Dissertation
Applications of portable XRF technology to the eruptive deposits of super volcanoes: a detailed study of the Otowi Member of the Bandelier Tuff
Doctor of Philosophy (PhD), Washington State University
01/2017
Handle:
https://hdl.handle.net/2376/111218
Abstract
Portable X-ray fluorescence (pXRF) calibrations were developed for geological materials using minimal sample preparation: one for the broad range of geologic compositions using rock powders, and three with an emphasis on volcanic rock samples. The calibrations use influence coefficients to correct for absorbance and secondary enhancement of one element by X-rays of other elements present in the sample. One of these calibrations, for powdered glass of rhyolitic composition, was then applied to a study of chemical variations in a large rhyolitic ignimbrite, the Otowi Member of the Bandelier Tuff.
The Valles caldera is the classic example of a resurgent caldera, formed during two ignimbrite super-eruptions. The older event, at 1.60 Ma, produced the chemically zoned Otowi Member of the Bandelier Tuff. The pXRF instrument was used to generate a massive chemical dataset of pumice and fine ignimbrite matrix samples (~4,000 in total) from the Otowi Member in order to study the chemical zonation of the ignimbrite in unprecedented detail.
Volcanologists and petrologists who study the geochemistry of ignimbrites have long been aware of potential problems arising from analyzing whole rocks. Ashy tuff matrices are typically enriched in crystals, and lithic fragments in ignimbrites also contribute to divergence of bulk and magmatic compositions. However, there have been no prior detailed studies aimed at addressing the magnitude of the problem. Concentrations of Ti, Fe, Ca, P, Sr, Ba, Zr and Ce in the ignimbrite matrix are variably and non-systematically enriched or depleted compared to those in pumice. Strongly incompatible elements (e.g. Rb, Nb) are least affected, and provide the most robust measure of magmatic evolution obtainable from ignimbrite whole-rock data.
Using the large data set, four statistically-robust compositional modes, corresponding to different degrees of magmatic evolution of rhyolitic melt, have been identified. The abundance of these compositions shifted from most-evolved to least-evolved as the eruption progressed, but with significant scatter increasing upwards through the tuff. These observations are used to construct a scheme of magma chamber evacuation, caldera subsidence, and pyroclastic flow deposition during the Otowi eruption.
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Details
- Title
- Applications of portable XRF technology to the eruptive deposits of super volcanoes: a detailed study of the Otowi Member of the Bandelier Tuff
- Creators
- Ashley Elizabeth Steiner
- Contributors
- John A Wolff (Advisor)Michael C Rowe (Committee Member)Catherine M Cooper (Committee Member)Scott Boroughs (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- School of the Environment (CAHNRS)
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Number of pages
- 234
- Identifiers
- 99900581715901842
- Language
- English
- Resource Type
- Dissertation