Combining U-series crystal ages with trace element and isotopic information to evaluate the thermal and chemical evolution of silicic magmatic systems
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 <5% of their storage time at temperatures 750oC and thus in a liquid dominated (i.e., mobile) state. In contrast, some plagioclase possibly spent >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.