Year of Graduation
2024
Level of Access
Open Access Thesis
Embargo Period
5-15-2027
Department or Program
Earth and Oceanographic Science
First Advisor
Rachel Beane
Second Advisor
Emily Peterman
Abstract
Determining how silicic magma is generated and how the crust accommodates large volumes of magma is challenging because only the erupted products of a magma system are accessible. One model being tested suggests that silicic magma is extracted from a crystal-rich, melt-bearing magma mush. Plutonic lithics (coarse-grained crystalline rock fragments) may be sourced from this magma mush or from surrounding crustal material. Extracted plutonic lithics were collected from the Ohakuri ignimbrite of the Central Taupō Volcanic Zone (TVZ), New Zealand, where high rates of rhyolitic magma erupt explosively forming large calderas. Some of the plutonic lithics have minor element signatures similar to the Ohakuri pumice and are interpreted as being sourced from the underlying reservoir of magma mush. Other plutonic lithics from the Ohakuri ignimbrite have geochemical signatures similar to Whakamaru, an older volcanic system, and are interpreted to be crustal fragments brought up by the force of the Ohakuri eruption. The "magma mush" lithics are enriched in compatible major elements relative to Ohakuri pumice and comprise a framework of phenocrysts surrounded by granophyre. Notably the composition of granophyre is more evolved than the pumice suggesting it may be the crystallized product of remnant interstitial melt. Pressures modeled for the granophyre using rhyolite-MELTs geobarometry are similar to the extraction pressures modeled for the Ohakuri pumice glass. The combination of the observed textures, geochemical signatures, and calculated pressures, provide the first evidence that granophyre may represent the once potentially extractable liquid component of magma mush within large silicic volcanic systems.