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Trace Element Analysis of Kimberlites and Associated

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Trace Element Analysis of Kimberlites and Associated TRACE ELEMENT ANALYSIS OF KIMBERLITES AND ASSOCIATED ROCKS AND XENOLITHS by NICHOLAS WILLIAM ROGERS B.Sc.(Hons.), M.Sc. A THESIS SUBMITTED IN FULFILMENT OF THE CONDITIONS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN THE UNIVERSITY OF LONDON University of London Reactor Centre Silwood Park, Sunninghill Ascot Berkshire SL5 7PY JANUARY 1980 OUT YONDER THERE WAS THIS HUGE WORLD, WHICH EXISTS INDEPENDENTLY OF US HUMAN BEINGS AND WHICH STANDS BEFORE US LIKE A GREAT, ETERNAL RIDDLE, AT LEAST PARTIALLY ACCESSIBLE TO OUR INSPECTION. ALBERT EINSTEIN. 1 A B S TRAC T The techniques of instrumental thermal and epithermal neutron activation analysis and radiochemical activation analysis as previously used at the University of London Reactor Centre, have been refined and applied to the determination of selected trace elements in samples from kimberlites and similar localities. This thesis describes the techniques as used in geochemical analysis and illustrates their precision and accuracy with determinations of nine rare earth elements (La,Ce,Nd,Sm, Eu,Tb,Ho,Yb and Lu) and six other trace elements (Th,U,Cs,Sc,Co and Cr) in the U.S.G.S. standard rocks BCR.l, AGV.1 and G.2 and in the I.A.E.A. inter-laboratory comparison sample, SL.l (Lake sediment). Subsequent analysis of kimberlites and similar volcanic rocks (alnoites and lamprophyres) revealed that, although their trace element abundances showed superficial similarities, those in the kimberlites were systematically different, suggesting differences in kimberlite petrogenesis. The alnoites and lamprophyres were related to a garnet lherzolite source that showed light rare earth element enrichment, ten- tatively ascribed to a metasomatic event. In order to compare the trace element composition of the model mantle with that of possible real mantle material, a suite of fertile and sterile garnet lherzolite and harzburgite xenoliths from South African kimberlites were analysed for 13 trace elements. The fertile mantle samples were shown to possess flat rare earth element patterns, with abundances ranging from ti 1-3x chondrite. No evidence for meta- somaticd activity was discerned. The analysis of the sterile samples, however, revealed a strong enrichment of the light rare earth elements. This is the opposite of the light rare earth element depletion expected, if the two groups of xenoliths are related by a simple partial melting episode. Trace element abundances were subsequently interpreted in relation to a two component mantle model. This model postulates initial depletion of the mantle in incompatible elements by partial fusion, accompanied, or followed, by metasomatic enrichment of the residue by a fluid with kimberlitic trace element abundances, prior to inclusion of the mantle fragments in the host kimberlite. Evidence for the higher level effects of any metasomatic activity was sought by the analysis of a suite of lower crustal garnet granulite xenoliths. Once again, different varieties were recognised from whole rock trace element abundances. In this case, however, the abundances were best explained using a conventional model of igneous crystal fraction- ation. It was concluded that metasomatic alteration by incompatible element rich fluids is an important process in the sub-continental mantle and plays a significant role in the petrogenesis of some continental magmas. Kimberlites are interpreted as an infrequent surface expression of this widespread mantle phenomenon. ii ACKNOWLEDGEMENTS I should like to express my gratitude to Dr. G.D. Borley and Mr. M. Kerridge for their joint supervision of this project. My thanks are due to the following people who have generously donated valuable samples for analysis:- Drs. S.W. Bachinski, D.A. Carswell, W.L. Griffin and P.H. Nixon. I am also indebted to Mr. A. Gray (University of Leeds) for his careful X.R.F. analyses of kimberlites and ultrabasic xenoliths. This work has benefitted greatly from discussions with Dr. P.H. Nixon. Finally, I should like to thank the staff and students (both past and present) of the University of London Reactor Centre, Silwood Park, for their friendship and hospitality over the past three years. They have tolerated the presence of a geologist in their midst with rare good humour! My thanks, particularly, to Joyce Vine for her typing. Computer programs for partial melting and least squares mixing models were kindly donated by Drs. I.L. Gibson and R.C.O. Gill respect- ively. 