-crustal depths (13-22 km). Inter- and intra-crystalline compositions range from Ti-magnesiohornblende → Ti-tschermakite → tschermakite → magnesiohornblende and some display gradual decreases in T from core to rim, both consistent with magma differentiation by cooling at depth. (2) The largest amphibole crystals from the basaltic-andesite to andesite display several core to rim increases in T (up to 70°C), indicating new hotter magma periodically fluxed the crystal mush. (3) The dominant population of amphibole (magnesiohornblende) from the rhyolite is small and bladed and crystallized at low P-T conditions (P: 0.3 GPa, T: 765°C), equivalent to the eruptive P-T conditions. Amphibole (tschermakite-magnesiohornblende) from the dacitic and low-silica rhyolitic pumice form two distinct populations, which nucleated at two different T (High: 820°C and Low: 750°C). These compositional variations, governed primarily by differences in T conditions during crystal growth, record the mixing of two distinct amphibole populations that approached a thermal equilibrium at the eruptive T. Therefore, the diversity in amphibole compositions can be reconciled as an exchange of crystals+liquid between the basaltic-andesite to dacite from the mid-crust and rhyolite from the upper-crust, which quenched against one another, modifying the dacite to low-silica rhyolite compositions as the eruption progressed.

The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Title from PDF of title page (University of Missouri--Columbia, viewed on October 5, 2009). Thesis advisor: Dr. Alan G. Whittington. Includes bibliographical references.

xii, 76 p. ： ill. (some col.) / Large-volume calderas are responsible for producing large deposits of rhyolite and high-silica rhyolite, but the mechanisms by which these deposits are produced are still poorly understood. The Kamchatka Peninsula of Russia contains several large calderas and is one of the most volcanically active areas on Earth. Karymshina Caldera, the largest (25 km x 15 km) caldera in Kamchatka, produced an estimated 800 km 3 of high-silica rhyolitic ignimbrites and post-caldera extrusions, which erupted 1.78 and 0.5-0.8 Ma, respectively. SiO2 content ranges from 66.27-71.89 wt% in the ignimbrites and 70.16-77.70 wt% in the post-caldera extrusions studied. Crystal content is primarily quartz and plagioclase, 0.5-2 mm in size, with other minerals. Values of δ18 O, δD, 87 Sr/86 Sr, and 144 Nd/143 Nd indicate little assimilation of crustal material, in contrast to modeling results. XRF analysis indicates a homogeneous magma. The rhyolite-MELTS program was used to model crystallization of a basaltic source with addition of amphibolite partial melt and hydrothermally-altered silicic rock to reproduce the observed compositions. This thesis contains both previously published and co-authored material. / Committee in charge: Dr. Ilya Bindeman， Chairperson； Dr. Paul Wallace， Member； Dr. Mark Reed， Member

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1976. / Microfiche copy available in Archives and Science / Bibliography: leaves 232-239. / by Rudolph Hon. / Ph.D.

The main rock unit exposed in the southern part of the Tucson Mountains, Pima County, Arizona, is the Cat Mountain rhyolite. It forms the eastward dipping slope and the western escarpment of the mountain range, capping the large fault blocks which make up the range. Petrographic and radiometric data combine to show that the Cat Mountain rhyolite, as originally defined, consists of two major ash flow eruption sequences. The lower sequence is less uniform and continuous than the highly welded characteristically jointed upper unit. A basal non welded unit is found along the western escarpment, a partly welded transition zone is found between the welded units, and a capping partly-to-non-welded unit is exposed in protected parts of the eastern slope. The volcanic history of the area began with the emplacement of a nuee ardente deposit forming the “chaos” unit. This was followed by two ash flow pulses through the same vents, and the sequence was terminated by the intrusion of spherulitic rhyolite sealing up the vents.

