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Silicic magmatic systems are fundamental to the production and evolution of continental crust, the recycling of volatile elements from mantle to atmosphere, and the production of economic deposits. Additionally, silicic systems are responsible for producing some the most catastrophic volcanic eruptions on Earth. The processes and timescales governing the production, evolution, and ultimately eruption of silicic magmatic systems remain poorly constrained but are critical to accurate volcanic hazard assessment. Active debate continues regarding many aspects of silicic magmatic systems, such as: 1) the architecture of the sub-volcanic system (i.e., physical distribution of diverse magmatic components throughout the crust that contribute to erupted magma bodies); 2) the longevity of storage of different magmatic components (i.e. melts, crystals, volatiles) prior to eruption; 3) the timescales over which the erupted magma body (or bodies) assemble prior to eruption; and 4) the predominant thermal (and by proxy physical) state in which silicic magma bodies are stored within the crust. In this dissertation I combine a variety of geochemical tools to explore the thermal the chemical evolution of silicic magmatic systems of different size and eruption magnitude to address this outstanding uncertainty. In detail this dissertation focuses on the two most recent eruptive episodes (1980-86 and 2004-05) from Mount St. Helens (MSH), USA, and six eruptions spanning the last 30 kyrs of activity at the Taupo Volcanic Center (TVC) in New Zealand.In Chapter 1 I use new 238U-230Th-226Ra disequilibria data, in-situ plagioclase trace-element data, and Sr-in-plagioclase diffusion timescales from Mount St. Helens 1980 cryptodome and 2004-2005 dacite domes to evaluate the thermal storage conditions and compositional diversity of recent MSH magmas. Plagioclase trace-element data and U-series characteristics reveal a compositionally heterogenous magmatic system beneath MSH and require multi-stage plagioclase growth histories. The data also show that 2004-05 dacites contain a different plagioclase population relative to the 1980 cryptodome dacite, comprised of either a compositionally distinct plagioclase component or composed of the same plagioclase components but in significantly different proportions. Coupling Sr diffusion timescales with U-series measurements indicates that significant fraction (at least ~40%) of modeled MSH plagioclase spend 25-50% of their storage time in similar conditions. Our data combined with data from other arc systems imply that the process of remobilizing magma in arc systems towards successive eruptions requires thermally rejuvenating largely crystalline material, as opposed to sequential tapping of a persistent liquid-dominated magma body, even if successive eruptions are spaced closely in time (decades). Also, rejuvenation events responsible for successive eruptions may sample spatially localized, but potentially overlapping, portions of the broader magma reservoir.In Chapter 2 I present new 238U-230Th ages and trace-element data for zircons separated from both whole-rock (WR) samples and from plagioclase separates from the 25.4 ka Oruanui eruption within the TVC. These data provide a more complete record of zircon crystallization than previous analyses, which were restricted to whole-rock interiors, and demonstrate that plagioclase-hosted zircons and WR hosted zircon rims (surfaces) both record processes obscured in WR zircon interior data. WR zircon surface 238U-230Th ages yield a single eruption age peak whereas the WR zircon interior and both plagioclase-hosted zircon surface and interior age spectra record more complex multi-modal age distributions. WR zircon surfaces are generally more restricted in composition than WR interiors and display two clear compositional groups, the preservation of which requires the two groups of zircons must have been incorporated into the erupted Oruanui rhyolite within ~2 kyrs of eruption. Systematic compositional variations with 238U-230Th crystallization age recorded in plagioclase-hosted zircon surfaces indicate a portion of the Oruanui magmatic system experienced a shift to higher temperatures and less evolved compositions occurred beginning at ~40-45 ka and suggests at portion of Oruanui plagioclase crystallized more than ~13 kyrs prior to eruption. These new data clearly demonstrate the additional insight available by detailed microanalytical investigations of zircons extracted from multiple hosts within a magmatic system. More broadly the work presented here suggests that large volumes of silicic magma accumulate in the crust via amalgamation of multiple smaller compositionally distinct portions of the broader magma reservoir, and that large silicic eruptions may result from a complex interaction of thermal priming (i.e., rejuvenation) and tectonic triggering/modulation.In Chapter 3 I present in-situ plagioclase trace-element data and Sr-in-plagioclase diffusion timescales from six eruptions spanning the last 30 kyrs of activity from the TVC. I use these data to better understand the compositional and thermal evolution of a large silicic magmatic system across a caldera-forming cycle. Results show that plagioclase compositions do not vary systematically across the caldera cycle, but within each eruption plagioclase interiors are far more compositionally diverse than corresponding