麻豆传媒

Thermoluminescence (TL) dating is one of few techniques that can be applied to a variety of geological material to accurately determine ages of Quaternary deposits. TL dating of solid Quaternary North American volcanic rocks such as obsidian, basalt and dacite, may be extremely useful for thoroughly understanding regional volcanism by allowing correct spatial and temporal correlation of young, solid volcanic deposits. TL dating applied to glass separates from North American Quaternary solid volcanic rocks has not previously been attempted. The aim of this research project was to characterize the TL behaviour and determine TL age estimates of glass extracts from five obsidians and eleven crystalline lavas collected from various localities in western North America. All samples were independently dated by other Quaternary dating methods. Preliminary analyses of the samples included major element analysis and thin section observations for petrographical classification, microprobe analysis to determine the glass chemistry of the rocks, and thick-source alpha-counting to determine the dose rates of the samples. A dose response test was designed to determine the intensity, reproducibility and short term stability of the TL signal of all the samples. Six of the sixteen samples analyzed showed well-behaved TL properties and were chosen for dating. An anomalous fading test on half of these samples showed that they suffered varying degrees of fading. Therefore, before TL analysis, all samples underwent a 16 day, 55^oC post-irradiation preheat treatment in an attempt to remove TL from unstable traps. TL age estimates of four of the six samples dated were significantly greater than their independently determined ages. However, two samples yielded TL ages which were concordant with expected ages (within卤 or 卤2 ). These promising results show that future research in this domain is justifiable and may eventually confirm if TL dating of glass extracts from obsidians and crystalline lavas can be reliably and accurately accomplished.

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Supervisor: Dorothy Godfrey-Smith

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