A Bayesian approach to understanding multiple mineral phase age spectra and a reassessment of the 40K decay modes

Carter, John N. (Jack) (2021) A Bayesian approach to understanding multiple mineral phase age spectra and a reassessment of the 40K decay modes. PhD thesis, University of Glasgow.

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The application of 40Ar/39Ar geochronology is extensive. Geological ages have been determine for events spanning from the Early Solar system to recent historical volcanic eruptions. The ubiquity of the parent isotope 40K means this method is also applicable across all of the terrestrial planets and their satellites. However, the technique has limitations. The aim of this thesis is to address two limitations; (1) Is it possible to obtain meaningful ages from multiple grain assemblages that contain multiple ages and phases? and (2) Does the 40K decay constant need revisiting?

To address this first question I develop a Bayesian modelling framework to fit both the age spectrum and cumulative release of 39Ar, the parameters that best describe the data (e.g., the age, diffusion kinetics, and mixture weights) are inferred. Chapter 2 sets out this modelling framework and shows that for simple mixtures with highly informative priors it is possible to infer geologically meaningful information. Despite this I make a number of limiting assumptions that are address in Chapter 3. Chapter 3 shows an extension of this method to a non-parametric framework where the number of distinct age components is also treated as an unknown. By modelling a range of mixtures I show that is it possible to get ages of geological meaning with the limited prior information available in nature. The final part of this research question involves establishing the sources of the glacial flood sediments from Moses Coulee and Ephrata fan. These sediments are linked to the catastrophic flooding of fresh water across the Columbia Basin, down the Columbia River and out-washing to the Pacific Ocean during the last ice-age. During the period from ~22000 to ~13000 years ago, there were potentially hundreds of such flooding events in the region. I use the non-parametric Bayesian model to estimate components age of both sediments to link back to known source lithologies. The determination of Mesoprotezeroic age components in both sam- ples requires that glacial Lake Missoula is a source of both of these samples. These data and model output may also suggest that the Moses Coulee formed before the Grand Coulee or that the glacial megafloods in the Pacific Northwest were larger than previous estimations.

The second concern address the 40K decay constant. In this section I create a theoretical argument based on energy conservation to illicit the inclusion of an electron capture (EC) to ground state decay mode. This decay mode has been ignored in previous studies due to a lack of experimental verification. I used the theory of β decay to estimate the strength of this decay mode and show that by the inclusion of the decay K-Ar ages (≤1 Ga) may shift by 2% to be younger than previously estimated.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Bayesian, geochronology, decay constant, provenance.
Subjects: G Geography. Anthropology. Recreation > G Geography (General)
Colleges/Schools: College of Science and Engineering > School of Geographical and Earth Sciences > Earth Sciences
Supervisor's Name: Mark, Professor Darren F. and Tremblay, Dr. Marissa M.
Date of Award: 2021
Depositing User: Mr Jack Carter
Unique ID: glathesis:2021-82351
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 13 Oct 2021 10:44
Last Modified: 26 Oct 2021 14:38
Thesis DOI: 10.5525/gla.thesis.82351
URI: http://theses.gla.ac.uk/id/eprint/82351
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