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Composition and Structure of Titanian Andradite from Magmatic And Composition and structure of titanian andradite from magmatic and hydrothermal environments by Elizabeth Hilton B.Sc. (Hons), Saint Mary's University, 1998. A. THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Geological Sciences Division, Department of Earth and Ocean Sciences) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA May, 2000. O Elizabeth Hilton, 2000. In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of (fariV* Oceav\ Sciences, The University of British Columbia Vancouver, Canada Date Kft^ JOOO. Abstract Titanian andradite provides a wealth of information about the environments in which they form. Zippa Mountain pluton and the Crowsnest volcanic rocks provide examples of titanian andradite formed indifferent environments (e.g., magmatic, skarn, volcanic, and hydrothermal). Electron-microprobe, petrographic, and geochemical analysis, coupled with X-ray techniques were used to determine the composition, structure, site occupancies, and to discriminate between titanian andradite formed in different environments. Site occupancies, determined from the study samples, are as follows: Ca2+ and Na+ are always assigned to the X site; Mn2+ is preferentially assigned to the X site, but may also be at the Y site; Al3+, Mg2+, Cr3+, V3+, Ti4 +, and Fe2f are always assigned to the Y site; Fe3+ may be assigned into the Y site or the Z site; and Si4+, Zr4^, and H4+ are always assigned to the Z site. The titanium substitution mechanism may be 3+ via the TiMgFe2. exchange component, indicative of octahedral Ti substitution and octahedrally controlled cell volume. Chemical zoning of magmatic and skarn titanian andradite is irregular. Garnet from volcanic samples have irregularly zoned cores and regularly zoned rims as does the magmatic cumulus sample. Hydrothermal samples show regular chemical zonation. EPMA data reveals titanian andradite zoning patterns within different rock types and different formation environments. Dark zones in the volcanic garnet contain more Ti02 and A1203 and less Fe203 than lighter zones. Melasyenite samples show positive correlation between Ti02 and Fe203. Conversely, pyroxenite samples show regular zoning with darker zones having more A1203 and less Fe203 than lighter areas. In these Ill samples, Ti02 and Fe203 are negatively correlated. In terms of Thompson components, magmatic samples have small norms, negative MgCa-, very small H4Si- component, and positive FeMg-, whereas hydrothermal samples have positive H4Si- components, zero to slightly positive FeMg-, and large norms. On average, skarn samples have equal amounts 3+ of both TiSi- and TiMgFe2_ components and are therefore intermediate between magmatic and hydrothermal samples. Oxygen fugacify and activity of silica are 3f correlated by TiMgFe2. and TiSi- components and indicate that the magmatic samples formed under low fQ2 conditions, skarn samples inherited \hefQ2 signature by interaction with early magmatic fluids, whereas the hydrothermal sample crystallised from a more evolved fluid which had a higher^ and asi02. Table of Contents Abstract List of Figures vi List of Tables viii Acknowledgments ix Chapter 1: Introduction 1 1.1 General Introduction 1 1.2 Purpose 2 Chapter 2: Literature Review of Titanium Andradite 4 2.1 Overview of Titanian Andradite 4 2.2 Anisotropy 5 2.3 Zoning in Titanian Andradite..... 6 Chapter 3: Sample Suite and Petrography 10 3.1 Geological Setting and Sample Suite 10 3.1.1 Geological Setting..., 10 3.1.2 Sample Suite 12 3.1.3 Whole Rock Geochemistry and Geochemical Methodsl2 3.2 Petrography.... 14 3.2.1 Sample Description 14 3.2.2 Petrography 16 3.2.3 Scanning Electron Microscopy 20 Chapter 4: Chemical Characteristics and Representation of Ti-Andradite....56 4.1 Chemical Mineralogy 56 4.1.1 Electron Microprobe Analyses 56 4.1.2 Compositional Zoning 56 4.2 Representation of Analyses 64 4.3 Summary : 71 Chapter 5: Other Chemical Techniques Used to Describe Ti-Andradite 101 5.1 Introduction 101 5.2 Wet-chemistry 101 5.3 FTIR and Estimates of OH 101 5.3.1 Literature Review of OH Site Occupancy 101 5.3.2 FTIR Methods : 103 Chapter 6: X-ray Diffraction Analysis 106 6.1 Introduction to Diffractometry 106 6.2 The Andradite Unit Cell by Powder Diffraction 107 6.3 Single Crystal Diffractometry 109 6.4 Summary 111 Chapter 7: An Analysis of Site Occupancy in Ti-Andradite 116 7.1 Recapitulation of Andradite Crystals 116 7.2 Literature Review of Site Occupancy in Ti-Andradite. 