Thorium substitution in monazite: case studies and forward modelling Megan A. Williams Department of Earth Sciences School of Physical Sciences University of Adelaide This Thesis is submitted in fullfi llment of the requirements for the degree of Doctor of Philosophy December 2019 Table of Contents Abstract vii Declaration viii Publications arising from this thesis ix Acknowledgements xi Chapter 1: Signifi cance and aims of this thesis xv Chapter outlines xviii Chapter 2: Thorium distribution in the crust: outcrop and grain scale perspectives 3 Abstract 3 1 Introduction 3 2 Geological background 3 3 Sample selection 5 4 Methods 5 4.1 Whole rock geochemistry 5 4.2 In fi eld Gamma ray Spectrometry (in fi eld GRS) 5 4.3 Mineral Liberation Analysis (MLA) 6 4.4 Electron Probe Microanalysis (EPMA) 6 5 Results 7 5.1 Whole rock geochemistry 7 5.2 In fi eld GRS 7 5.4 EPMA 12 6 Discussion 15 6.1 Melt loss and the preservation of monazite 15 6.2 Bulk rock trends in thorium distribution 15 6.3 Monazite distribution at Mt Staff ord 16 6.4 Monazite forming reactions 17 6.5 Grain scale trends in thorium distribution 18 6.6 Retention of thorium in granulite facies terranes 19 7 Conclusions 20 Acknowledgements 20 Supporting Information 20 References 20 Appendix S2.1: Whole rock geochemistry for Mt Staff ord samples. 22 Chapter 3: Thorium zoning in monazite: a case study from the Ivrea Verbano Zone, NW Italy 29 i Abstract 29 1 Introduction 29 2 Geological setting 30 3 Mineral assemblages and textures in metapelites 32 4 Sample selection 32 5 Methods 33 5.1 Whole rock geochemistry 33 5.2 In fi eld gamma ray spectrometry 33 5.3 Mineral Liberation Analysis (MLA) 33 5.4 Electron Probe Microanalysis (EPMA) 33 5.5 Laser Ablation–Inductively coupled plasma–Mass Spectrometry (LA–ICP–MS) 34 6 Results 34 6.1 Bulk rock composition 34 6.2 Accessory mineral petrography and volume proportions 36 6.3 Monazite composition 37 6.4 Monazite U–Pb geochronology 42 7 Discussion 42 7.1 Whole rock Th budget 42 7.2 Monazite stability 44 7.3 Grain scale variation of Th 44 7.4 Monazite ages 47 7.5 Changes to monazite Th end member fractions with metamorphic grade 48 7.6 Mechanisms of monazite formation 49 7.7 Diff erentiation of continental crust through partial melting 50 8 Conclusions 51 Supporting information 51 References 51 Appendix S1: Detailed analytical technique for LA–ICP–MS monazite geochronology and trace element analysis 57 Appendix S3.3: Whole rock geochemistry for Ivrea–Verbano metapelite samples. 60 Chapter 4: Temperature dependence of thorium substitution mechanisms in monazite 65 Abstract 65 1 Introduction 65 1.1 Thorium substitution mechanisms in monazite 67 2 Dataset collation 67 3 Results 68 ii 4 Discussion 68 4.1 P–T dependence of Th in monazite 68 4.2 Possibile cheralite huttonite solvus? 71 4.3 Monazite composition in crustal sections 72 4.4 Dataset limitations 73 4.5 Implications of the dataset 74 4.5.1 Understanding monazite formation and chemistry 74 4.5.2 Understanding limits to monazite stability 75 4.5.3 Implications for heat production in metamorphic rocks 76 4.5.4 Development of phase equilibria models 76 5 Conclusions 77 Supporting information 77 References 77 Appendix S4.1: Summary of literature included in monazite database 81 Appendix S4.3: Alternative dataset visualisations 89 Chapter 5: Phase equilibria modelling of monazite in a Th bearing system 95 Abstract 95 1 Introduction 95 2 Methods 96 2.1 Thermodynamic model development 96 2.2 Phase equilibria modelling 100 3 Equilibrium assemblage diagrams 102 4 Discussion 105 4.1 P–T extent of monazite stability 105 4.2 Th in monazite 106 4.3 Bulk rock composition 108 4.3.1 Bulk P₂O₅ 108 4.3.2 Bulk CaO and Al2O3 110 4.3.3 Bulk LREE and Th 110 4.4 Comparison to natural data 114 4.5 Application to Mt Staff ord and the Ivrea–Verbano Zone 115 4.6 Implications for accessory mineral petrology 117 5 Conclusions 118 Supporting information 118 References 119 Appendix S5.1: Spear & Pyle (2010) (C1) and Spear (2010) (C2) diagrams 121 iii Appendix S5.2: Additional diagrams and bulk compositions 122 Appendix S5.3: Mt Staff ord and Ivrea–Verbano Zone modelling methods and diagrams 122 Chapter 6: Summary and Conclusions 135 Thesis summary and conclusions 135 Appendix A1: Additional publications by the author 143 Appendix A2: Additional data for Mt Staff ord samples 173 Appendix A3: Additional data for Ivrea–Verbano Zone samples 177 iv Abstract The accessory mineral monazite [(REE, Th, U, Y, Ca)(P, Si)O4] 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. vii Declaration I certify that this work contains no material which has been accepted for the award of any other degree or diploma in my name, in any university or other tertiary institution and, to the best of my knowledge and belief, contains no material previously published or written by another person, except where due reference has been made in the text. In addition, I certify that no part of this work will, in the future, be used in a submission in my name, for any other degree or diploma in any university or other tertiary institution without the prior approval of the University of Adelaide and where applicable, any partner institution responsible for the joint-award of this degree. The author acknowledge that copyright of published works contained within this thesis resides with the copyright holder(s) of those works. I also give permission for the digital version of my thesis to be made available on the web, via the University’s digital research repository, the Library Search and also through web search engines, unless permission has been granted by the University to restrict access for a period of time. I acknowledge the support I have received for my research through the provision of an Australian Government Research Training Program Scholarship. MEGAN WILLIAMS DATE viii Publications arising from this thesis Journal articles Williams, M. A., Kelsey, D. E., Baggs, T., Hand, M., & Alessio, K. L. (2018). Thorium distribution in the crust: Outcrop and grain-scale perspectives. Lithos, 320, 222-235. Williams, M. A., Kelsey, D. E., & Rubatto, D. (submitted and reviewed). Thorium zoning in monazite: a case study from the Ivrea–Verbano Zone, NW Italy. Journal of Metamorphic Geology. Williams, M. A., Kelsey, D. E., Hand, M., Raimondo, T., Morrissey, L. J., Tucker, N. M., & Dutch, R. A. (2018). Further evidence for two metamorphic events in the Mawson Continent. Antarctic Science, 30(1), 44-65. Alessio, K. L., Hand, M., Kelsey, D. E., Williams, M. A., Morrissey, L. J., & Barovich, K. (2018). Conservation of deep crustal heat production. Geology, 46(4), 335-338. Conference abstracts Williams, M. A., Kelsey, D. E., Hand, M., Rubatto, D., Alessio, K. L., 2018. Keeping the deep crust hot: the role of monazite. Australian Geoscience Council Convention 2018, Adelaide, Australia. Williams, M.