Zircon Petrochronology and Thermochemical Modeling of Quaternary Magmatic Systems in Western North America (The Geysers, Eastern Snake River Plain, Blackfoot)

Zircon (ZrSiO4) is an accessory mineral commonly present in a wide range of igneous, metamorphic, and sedimentary rocks. When crystallized in-situ, it is a powerful tool for reconstructing chemical and thermal processes in the Earth’s crust at high fidelity and with fine temporal resolution. In this dissertation, zircon from a range of mafic to silicic volcanic fields in the western United States was investigated by combining U-Th-Pb geochronology, trace element geochemistry, and δ18O and ɛHf isotopic data. This integrated petrochronological approach of studying zircon age and composition in combination with textural constraints yielded new insights into magma origins, evolution, and geothermal potential associated with Quaternary volcanism. The first part of this study investigates zircon crystallization in the Eastern Snake River Plain (ESRP) and the adjacent Craters of the Moon volcanic field (COM) in southern Idaho. While the ESRP is dominated by voluminous olivine tholeiite lava flows with subordinate trachyandesite and rhyolite lavas, the COM field comprises mainly Fe-rich trachybasalts and rare evolved basaltic trachyandesites. In both settings, zircon is present in intermediate–silicic host rocks. High-resolution U-Pb and U-Th dating and trace element analysis confirm that zircon in these magmas formed autocrystically during protracted fractional crystallization, rather than being inherited from interstitial melts in a crystal mush. Equivalent evolutionary trends in zircon and whole-rock chemistry support this conclusion. Ultimately, zircon saturation in these systems was facilitated by strong enrichment of Zr via fractional crystallization. In the resulting high-Zr, compositionally intermediate ESRP and COM melts, zircon became saturated at unusually high temperatures and in comparatively primitive magma compositions. Since ESRP and COM zircon formed demonstrably in situ, zircon δ¹⁸O and εHf isotopic compositions are also reliable source indicators. These data suggest that ESRP and COM magmas originated from a similar mantle source, but subsequently evolved along different pathways. In the case of the ESRP, magma assimilated hydrothermally altered low-18O felsic rocks in the upper crust, whereas for the COM magmas, two sequential stages of assimilation occurred involving Archean lower crust and normal-δ¹⁸O upper crustal rocks, respectively. These findings underscore the importance of detailed petrochronological analyses of zircon and demonstrate that its presence in magmatic systems dominated by mafic melts has petrogenetic significance. Building on this framework, the second paper shifts focus to the Geysers Plutonic Complex (GPC) in California, where a rare combination of deep plutonic and cogenetic volcanic rocks is accessible due to extensive geothermal drilling. The GPC and its volcanic equivalents represent a compositionally evolved intrusive complex emplaced between ~700 and nearly 4000 m depth, based on geothermal drill well penetration. Zircon in these rocks records a history of magma evolution marked by early formation of highly fractionated microgranite porphyry, followed by emplacement of larger volumes of less evolved granite and granodiorite. Trace element and isotopic data (δ¹⁸O and εHf) from zircon reveal multiple magma types with different degrees of crustal assimilation. Using previously reported zircon age distributions, a thermal model was developed to reconstruct the magmatic history of the GPC. The model relates zircon saturation to magma temperature and successfully reproduces the observed zircon age distributions for the GPC. In the best-fit model scenario, the main volume of the GPC intruded during a brief, intense flare-up phase lasting for c. 50,000 years. This magma pulse was embedded into a longer interval of low-flux magmatism which lasted for at least c. 900,000 years. The agreement between the modeled volume of the intrusion and the extrapolated GPC volume validates the potential of zircon-based thermal modeling for reconstructing upper crustal magma reservoirs and their thermal histories. The final case study returns to southern Idaho and evaluates the geothermal potential of the Blackfoot Volcanic Field (BVF), a Pleistocene bimodal volcanic field located ~60 km southeast of the Snake River Plain-Yellowstone hotspot track. Despite hosting some of the youngest topaz rhyolites globally, the geothermal significance of this field remained uncertain. Zircon geochronology and geochemistry were used to constrain its eruptive and magmatic history. U-Pb and U-Th dating revealed two distinct eruptive phases: an early (1,006–785 ka) episode forming mainly the northern domes and a late (63–55 ka) pulse during which the southern domes erupted. Corresponding isotopic data for the BVF indicate a strong Archean crustal component in the parental magmas, consistent with their off-axis position relative to the hotspot. A thermal model for a 120 km³ magma reservoir emplaced 95,000 years ago and at 6 km depth successfully reproduces the timing of zircon crystallization and magma volumes for the southern domes and their subvolcanic equivalents. It also predicts that temperatures at ~4 km depth still exceed 300 °C today. The lack of surface geothermal manifestations within the BVF is attributed to a structurally complex hydrologic system that dilutes and diverts geothermal fluids. Collectively, these three case studies illustrate the broad applicability of zircon petrochronology in characterizing the temporal, thermal, and compositional evolution of magmatic systems underneath Quaternary volcanic fields. Zircon saturation can occur in intermediate magma compositions associated with voluminous basaltic magmas, extending the potential of zircon petrochronology for refining models of magma generation, evolution, and emplacement to systems beyond those conventionally investigated. When paired with thermal modeling, zircon data can provide insights into the thermal state of crustal magmatic systems, including those with geothermal energy potential. This thesis demonstrates the integrative power of zircon as a tool for decoding crustal magmatism and guiding geothermal exploration.