How similar are silicic volcanic and plutonic systems? New insights into upper crustal storage conditions and formation processes of ignimbrites

How are large, silicic volcanic and plutonic systems related? Are their magmas stored in a similar way? Do rhyolitic and granitic magmas form by identical processes? While these are highly relevant questions when trying to predict volcanic eruptions, the debate regarding the plutonic-volcanic (P-V) connection is ongoing with no clear answer provided to date. The reason for the ongoing debate stems from the fact that as our observations regarding silicic igneous rocks increase, our interpretations become more disparate, providing controversy and debate. Hence, current interpretations of the P-V relationship range from plutonic and silicic volcanic rocks being completely unrelated to them forming from the same magma system. Assuming the latter, if pre-eruptive magma storage for silicic volcanic systems occurs in a pluton-like (low temperature, crystal-rich) state, then these volcanic magmas would need to be re-heated or require an unknown mechanism to melt and erupt sometimes aphyric magmas. To answer these questions, my dissertation work seeks to compare silicic volcanic rocks from the Southern Rocky Mountains Volcanic Field (CO) to plutonic rocks. First, I applied Ti-in-Quartz thermometry and diffusion modelling to the immense (5000 km3) Fish Canyon Tuff (FCT). Temperatures for FCT are extremely homogenous throughout the Tuff at 737°C or 673°C (depending on choice of TiO2 activity). Comparison of multiple recent Ti-in-Quartz diffusion coefficients to the geologic context of FCT show that the most recent diffusion coefficients by Audétat et al. (2021) cannot be applied to the FCT. Those of Jollands et al. (2020) and Cherniak et al. (2007) suggest storage of FCT mush at or significantly below 737°C in a crystal-rich state for most of its upper crustal lifetime. To further gain insight into the P-V connection, Fe isotope ratios (δ56Fe) of magnetite and whole rock from the FCT and co-magmatic plutonic, volcanic, and hydrothermal rocks of the Latir Volcanic Field (NM) were analyzed. The δ56Fe of FCT magnetite is comparable to that of plutonic magnetite, implying that FCT may be an erupted plutonic mush. Samples from the Latir Volcanic Field show that plutonic rocks consistently have magnetite isotopically heavier than whole rock, whereas volcanic rocks have magnetite similar to whole rock. Hydrothermal rocks tend to have magnetite lighter than whole rock. This is consistent with the idea that a near-solid mush is percolated by water-rich liquids, which can fractionate Fe isotopes. These water-rich liquids could aid as a trigger to remelt and erupt the top of the pluton-like mush. Lastly, FCT granophyre (quartz-feldspar intergrowth) was imaged using synchrotron X-ray microtomography to gain insight into its 3D texture. This imaging shows directed granophyre fabric leading into phenocrysts, interpreted to reflect low temperature, water-rich liquids feeding components into growing crystals, rather than the traditional interpretation of granophyre forming by rapid crystallization upon eruption. Results from my dissertation work imply that silicic volcanic systems form in the upper crust at low temperature, crystal-rich mush. A crystal-rich storage of volcanic systems is consistent with other recent geophysical, geochemical and petrological observations and promotes movement away from the original view of liquid-rich upper crustal magma chambers. Additionally, my research suggests that silicic plutonic and volcanic rocks form from identical highly crystalline mushes, for which the top will erupt as volcanic rocks while the bottom remains in the crust as pluton. This has direct impact with serious implications for our understanding of igneous systems and how we view eruption triggering.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Geology

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2023

Author dc:creator

• Bruckel, Karoline Marie

Contributors dc:contributor

• Lundstrom, Craig C
• Gregg, Patricia M
• Johnson, Thomas M
• Stewart, Michael

• Fish Canyon Tuff
• Amalia Tuff
• Fe Isotopes
• Quartz
• Feldspar
• Diffusion
• Thermometry
• Granophyre
• Silicic Magma

Statement dc:rights

• Copyright 2023 Karoline Bruckel

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/120395

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F120395