Understanding continental crust formation and modification is a fundamental and longstanding geologic problem. Influx of mantle-derived basaltic magma and partial melting of the crust are two ways to drive crustal differentiation. This process results in a low density upper crust and denser, more refractory lower crust, creating significant and vastly different geochemical reservoirs over time. The High Lava Plains (HLP) and Northwestern Basin and Range (NWBR) in central and eastern Oregon provide an excellent example of intraplate volcanism where we can examine the beginnings of segregation of a relatively young, recently accreted crust. The origins of continental magmatism and its relationship to plate tectonics, especially away from the continental margins, are only slowly becoming revealed. The western United States is the most volcanically active part of North America during Cenozoic time, and this activity includes the enigmatic volcanism of the HLP and NWBR. Rhyolitic volcanism in the HLP and NWBR is age-progressive but in a direction that is nearly perpendicular to North American Plate motion. Despite being erupted through a similar crust and with a similar composition of mafic input, the HLP province is strongly bimodal (basalt-rhyolite) while the NWBR province exhibits a continuum of compositions. High silica rhyolites are commonplace in the HLP, with approximately a 1:1 ratio of rhyolite to basalt, even though the crust is comprised of mafic accreted terranes. Asthenospheric flow, mantle melting and crustal extension coupled with southwesterly North American plate motion explain the age-progressive volcanism of the HLP and NWBR. Differential asthenopheric counterflow and mantle upwelling created by the down-going Cascadia slab, coupled with transtensional stresses related to the rotation of the North American plate and Basin and Range extension, decreasing to the north, can produce the observed variations in rhyolite compositions and volumes in the two adjacent provinces. These differences are caused by fundamentally different petrogenetic processes that take place in the crust. In the HLP, an increase in mantle-derived magma flux into the lower crust has created low silica rhyolite via partial melt that separated, coalesced and rose buoyantly. This low silica rhyolite may erupt, solidify in the upper crust, or differentiate by fractional crystallization to produce high-iron, high-silica rhyolite containing an anhydrous phase assemblage. In the NWBR, a smaller flux of basaltic magma, coupled with greater transtension resulted in small crustal processing zones where fractional crystallization coupled with magma mixing and recharge created a wide range of compositions. Partial melting to form rhyolites was limited. These rhyolites have lower iron, and hydrous phases (biotite, amphibole) are common. These processes modify the crust in different ways, leaving a stratified crust in the HLP but a less modified crust in the NWBR. Recent geophysical and isotopic studies bear out these differences and allow for a unified, internally consistent model for both provinces, one that relies only on partial melt generation driven by current plate movements and do not require a mantle plume contribution. The bimodal volcanism of the HLP is a direct consequence of the processes that cause the gravitational differentiation of the continental crust into upper and lower units. The model for the HLP is generally applicable to other localities that have predominantly mafic crust and a similar balance of crustal transtension and mantle-derived basaltic flux. One such place is Iceland, which has strongly bimodal (basalt – rhyolite) volcanism. In areas where silicic crust has become substantially more mafic due to a high flux of intraplated basalts, such as in the bimodal Snake River Plain, the model is also applicable. / Graduation date: 2012 / In order for the .age files to run, the add-in called ArArCalc for Excel (version 200 or 2003) must be installed. ArArCalc is available from the website Earthref.org

