Durham E-Theses Shock metamorphism of potassic feldspars Robertson, P. B. How to cite: Robertson, P. B. (1973) Shock metamorphism of potassic feldspars, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/8594/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk SHOCK METAMORPHISM OF POTASSIC FELDSPARS A thesis submitted for the degree of Doctor of Philosophy in the University of Durham P.B. Robertson Graduate Society October, 1973 ABSTRACT Hypervelocity meteorite impact produces transient pressures as high as several megabars and temperatures in excess of 1500°C. Shock metamorphism describes the effects upon the target rocks, effects most distinctive in the range approximately 100-600kb. Shock deformation produced in potassic feldspars at three terrestrial craters and in experimentally shocked K-spar have been examined. Pressures in natural material were estimated from deformation of coexisting quartz and plagioclase, and in experiments pressures were calculated using impedance matching. At Charlevoix crater, Quebec, pressures above 170kb have induced chemical remobilization partially converting film perthite to spindle microperthite in orthoclase and microcline. In ortho- clase, shock produced cleavages form parallel to (210) and (120) at 170kb, and above 300kb parallel to (112), (031), (Oil) and (101). Planar deformation features resulting from lattice gliding, form initially at 200kb, and are well developed at 230-270kb. In orthoclase they are generally parallel to (241) and (241), and (131), (110) and (120) in microcline. Above 300kb some sets become glide twins. Structural state of the feldspars is apparently not affected in the pressure range examined (<_ 360kb). In the breccias of Lac Couture crater, Quebec, strong deformation twins and planar features develop in microcline above 150kb, and some of the twins may obey a 'diagonal' twin law. Single crystal microcline was experimentally shocked at pressures from 37 to 417kb. Three shock regions were recognized: Regime I (<100kb) normal elastic compression, Regime II (100-300kb) mixed phase region, Regime III (>300kb) high pressure phase region. Shock cleavages, (111), (111) and (hkO), developed initially below 60kb, contain glass at highest pressures. Planar features, generally parallel to (111) and (111) also, appear at 154kb becoming more pronounced with increasing pressure, with poorly defined deformation twins occurring above 175kb. X-ray patterns show that long range structural order diminishes above 150kb accompanied by formation of a low-birefringent, unidentified phase, possibly a high-pressure, disordered modification of K-feldspar. Differences in the pressure required for formation of particular shock effects between craters, and between natural and experimental systems are due to differences in structural state of the feldspars. At Brent crater, Ontario, partially recrystallized alkali feldspars in shocked basement gneisses show compositional variations due to incomplete homogenization. This is not a shock effect but results from thermal metamorphism by the overlying melt layer. A similar thermal effect has produced chemical variations in some highly shocked orthoclases at Charlevoix. TABLE OF CONTENTS Page CHAPTER I INTRODUCTION 1 1.1 Meteorite Impact Phenomena 1 1.1.1 Terrestrial Meteorite Craters 1 1.1.2 Shock Metamorphism 3 I.1.2A Shock Environments 3 I.1.2B Typical Shock Effects 4 I.1.2C Shock Effects in K-feldspars 10 1.2 Research Objectives 14 1.3 Acknowledgements 16 CHAPTER II STRUCTURAL DEFORMATION IN ORTHOCLASE AND MICROCLINE 18 II. 1 Charlevoix 18 II. 1.1 Introduction 18 II. 1.2 Chamockitic Rocks 27 11.1.3 Levels of Shock Metamorphism Preserved 36 11.1.4 Shock Metamorphism of Potassic Feldspars 42 II.1.4A Structural State 42 II.1.4B Microscopic Evidence for Deformation 70 11.2 Lac Couture Crater 101 11.2.1 Introduction 101 11.2.2 Shock Metamorphism of Microcline 104 11.3 Discussion 114 CHAPTER III THE EFFECT OF SHOCK METAMORPHISM ON THE COMPOSITIONS OF ALKALI FELDSPARS, CHIEFLY AT BRENT CRATER 127 111.1 Introduction 127 111.2 Brent Crater 129 III.2.1 Geology 129 111.3 Petrography, Composition and Genesis of the Igneous Rocks of Brent Crater 135 :•y..:. III.3.1 Composition of Feldspars Beneath the Brent Crater 139 III. 4 Discussion 156 CHAPTER IV SHOCK-LOADING EXPERIMENTS ON MICROCLINE 170 IV. 1 Introduction 170 IV.1.1 Equation of State and Recovery Shock Experiments 170 IV.1.2 Experimental Deformation of Feldspars 172 IV.1.2A Static Deformation 172 IV.1.2B Shock Deformation. 