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Type: Thesis
Title: Thorium substitution in monazite: case studies and forward modelling
Author: Williams, Megan Alice
Issue Date: 2019
School/Discipline: School of Physical Sciences
Abstract: The accessory mineral monazite [(REE, Th, U, Y, Ca)(P, Si)O] is the major host of the heat producing element Th in the high temperature (>500 C) continental crust, hosted predominantly in peraluminous rock types. It is also an important geochronometer for high temperature crustal processes. As monazite forms, it often preserves multiple chemical and isotopic zones which can be used to infer the timing and conditions of formation. These zones can be preserved through multiple cycles of metamorphism and partial melting. While some aspects of monazite chemistry (e.g. LREE and Y) are well understood, studies which have focussed on Th in particular are few. This has resulted in a lack of clarity on the partitioning of Th into monazite with progressive metamorphism as well as a limited understanding of the solid-solution behaviour of the two Th-bearing endmembers of monazite, cheralite and huttonite. To expand the utility of this mineral, this thesis fi rst presents two detailed and comprehensive case studies of chemical zoning in monazite from compositionally homogeneous suites of progressively metamorphosed metasediments, Mt Staff ord, central Australia and the Ivrea–Verbano Zone, Italy. These studies also present the chemistry of associated minerals, mo dal abundance of accessory minerals, bulk rock chemistry and mineralogy. These case studies have a particular focus on Th, and compare trends observed in monazite from progressively metamorphosed terranes to bulk rock Th and mineralogy trends. These studies show that monazite in granulite-facies and UHT rocks is not depleted in Th with respect to amphibolite-facies monazite. In all samples, cheralite is the dominant Th-endmember of monazite. Monazite modal proportion is also observed to increase with metamorphic grade in both terranes. The case studies are then integrated with a global dataset of over 5000 monazite chemical analyses spaning a wide range of pressure and temperature conditions. This analysis shows that Th in monazite shows systematic behaviour with temperature with limited eff ect from pressure and that the trends observed in the case studies can be considered universal. This new understanding of Th partitioning in monazite is used to build and calibrate a predictive and readily adaptable thermodynamic framework for modelling the chemistry and abundance of monazite and associated minerals. This framework is tested on representative pelite compositions to explore the bulk compositional and pressure–temperature controls on monazite stability and composition. Closed- and open-system melting scenarios are also explored. Finally, the thermodynamic framework is used to calculate models for one sample from each case study to provide the proof-of-concept that these models adequately predict the complexity of monazite compositions in natural systems and to provide new insights into the formation of this mineral.
Advisor: Kelsey, David
Glorie, Stijn
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 2020
Keywords: Accessory mineral
monazite chemistry
electron probe microanalysis
phase equilibria
Provenance: This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at:
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