Open Research Online The Open University’s repository of research publications and other research outputs Volatiles in the Moon: A sulfur and chlorine perspective Thesis How to cite: Faircloth, Samantha Jane (2020). Volatiles in the Moon: A sulfur and chlorine perspective. PhD thesis The Open University. For guidance on citations see FAQs. c 2020 The Author https://creativecommons.org/licenses/by-nc-nd/4.0/ Version: Version of Record Link(s) to article on publisher’s website: http://dx.doi.org/doi:10.21954/ou.ro.00011603 Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright owners. For more information on Open Research Online’s data policy on reuse of materials please consult the policies page. oro.open.ac.uk Volatiles in the Moon: A sulfur and chlorine perspective Samantha Jane Faircloth MSc, BSc (Hons), BA (Hons) This thesis was submitted to The Open University for the degree of Doctor of Philosophy School of Physical Sciences February 2020 ii Abstract Sulfur is a key volatile element in magmatic systems that exists in many phases (e.g. melt or gas), in multiple-oxidation states (S2-, S4+ and S6+), and has more than one stable isotope (e.g. 32S and 34S). Therefore, by measuring S, information regarding the conditions of a magma can be acquired. The aim of this work is to investigate what S can tell us about the behaviour of late-stage lunar basaltic magmas. An analytical protocol was developed to simultaneously measure S and Cl abundances and isotopes of lunar apatite in eleven lunar basalts with nano-scale secondary ion mass spectrometry (NanoSIMS). Additionally, a method was developed to measure the oxidation state of S in apatites of five mare basalts with X-ray absorption near-edge structure (XANES) spectroscopy at the S K-edge, making it possible to compare S oxidation state and S isotopes of lunar apatite for the first time. Lunar apatites contain ~20–2,800 ppm S, with δ34S values between -33.3 ± 3.8‰ and +36.4 ± 3.2‰ (2σ). The Cl abundance is ~350–7,230 ppm, with δ37Cl values of +6.5‰ ± 6+ 0.9‰ to +36.5‰ ± 1.1‰ (2σ). All of the apatites have S /ΣStot ratios of >0, with average 6+ S /ΣStot values between 0.05 and 0.55. An absence of correlation between S and Cl isotopes suggests a lack of evolutionary relationship between S and KREEP-rich components. The direction of S isotope fractionation, negative or positive, can be explained by degassing of H2S and SO2 from a 2- 2- relatively reduced (S ) or oxidized (SO4 ) late-stage silicate melt, respectively. The historical existence of relatively oxidized late-stage silicate melts is also evidenced by the 6+ 6+ 34 presence of S in lunar apatite. A positive trend is apparent between S /ΣStot and δ S which is indicative of the dependence of S isotope fractionation on the oxygen fugacity of the late-stage silicate melt. iii Acknowledgements I would very much like to thank my three supervisors, Mahesh Anand, Ian Franchi (The Open University) and Sara Russell (The Natural History Museum) for their time, guidance and for all of the opportunities that they have made available to me throughout my PhD. The ideas and proposals that I suggested throughout the research project (the majority of which were not planned at the beginning of the PhD) have been welcomed and encouraged (obviously after much scrutiny!). My supervisors ensured that there were no obstacles to the research that I wanted to do and had every faith in me, for which I am very grateful. The most staggering commitment came from Mahesh, Ian and Richard C. Greenwood who took it in turns to travel to and work with me at Diamond Light Source (DLS), staying conscious until the small hours (4am was the record) to ensure that we made the most of the beamtime. A special thanks go to Richard who made an additional trip to collect some extra samples at the last hour. I would also like thank Tina Geraki (DLS) for her continual support during and long after the experiments at beamline I18. A big thanks goes to Xuchao Zhao for endless hours of technical training and support with the nano-scale secondary ion mass spectrometer (NanoSIMS) at The Open University. Patience and cheerfulness are definitely good traits to have when using the NanoSIMS. I thank Pete Landsberg for all of the little bits and pieces and tinkering that he has done in the labs for me over the course of my project. Thank you also to Diane Johnson (previously of The Open