rims. Additionally, plagioclase rims appear to have grown in equilibrium with the erupted melt, with the possible exception of the Omega dacite. These data require plagioclase interiors to have crystallized within a more compositionally heterogenous portion of the TVC system (possibly the deeper-seated crustal mush) and have been subsequently transported to the final erupted magma body, where crystal rims grew. Diffusion timescales recovered from plagioclase rims are also remarkably similar across all six eruptions, suggesting that the final crystal-poor erupted magma bodies were constructed within years to decades (possibly centuries) prior to eruption, regardless of eruption size. These data suggest that the accumulation of large volumes of mobile silicic magma within the upper crust may not require significantly longer accumulation times relative to small volume melt bodies. Instead, other explanations are needed – for example, it may be possible that larger volumes of mobile magma accumulate when the mush system is more interconnected (i.e., more developed melt pathways), which could allow for more efficient melt extraction, or access to a greater volume of mush.Collectively, these chapters provide novel insights into the processes and timescales governing the construction and evolution of silicic magmatic systems. The work on MSH demonstrates silicic reservoirs at many arc systems are comprised of heterogenous crystal rich mushes that appear to exist predominantly at low temperatures. The MSH work also demonstrates that successive eruptions can sample compositionally distinct portions of the magma reservoir even when spaces closely in time (within decades). The work presented herein on the TVC emphasized the importance of detailed investigations of crystalline cargo in detangling the complex processes operating within silicic magmatic systems. Additionally, this work demonstrates that the production of large volumes of mobile silicic melt within the upper crust can occur on timescales of years to centuries and thus does not require protracted accumulation timescales relative to smaller volume melt bodies. Instead, larger volumes of mobile magma may accumulate when the mush system is more interconnected (i.e., more developed melt pathways), which could allow for more efficient melt extraction, or access to a greater volume of mush.

The development of soil is driven by environmental conditions, and soil properties are reflective of those conditions. Pedogenic carbonate is widespread in arid and semi-arid soils and its stable C and O isotope composition is often used as a paleoclimate indicator. However, questions remain about the precise meaning of those signals, and most importantly the season(s) that the isotopic composition most closely reflects. Chapter 1 examines the soil conditions that lead to carbonate formation, and describes research on a climosequence of four sites on Holocene soils in Fish Lake Valley, Nevada (western USA), where Mean Annual Precipitation ranges from ~80mm yr-1 to ~220mm yr-1. Continuous measurements of soil temperature, soil volumetric water content, air temperature, and air humidity were recorded in 0.5hr intervals. Precipitation and samples of soil atmosphere from 10, 25, 50 and 100cm were collected at various intervals for CO2 concentration, δ13C and δ18O values (which are a proxy for soil water δ18O). This climosequence indicates that the C and O stable isotope composition of soil CO2, H2O and carbonate change systematically with elevation and climate. At the lowest elevation site, soil carbonate δ13C and δ18O values reflect constant conditions nearly year round, especially in the 25-50cm soil depth interval. At the mid-elevation sites, spring through summer conditions appear to be recorded in δ13C and δ18O values of pedogenic carbonate. At the highest elevation site of 2602m, the C and O isotopes of carbonate never reflected the measured soil CO2 and H2O isotopes. These results indicate that there is inherent difficulty or complexity in assigning a definitive timing of carbonate formation based on short-term (one or two year) field measurements of soil CO2 and H2O, but also suggests that short-term monitoring, despite its weaknesses, helps to reveal the nature of the long term record isotopically recorded in soil carbonate. Chapter 2 presents the first continuous millennial-scale paleoclimate reconstruction based on O, C and U isotopes in dense, laminated pedogenic carbonate. The record is from the Wind River Basin of Wyoming, and provides isotopic proxies for climate and vegetation conditions in the North American mid-continent for the last 120ka. Accurate and precise 230Th/U ages were obtained from laser ablation ICP-MS spots of 93um diameter size. The resulting carbonate growth calculations indicate that ~100μm sampling regions represent 10ka to 0.1ka at growth rates of 10 to 1000μm ka-1. The U-series transects were coupled to δ13C and δ18O values obtained by ion probe on 10um diameter spots. Modern soil carbonate in the region has δ18O and δ13C values that are similar to the youngest (early Holocene) samples acquired in the laminations, strengthening the interpretation that the carbonate reflects soil isotopic conditions at the time of formation. The carbonate lamination record from 120 to 80ka is somewhat sparse due to slowly forming carbonate, but records conditions that change in phase with summer solar insolation, and indicate that relatively