111 Financial support was received in the form of a research studentship from the Science Research Council for two years and a grant for one year from the U.L.R.C. Discretionary Fund. My thanks, again, to Mr. M. Kerridge for this extra funding. iv CONTENTS Page CHAPTER 1 INTRODUCTION 1.1 The Use of Trace Elements in Geochemistry 1 1.2 The Geochemistry of the Rare Earth Elements 7 1.3 Rare Earth Element Analysis 12 1.4 The Application of Trace Element Analysis to Kimberlite 14 Geology 1.5 Bibliography 17 CHAPTER 2 NEUTRON ACTIVATION ANALYSIS 2.1 Theory 19 2.2 Modes of Radioactive Decay of Use in NAA 22 2.3 The Detection of Gamma Rays 24 2.4 Gamma Ray Spectrometry 30 2.5 The Analysis of Gamma Ray Spectra 33 2.6 SPEC5 - An Interactive Graphics Computer Program for the 36 Rapid Reduction of y-Ray Spectra 2.7 Analytical Technique 45 2.8 Sources of Error and their Evaluation 47 2.9 Radiochemical Neutron Activation Analysis 57 2.10 The Use of Epithermal Neutrons in Activation Analysis 62 2.11 Summary 68 V Page CHAPTER 3 TRACE ELEMENT ABUNDANCES OF KIMBERLITES AND SIMILAR VOLCANIC ROCKS 3.1 Introduction 70 3.2 Results 76 3.3 Alnoites 82 3.4 Lamprophyres 97 3.5 Kimberlites 105 3.6 The Relationship between Kimberlite and other Volcanic 118 Rocks 3.7 Conclusions 124 CHAPTER 4 TRACE ELEMENT ABUNDANCES OF ULTRABASIC XENOLITHS 4.1 Introduction 125 4.2 Sample Descriptions 130 4.3 Results 130 4.4 Comparison with Analyses of other Ultrabasic Rocks 132 4.5 Petrogenesis of the Lherzolite and Harzburgite Xenoliths 143 4.6 Conclusions 165 CHAPTER 5 TRACE ELEMENT ABUNDANCES OF GRANULITE XENOLITHS 5.1 Introduction 167 5.2 Whole Rock Analyses: Results and Discussion 171 5.3 Separate Mineral Analyses: Results and Discussion 178 5.4 The Origin of the Granulite Xenoliths and their Protoliths 194 5.5 Discussion 209 Vi Page CHAPTER 6 GENERAL DISCUSSION AND CONCLUSIONS 212 APPENDICES 2.1 Program SPECS 233 3.1 Trace Element Analyses of Alnoites, Lamprophyres and 246 Kimberlites 4.1 Major and Trace Element Analyses of the Ultrabasic 252 Xenoliths 5.1 Trace Element Analyses of Whole Rock Granulite and 254 Pyroxenite Samples 5.2 Trace Element Analyses of separated minerals 257 REFERENCES 259 vii DIAGRAMS Page 1.1 The Variation of Kd with Rare Earth Element Atomic Number 11 2.1 A Typical y-Ray Spectrum 26 2.2 A Cut-away Diagram of a Ge(Li) Detector 29 2.3 A Block Diagram of an Analyser System 31 2.4 The Trapezoid or Covell-type Background Correction Procedure 38 2.5 Continuum or Background Fitting Correction Procedure 41 2.6 An Example of the Covell-type Correction from SPEC5 43 2.7 An Example of the Background Fitting Procedure from SPEC5 44 3.1 REE Analyses of Six Kimberlites 77 3.2 REE Analyses of Four Kimberlites 78 3.3 Representative Lamprophyre REE analyses 79 3.4 REE Analyses of the Alnoites 80 3.5 Graph of Sm vs La/Yb for the Analysed Rocks 83 3.6 Graphs of La vs Mg0, A1203, Zr and CaO for the Alnoites 85 3.7 a 88 & Results of the Differential Dissolution Experiments 3.7 b 89 3.8 Comparison of Petrogenetic Models for the Alnoites and 93 S. Australian Volcanics 3.9 Comparison of Lamprophyre Analyses with partial melting models 100 viii Page 3.10 Comparison of Lamprophyre REE Abundances with Three Partial 101 Melting Models 3.11 Graph of Sm vs Co for the Arizona Lamprophyres 104 3.12 Sm vs La/Yb Plot for Kimberlites 107 3.13 Graph of La vs Th for Kimberlites 113 3.14 Sm vs La/Yb Plot for Carbonatites and Ultra-Potassic Rocks 120 3.15 Graph of La vs Th for Potassic Basalts, Alnoites and 122 Lamprophyres 4.1 REE Abundances of Four Relatively Fertile Xenoliths 135 4.2 REE in Sterile Xenoliths from N. Lesotho 136 4.3 REE in Sterile Xenoliths from the Kimberley Area and 137 Monastery Mine 4.4 REE in a Mica Wehrlite and an Ilmenite Orthopyroxenite 138 4.5 Graph of (La/Lu)N vs Lu 142 4.6a Graph of Ca0 vs Lu 144 4.6b Graph of Ca0 vs Sc 145 4.7 Xenolith Partial Melting Models for Ni, Sc and Lu 148 4.8 Partial Melting Model for Ca0 vs Lu Variation 152 4.9 REE Partial Melting Models Based on PHN2838 155 4.10a Graph of Lu vs Na20 157 4.10b Graph of Lu vs TiO2 158 4.11a Graph of Th vs U 160 4.Ilb Graph of La vs Th 160 ix Page 5.1 REE Abundances of Group 1 Granulite Xenoliths 172 5.2 REE Abundances of Group 2 Granulite Xenoliths 173 5.3 REE Abundances of a Felsic Granulite Xenolith 173 5.4 REE Abundances of Garnet Pyroxenite Xenoliths 174 5.5 Graph of Sm vs Zr 176 5.6 Graph of Sm vs Eu 177 5.7 REE Abundances of Clinopyroxenes from Granulite Xenoliths 180 5.8 REE Abundances of Garnets from Granulite Xenoliths 181 5.9 a REE Abundances of Garnets and Clinopyroxenes from Garnet 182 Pyroxenite Xenoliths 5.9 b y 5.10 197 - Partial Melting Models Based on REE Abundances a,b & c 199 5.11 Fractional Crystallisation Model 1 204 5.12 Fractional Crystallisation Model 2 206 6.1 Zone Melting Model 1 220 6.2 Zone Melting Model 2 221 6.3 Sm vs La/i'b Plot of Zone Melting Models 223 6.4 Theoretical Dependence of Element Enrichment on 226 Distribution Coefficient and Distance Travelled by Melted Zone x TABLES Page 2.1 Relevant Data on the Nuclides used for Activation 46 Analysis in this Work 2.2 Concentrations of Elements in the Multi-Element 50 Standard Solutions 2.3 Ratio Correction Factors for Photo-peak Interferences 53 2.4a BCR.1 Analyses 55 2.4b Analyses of I.A.E.A.
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