The Peach Spring Tuff (PST), a large Miocene ignimbrite located in the southwestern USA, is unique in its abundance of U, Th, and REE concentrating accessory minerals (zircon, sphene, allanite, chevkinite). Textures and compositions of these accessory minerals and magnetite were studied by a variety of methods.
Samples of pumice clasts and fiamme were collected from locations in the outflow sheet and intracaldera deposits. Samples range in composition and crystal content, from relatively crystal-poor rhyolites to crystal-rich trachytes, with intracaldera fiamme on the more mafic end of this spectrum. REE trends in zircon and sphene grains reveal a simple fractionation history in rhyolites, but MREE enrichment in sphene edges in trachytes suggest final growth from a less evolved melt. Ti-in-zircon and Zr-in-sphene thermometry reveals lower temperature growth at edges of grains from rhyolites, while edges from trachytes record warmer temperatures. Trace element variations and temperatures also suggest that zircon has a more protracted history of growth than other accessory phases. Textures corroborate the geochemical results. Phenocrysts in rhyolites tend to be euhedral, while those from intracaldera trachytes display resorption features. Sphene size distributions of accessory minerals are kinked, suggestive of growth during decompression.
These results suggest that the PST was a zoned system affected by a late-stage heating event, which may have triggered eruption, followed by eruptive decompression. One outflow pumice clast of trachyte composition follows compositional trends of intracaldera trachytes and textural trends of outflow rhyolites, suggesting that these events affected different regions of the chamber to different extents.
| Identifer | oai:union.ndltd.org:VANDERBILT/oai:VANDERBILTETD:etd-07172010-160944 |
| Date | 02 August 2010 |
| Creators | Pamukcu, Ayla Susan |
| Contributors | Dr. Guilherme Gualda, Dr. Calvin Miller, Dr. Molly Miller |
| Publisher | VANDERBILT |
| Source Sets | Vanderbilt University Theses |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.library.vanderbilt.edu/available/etd-07172010-160944/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to Vanderbilt University or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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