Master of Science / Department of Geology / Matthew E. Brueseke / Previous studies of the northern Great Basin have indicated that mid-Miocene epithermal gold and silver ore deposits distributed regionally are temporally related to the magmatic activity associated with the onset of widespread extension and the Yellowstone hotspot (Saunders and Crowe, 1996; Kamenov et al., 2007). This study is focused on the volcanic rocks and ore deposits from the Silver City district (SCD), ID to address the petrogenesis and magmatic evolution that was influential in forming local precious metal deposits. The goal is to understand the tectonomagmatic conditions that contributed to the petrogenesis of the volcanic suite in the Silver City district, which can be used to provide details on the relationship between coeval mid- Miocene magmatism and mineralization across the northern Great Basin and Oregon Plateau. In order to better constrain the magmatic evolution of the SCD and potential sources of the precious metals, we have undertaken detailed sampling of local crust and mid-Miocene volcanic units to constrain their physical, geochemical, isotopic, and geochronological characteristics, as well as provide constraints on the petrogenesis of the mid-Miocene volcanic package. Prior studies of the local volcanism have yielded K-Ar and [superscript]40Ar/[superscript]39Ar ages of ~16.6 to 14 Ma (Bonnichsen, 1983), while others have dated adularia from one SCD mineral vein and obtained [superscript]40Ar/[superscript]39Ar ages of between 15.6 and 16.3 Ma (Hames et al., 2009; and Aseto et al., 2011). Field observations are consistent with earlier work (Lindgren, 1900; Asher, 1968; Pansze, 1975; Halsor et al., 1988; Bonnichsen and Godchaux, 2006; Camp and Ross, 2009) and reveal a sequence of basalt consisting of regionally prevalent Steens Basalt that pre-dated precious metal mineralization. Some of the basalt appears to have been erupted locally, based on the presence of mafic dikes and thick pyroclastic deposits similar to other regional mid-Miocene magmatic systems. Stratigraphically overlying this lower basalt suite is a complex package of rhyolite flows and domes, thin silicic pyroclastic units, additional basaltic lava flows, intermediate lava flows, and mafic/silicic shallow intrusives. Geochemical analysis indicates that the basaltic and basaltic andesite lava flows are locally erupted flows of Steens Basalt while the intermediate and silicic volcanism in SCD can be classified into nine distinct units including two andesites, one dacite, four rhyolites and two rhyolite tuffaceous units. Geochemical modeling suggest that the intermediate and silicic magmas were formed by a combination of open system processes, including low pressure partial melting and assimilation of mid to upper crustal granitoid basement rock, and magma mixing between silicic and basaltic endmembers. The formation of silicic volcanism in the SCD is similar to other regional mid-Miocene silicic volcanic systems (e.g. Santa Rosa-Calico volcanic field and Jarbidge Rhyolite). Based on new [superscript]40Ar/[superscript]39Ar geochronology of both volcanic units and epithermally emplaced mineralization, SCD volcanism appears to have erupted over a relatively short amount of time that overlaps with local epithermal Au-Ag mineralization.

The Miocene rhyolites of the Spor Mountain Formation hosts the world's largest beryllium deposit which produced 85% of the world's beryllium in 2010. The fresh lava is extremely enriched in Be (up to 75 ppm in matrix glass). We have examined the rhyolite to understand the Be enrichment. The Spor Mountain rhyolite contains ~40% quartz, ~40% sanidine, ~10% biotite, and ~10% plagioclase, along with accessory fluorite, columbite, euxenite, fergusonite, monazite, thorite, and zircon. Two types of rhyolite erupted within the Spor Mountain Formation, a less evolved magma (1150 ppm Rb, 42 ppm Be, 0.68 wt% F in matrix glass) and an evolved magma (1710 ppm Rb, 75 ppm Be, 1.56 wt% F in matrix glass). Eruption temperatures estimated using zircon saturation, feldspar-liquid, two feldspar, and Ti-in-quartz geothermometers converge on 718 °C for the less evolved magma and 682 °C for the evolved magma. Using the Ti-in-Qz equation of Huang and Audetat (2012), the pressure of the Spor Mountain rhyolite system is estimated to be around 2 kbar at 700°C. Water content of the rhyolite melt was less than <5 wt%, based on the presence of all four major mineral phases at 700°C and the magma was water undersaturated (Webster et al., 1987). Viscosity of the rhyolite was about 6.2 log Pa·s for the less evolved rhyolite and 5.8 log Pa·s for the evolved rhyolite. Magma viscosities calculated using the Einstein-Roscoe question suggest the evolved magma has a slightly higher viscosity than the less evolved magma (7.0 log Pa·s in the evolved magma vs 6.7 log Pa·s in the less evolved magma) because of higher phenocryst content. Fluorine lowered the melt viscosity, though not by a significant amount (less than 0.5 log units at 1.7 wt% F). Partition coefficients for 32 elements have been calculated for biotite, for 21 elements for sanidine and plagioclase, and for 6 elements for quartz, using data acquired by laser ablation inductively coupled plasma mass spectrometry. Partition coefficients for feldspars in the Spor Mountain rhyolite are generally higher than other silicic magmas, and lower for biotite. Beryllium is one of the most incompatible trace elements in the Spor Mountain rhyolite, with a bulk partition coefficient <0.1. Volatile content of the melt (specifically F), melt composition, and the low temperature of crystallization act as the major controls of trace element partitioning. Trace element models using these partition coefficients suggests that crystal fractionation is the dominant magmatic enrichment process within the rhyolite, requiring ~45% crystallization (f = 55%) of the observed phenocrysts to get compositions from the less evolved to evolved rhyolite. Accumulation of batches of melt formed by different degrees of partial melting cannot explain the great depletion of compatible elements.