173 IV.2 Recovery Shock Experiments on Microcline 183 IV.2.1 Procedure 183 IV.2.2 Post-Shock Examination 189 TABLE OF CONTENTS (CONT'D) .IV.2. 2A General Observations 189 IV.2.2B Microscopic Observations 190 IV.2. 2C Shock Deformation Effects 199 IV.2.2D X-ray Examination 228 IV.2. 2E Electron Microprobe Analysis 238 IV.3 Discussion and Summary 242 IV.3.1 Regime - Stage Subdivisions of Shock Effects 243 IV.3.2 Comparison with Observations of Others on Shocked Feldspars 250 IV.3.3 High-Pressure Phase 251 CHAPTER V CONCLUSIONS 255 V.l Changes in Outward Physical Appearance, Visible either in Hand Specimen or Microscopically 256 V.l.l Fracturing 256 V.l.2 Planar Deformation Features 258 V.l. 3 Deformation Twinning 261 V.l.4 Colour Changes 263 V.2 Crystal Structure Changes 263 V.2.1 Disordering 264 V.2.2 Phase Transformations, Disordering and Diaplectic Glass 265 V.2.3 Densityu 267 V.3 Changes in Optical Properties 267 V.3.1 Refractive Index 268 V.3.2 Birefringence 269 V.3.3 Optic Axial Angle 269 V.4 Chemical Changes 269 V.5 Experimental vs. Natural Shock Deformation 271 V.6 Loose Ends 274 APPENDIX A STRUCTURAL PARAMETERS OF CHARLEVOIX K-FELDSPARS 277 APPENDIX B "BEST FIT" METHOD 278 APPENDIX C RECOVERY SHOCK EXPERIMENTS 283 APPENDIX D HUGONIOT EQUATION OF STATE 291 REFERENCES 308 LIST OF FIGURES Figure Page 11.1 Location of Charlevoix, Lac Couture and Brent craters 20 11.2 Topography of the Charlevoix crater 22 11.3 Geology of the Charlevoix crater 25 11.4 Sample localities, Charlevoix crater 31 11.5 Optic angle vs_. composition for potassic feldspars 51 11.6 Standard deviation of 2Va vs_. distance from crater centre 54 11.7 202o4 vs* 2^060 for potassic feldspars 59 11.8 b vs. £ for potassic feldspars 63 11.9 Per cent ordering of potassic feldspars 67 11.10 Shock-induced cleavages in potassic feldspars 80 11.11 Planar deformation features in orthoclase, Charlevoix 394 11.12 Planar deformation features in microcline, Charlevoix 98 11.13 Lac Couture crater 103 11.14 Planar deformation features in microcline, Lac Couture 111 111.1 Geology of the Brent crater 132 111.2 Cross-section of the. Brent crater 134 111.3 Composition of feldspars beneath Brent crater 153 111.4 Compositions of feldspars in melt layer, Brent 158 111.5 Compositions of feldspars, sample 94, Charlevoix 167 IV.1 Partial Hugoniot of microcline 176 IV.2 Composite feldspar Hugoniot 179 IV. 3 Shock pressure vs_. shock temperature of tectosilicates 181 IV.4 Shock experiment design 185 IV.5 Target and projectile assembly 187 IV.6 Density of shocked microcline vs_. pressure 193 IV. 7 Refractive index of shocked microcline vs_. pressure 197 IV.8 Cleavages in experimentally shocked microcline 208 IV.9 Twins in experimentally shocked microcline 214 IV. 10 Planar deformation features in experimentally shocked microcline 223 IV.11 Lattice changes in experimentally shocked microcline 227 IV. 12 Schematic Debije-Scherrer pattern of shocked microcline 235 B.l Stereograms using optic directions as axes 281 D.l Typical Hugoniot curves 294 D.2 Hugoniot typical of some geologic materials 296 D.3 Propagation of shock waves in experiments 299 D.4 Graphical method of impedance matching method 303 D.5 Shock-wave propagation and impedance matching solution in Recovery experiments 306 LIST OF TABLES Table Page 11.1 Modal analyses of charnockitic rocks of Charlevoix 33 11.2 Shock metamorphism of quartz of Charlevoix 41 11.3 Composition and 2Va of K-feldspars of Charlevoix 48 11.4 Triclinicity of K-feldspars of Charlevoix 56 11.5 Normal and anomalous K-feldspars of Charlevoix 60 11.6 Per cent order of K-feldspars of Charlevoix 68 11.7 Deformation planes in K-feldspars of Charlevoix 81 11.8 Shock metamorphism of quartz from Lac Couture 106 11.9 Planar deformation features in microcline from Lac Couture 113 111.1 K20.and Na20 (wt%) in Brent crater rocks 138 111.2 Composition of potassic feldspar beneath Brent crater 147 111.3 Composition of plagioclase in mesoperthites beneath Brent 148 111.4 Composition of plagioclase beneath Brent crater 149 111.5 Composition of feldspars in impact melt, Brent crater 163 111.6 Composition of potassic feldspar, sample MBP 94, Charlevoix 165 IV.1 Characteristics of Shock Regimes § Shock Stages for tecto- silicates 182 IV. 2 Density and. triclinicity of experimentally shocked microcline 191 IV.3 Optical properties of experimentally shocked microcline 195 IV.4 Planar deformations in experimentally shocked microcline 224 IV.5 Interplanar spacing for reflections from high-pressure phases 237 IV.