University) for comprehensive scanning electron microscopy (SEM) training at the outset of my project. Finally, I thank all of the people around me who have supported me and put up with me for the last three years and five months. I’m sure it cannot have been easy! Statement of original authorship All of the primary research presented in this PhD thesis is original and has not been submitted for a previous degree course or for publication in a peer-reviewed scientific journal. iv Table of contents Abstract iii Acknowledgements iv Statement of original authorship iv Table of contents v List of figures x List of tables xiii List of acronyms xiv List of mineral abbreviations xv Chapter One: Introduction 1 1.1. Moon formation 2 1.2. Sample return missions 4 1.3. Geological history of the Moon and lunar rock types 6 1.4. Lunar volatiles 8 1.4.1. Quantity and delivery of lunar volatiles 9 1.4.2. Sulfur behaviour during Moon formation and evolution 11 1.4.3. Chondritic delivery of sulfur 13 1.5. Previous laboratory analyses of lunar volatiles 14 1.5.1. Chlorine in lunar samples 14 1.5.2. Hydrogen in lunar samples 19 1.6. Sulfur in magmas 22 1.6.1. Sulfur abundance of lunar samples 22 1.6.2. Sulfur isotopes of lunar samples 24 1.6.3. Oxidation state of sulfur 28 1.7. Research justification 30 1.7.1. Research aim 31 1.7.2. Research objectives 31 1.8. Personal contribution 32 Chapter Two: Analytical methods 33 2.1. Optical microscopy 34 2.2. Secondary Electron Microscopy 34 2.2.1. Fundamental principles 34 v 2.2.1.1. Backscattered electrons 35 2.2.1.2. Energy dispersive X-ray spectroscopy (EDS) 36 2.2.1.3. Secondary electrons 37 2.2.1.4. Cathodoluminescence 38 2.2.2. Analytical methods and sample preparation for SEM-CL 41 2.3. Electron probe microanalysis (EPMA) 46 2.3.1. Fundamental principles 46 2.3.2. Analytical methods and sample preparation for EPMA 47 2.4. Nano-scale Secondary Ion Mass Spectrometry (NanoSIMS) 50 2.4.1. Fundamental principles 50 2.4.2. Analytical method 57 2.4.2.1. Quantification of standards used for NanoSIMS 57 analyses 2.4.2.2. Sample and standard preparation 58 2.4.3. Analytical techniques 59 2.4.3.1. Chlorine protocol 62 2.4.3.2. Sulfur protocol 63 2.4.3.3. Image processing 64 2.4.4. Data Reduction 66 2.4.4.1. Calibrating for abundance and assessment of 66 background 2.4.4.2. S and Cl abundance measurements and 70 calculations 2.4.4.3. Measurement and calculation of the unknown S 71 and Cl isotope values 2.4.5. Reproducibility: abundance and isotope measurements 73 2.4.5.1. Sulfur 73 2.4.5.2. Chlorine 75 Chapter Three: The Apollo samples 79 3.1. Sample significance 80 3.2. Sample selection 80 3.3. Sample descriptions and mineralogy 84 3.3.1. Ilmenite basalt: 10044,645 84 vi 3.3.2. Pigeonite Basalt: 12031,7 87 3.3.3. Pigeonite Basalt: 12039,45 90 3.3.4. High-Al Basalt: 14053,19 94 3.3.5. Olivine basalt: 15016,7 96 3.3.6. Pigeonite basalt: 15058,15 100 3.3.7. Gabbroic basalt: 15065,85 104 3.3.8. Pigeonite Basalt: 15085,15 109 3.3.9. KREEP basalt: 15386,46 111 3.3.10. Porphyritic Pigeonite Basalt: 15475,17 113 3.3.11. Olivine normative basalt: 15545,7 116 3.3.12. Olivine normative basalt: 15555,991 118 3.3.13. Vesicular olivine normative: 15556,137 123 3.3.14. Feldspathic granulitic impactite: 79215,50 126 Chapter Four: Sulfur and chlorine in lunar apatite 131 4.1. The abundance and isotopic composition of sulfur and chlorine 132 in lunar apatite 4.1.1. Sulfur results 136 4.1.2. Chlorine results 139 4.2. A comparison of sulfur and chlorine systematics in lunar 143 apatite 4.2.1. Chlorine isotopes and abundances compared with the 143 existing literature 4.2.2. Sulfur and chlorine in lunar apatite: a comparison 148 4.3. Sulfur in lunar apatite 154 4.3.1. A comparison of sulfur abundance in lunar apatite 154 4.3.2. A comparison of sulfur isotope values 156 4.4. Secondary processes affecting indigenous S in lunar apatite 161 4.4.1. Terrestrial contamination 161 4.4.2. Vaporization by micrometeorite impacts 162 4.4.3. Solar wind and cosmic ray spallation 163 4.4.4. Post-crystallisation alteration of S in apatite 163 4.5. Sulfur in the basaltic source region(s) 164 4.6. The behaviour of sulfur in late-stage lunar magmas 166 vii 4.6.1. Sulfide separation: immiscible FeS 166 4.6.2. Sulfur degassing and isotope fractionation from silicate 167 melts 4.6.3. Sulfur in lunar apatite: late-stage melts and gaseous S 169 species 4.6.4.