warm and dry conditions prevailed during the penultimate deglaciation and last interglacial period. The paleoclimate signal in the soil carbonate δ18O values during this time period are similar to that recorded in the well-studied Devils Hole phreatic carbonate δ18O record from the Mojave Desert in southwestern North America. From 75 to 55ka, carbonate δ18O values sharply increase by ~2‰, a change also recorded by speleothems from central North America, but not seen in the Devil’s Hole record. Atmospheric circulation during this time appears to have changed from a state dominated by westerly winds to one influenced by meridional flow from the south along the western limb of a strong and west-based North Atlantic Subtropical High, a condition that increased the proportion of summer to winter precipitation by 20-33%. δ13C and 234U/238Ui values from 75 to 55ka also indicate a shift towards wetter conditions and are supported by regional fossil pollen records suggesting a change to prairie-like ecosystems with wetter summers during this time. Subsequently, the carbonate record suggests that climate in mid-latitude central North America became progressively more arid during the onset of the last glaciation (both δ13C and 234U/238Ui values become more positive), culminating in peak aridity during the Last Glacial Maximum. Carbonate formation rates during these colder conditions increased and the temporal resolution of the Wind River record increased as a result, allowing it to capture sub-millenial abrupt climate changes associated with Dansgaard-Oeschger cycles 6 through 3, Heinrich Stadials 2 through 0 (Younger Dryas), as well as the dramatic Bolling-Allerod warming at 14.5ka. The Wind River pedogenic carbonate record shows the potential to develop similar high temporal resolution paleoclimate records form soil carbonate deposits that are relatively common worldwide.The Cenozoic paleoclimate of the Atacama Desert is not well known. Chapter 3 presents the results of a study of 14 Oligocene to early Miocene paleosols exposed in the El Tesoro Mine, near Calama, Chile. The paleosols developed on an aggrading alluvial fan system, and lie above mineralized gravels that host a copper ore body. Soil forming conditions that oscillated between chemical weathering and clay production (humid: analogous to modern Alfisols) to environments favoring the accumulation of pedogenic carbonate (arid to semi-arid: analogous to modern Aridisols) are indicated. In contrast, the region is presently hyperarid, and soils accumulate sulfate, chlorides, and nitrates. While total chemical analyses clearly reveal the accumulation of Ca by the carbonate rich paleosols, none of the soils revealed significant losses of elements by leaching. The δ18O values of the carbonates range from -13.81‰ to -3.16‰ (VPDB). The O isotope data, when combined with published data from the region, revealed a significant divergence in the O isotope composition of precipitation in the eastern and western margins of the Andean plateau since the Oligocene, suggesting that simple interpretations of declines in δ18O values of carbonate with increasing elevation may not be appropriate. These paleosols clearly indicate that wetter conditions prevailed in what is now the Atacama Desert during the Oligocene to early Miocene.Chapter 4 presents findings from a study examining a special case of interactions between soil water oxygen isotopes and cations adsorbed to clay minerals in the soil matrix. In isotope-enabled hydrology, soil and vadose zone sediments have been generally considered to be isotopically inert with respect to the water they host. This is inconsistent with knowledge that clay particles possessing an electronegative surface charge and resulting cation exchange capacity (CEC) interact with a wide range of solutes which, in the absence of clays, have been shown to exhibit δ18O isotope effects that vary in relation to the ionic strength of the solutions. To investigate the isotope effects caused by high CEC clays in mineral-water systems, we created a series of monominerallic-water mixtures at gravimetric water contents ranging from 5 to 32%, consisting of pure deionized water of known isotopic composition with homoionic (Mg, Ca, Na, K) montmorillonite. Similar mixtures were also created with quartz to determine the isotope effect of non-, or very minimally-, charged mineral surfaces. The δ18O value of the water in these monominerallic soil analogs was then measured by isotope ratio mass spectrometry (IRMS) after direct headspace CO2 equilibration. Mg- and Ca-exchanged homoionic montmorillonite depleted measured δ18O values up to 1.55‰ relative to pure water at 5% water content, declining to 0.49‰ depletion at 30% water content. K-montmorillonite enriched measured δ18O values up to 0.86‰ at 5% water content, declining to 0.11‰ enrichment at 30% water. Na-montmorillonite produces no measureable isotope effect. The isotope effects observed in these experiments may be present in natural, high-clay soils and sediments. These findings have relevance to the interpretation of results of direct CO2-water equilibration approaches to the measurement of the δ18O value of soil water. The adsorbed cation isotope effect may bear consideration in studies of pedogenic carbonate, plant-soil water use and soil-atmosphere interaction. Finally, the observed isotope effects may prove useful as molecular scale probes of the nature of mineral-water interactions.