116 7.3 Results and Ideas from This Study 118 Chapter 8: General Conclusions and Petrogenesis 126 References 130 Appendix A EPMA Error Analysis 137 Appendix B Cation Normalisation Routine: Algorithm and Matlab Code... 163 Appendix C CD Rom of Complete EPMA Tables back cover pocket vi List of Figures Figure 1 Zippa location map (BC) 3 Figure 2 Zippa sample location map 24 Figure 3 Cliff sample location map 25 Figure 4 Glacier sample location map 26 Figure 5 Bartnick sample location map 27 Figure 6 Geochemical data plotted as alkali-silica, AFM diagrams 30 Figure 6 (cont'd) Trace element variation diagrams 31 Figure 7 Polished thin section pictures 32 Figure 8A SEM images of zoning of Zippa magmatic samples 43 Figure 8B SEM images of zoning of cumulus and Crowsnest samples 46 Figure 8C SEM images of zoning of dyke samples 49 Figure 8D SEM images of zoning of skarn samples 51 Figure 9 X-ray maps for ZM39B-B 55 Figures 10-24 Line traverses plotted 76 Figure 25 SEM images showing line traverses on grains 91 Figure 26 TCS component plots 96 Figure 27 TCS compilation plots 99 Figure 28 TCS compilation plots for norm of the vector 100 Figure 29 Comparison of duplicate FeO analyses 105 Figure 30 Z95-1 2 theta correction factors 113 Figure 31 Measured cell volume plots 115 Vll Figure 32 Portion of the garnet structure projected down the c-axis 122 Figure Al EPMA analyses for the non-calibration standard plotted over time.. 140 Figure A2 Grid of analyses on the non-calibration standard 141 Figure A3 Analytical uncertainty plotted against the IS variation for EPMA analyses for the non-calibration standard 142 Figure A4 EPMA analyses of Andr point 1 from the non-calibration standard, over time 143 Figure A5 EPMA analyses of Andr point 2 from the non-calibration standard, over time 144 Figure A6 EPMA analyses of Andr point 3 from the non-calibration standard, over time 145 Figure A7 EPMA analyses of Andr point 4 from the non-calibration standard, over time 146 Figure A8 EPMA analyses of Andr point 5 from the non-calibration standard, over time 147 Vlll List of Tables Table 1 Sample suite from Zippa Mountain and related assemblages .23 Table 2 Geochemical data with norms 28 Table 3 Petrography : 37 Table 4 Representative microprobe analyses 72 Table 5 Zoning patterns summary 94 Table 6 Wet-chemical analyses of FeO in select garnet samples 104 Table 7 Calculated unit cells for selected andradite crystals from this study. 114 Table 8 J Widely accepted cation site occupancies 123 Table 9 Cation site assignments and totals '. 124 Table A1 EPMA analyses for calibration standards over time 148 Table A2 Calculated wt% for the andradite non-calibration standard 153 Table A3 EPMA analyses for the andradite non-calibration standard over time •. 154 Table A4 EPMA analyses for the andradite non-calibration standard grid 155 Table A5 EPMA analyses for the andradite non-calibration standard, organised by point, over time 156 Table A6 Calculated mean and standard deviation for the andradite non-calibration standard 162 IX Acknowledgments The author wishes to thank Dr. Kelly Russell, Dr. Greg Dipple, and Dr. Lee Groat for guidance, supervision, and improvements of this work. Special thanks to Dr. Mati Raudsepp for guidance and helpful suggestions for instrumentation. Dr. Ian Coulson is thanked for reviewing the thesis and for many helpful suggestions. Most of all I wish to thank my husband, my sisters, and parents for their support and encouragement. 1 Introduction 1.1 Introduction Titanian andradite forms in both magmatic and hydrothermal environments and is most commonly associated with silica-undersaturated rocks (Howie and Woolley 1968). Titanian andradite forms an important link between magmatic and hydrothermal environments. Variations in its composition can be used to track variables such as fQ2 and aSi02, making it an essential phase for studying the chemical dynamics of magmatic and hydrothermal systems and also for establishing records of fluid evolution between the two systems (Russell et al. 1999). Occurrences of purely magmatic titanian andradite from western Canada include the Crowsnest volcanics, Alberta (Dingwell and Brearley 1985) and the Zippa Mountain pluton, British Columbia (e.g. Lueck and Russell 1994) (Figure 1). Andradite from both these localities is examined in this thesis. The Crowsnest volcanics contain examples of titanian andradite formed in extrusive alkaline igneous rocks (Dingwell and Brearley 1985). The garnet occurs as phenocrysts along with aegirine-augite, sanidine, analcite, and rare plagioclase in trachyte and phonolite flows, agglomerates and tuffs (Dingwell and Brearley 1985). The garnet is chemically zoned with Ti and Fe contents decreasing from core to rim (Dingwell and Brearley 1985). The Zippa Mountain pluton is located in the Iskut River map area, in northern British Columbia about 10 km southwest of the confluence of the Iskut and Craig Rivers.
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