High-Resolution Numerical Thermal Modeling of Timescales and Mechanisms of Rhyolite Genesis and Transport as a Consequence of Basalt Influx: Applications to the Long Valley Volcanic Field, CA
Creators: name: Calogero, Meredith
Publisher: name: My University
Year: 2021
Subjects: subject: Volcanology
subject: Thermal Modeling
subject: Crustal Evolution
subject: Partial Melting
subject: High-Resolution
subject: Geology and Earth Sciences
subject: Science

description: High-silica rhyolites (&gt;75 wt% SiO2) are the most differentiated magmas on Earth. Though rare in subduction volcanism, they erupt in large volumes (100–1000s km3) in extensional settings. Understanding their generation is crucial as rhyolite formation substantially alters the continental crust, and such voluminous eruptions have global implications. Two supervolcano eruptions of high-silica rhyolite have occurred in the Western U.S. within 1 Myr, Long Valley, California, and Yellowstone, Wyoming. This thesis focuses on Long Valley caldera, where 800 km3 rhyolite erupted over the last 2.2 Myr, including the catastrophic caldera-forming Bishop Tuff eruption resulting in ≥600 km3 of zoned rhyolite in under a week. The second-most eruptive magma type in Long Valley is basalt. The formation of silicic magmas in response to basalt influx has been addressed by numerous numerical models in the literature. This study differs in that I present a novel numerical model with high spatial (~1 m) and temporal (&lt; day) resolution, tracking both kilometer- and meter-scale thermal and compositional evolution as consequences of basaltic sill emplacements and secondary rhyolitic melt segregation and transport. In Chapter 2, I investigate the crustal-scale response of basaltic sill emplacements into thick (32 km) granitoid crust, and quantify the importance of emplacement rate, depth range, and ambient temperature for causing substantial localized heating. I constrain the timescales for volatile release from a basaltic sill with finite H2O during crystallization and for the formation of transient wall-rock partial melts on the order of sill thickness. These timescales are both dependent on local ambient temperature and overlap within 10–100s of years after sill emplacement. Based on the results presented in Chapter 2, I hypothesize a mechanism for secondary melt migration, wherein melting of pre-existing aplite dikes enables transport of rhyolite melt. To address the consequences of secondary rhyolitic melt transport, I developed a new numerical model in Chapter 3, which I have named TIRAMISU (Transient hIgh-Resolution pArtial MeltIng Simulation of the crUst). The core of TIRAMISU is similar to that presented in Chapter 2, but in TIRAMISU melts may ascend if the sill center attains 80% crystallization and there is enough cumulative partial melt within the wall rock adjacent to the sill to be captured by model resolution. Secondary transport driven by a 50-m thick basalt sill invading the deep crust every 5 kyr results in about 60ºC cooling of the lower crust, and &lt;60ºC warming of the crust around the shallower rhyolite sills. For 3 km of basalt emplaced, about 1.7 km of rhyolite was generated, with about 30% of rhyolite from basalt and 70% from the granitoid crust. TIRAMISU tracks the evolving composition of the crust, including the partitioning of Sr. The Sr signature of the volcanic products in Long Valley is somewhat paradoxical. In these simulations, I find average Sr concentrations in the first-generation rhyolites of 380 ppm, with about 60% of the cumulative Sr from basaltic sources. Using melt contribution and Sr contribution percentages, I can estimate further drops in Sr concentration in second- and third-generation rhyolites, which would form through subsequent stages of melt transport. I intend further expansion of TIRAMISU to further investigate Long Valley, estimating the range of basalt influx histories needed to generate the observed volcanic products, as well as for exploring additional volcanic plumbing systems.