Jul 9, 2009 - from group I kimberlites worldwide. However, there are certain textural ...... of 2.7-2.5 Ga (Petters, 1991; Wilson et al., 1995a; Kusky, 1998).
UNIVERSITY OF BRISTOL
Trace element and isotope geochemistry of perovskites from kimberlites of Southern Africa
Chiranjeeb Sarkar
A dissertation submitted to the University of Bristol in accordance with the requirements of the degree of Doctor of Philosophy in the Faculty of Science Department of Earth Sciences
October 2011
Supervisors
Dr. Craig D. Storey Prof. Chris J. Hawkesworth Prof. R. Stephen J. Sparks
Word count: ca. 45,000
Abstract
In-situ trace elements along with Sr, Nd and stable O isotope data have been employed to assess the effects of contamination on perovskites from Orapa and Wesselton kimberlite and finally to determine pre-shallow level contamination kimberlite magma signature. Perovskites from both kimberlites are in general similar to other perovskites reported from group I kimberlites worldwide. However, there are certain textural features in Orapa perovskites which show that they crystallised until a late stage. Trace element contents in late crystallising perovskites are similar to those in earlier ones except for Nb and Zr. Crustal contamination had a minimal role as the samples have an extended range in La/Yb and Sr/Yb rather than scatter around low values. Moreover, the lack of any perovskites with elevated δ 18 O also suggests minimal interaction with upper crust. Relatively large variation in
87 Sr/86 Sr
ratio in Orapa perovskites is probably not due
to crustal assimilation into magma, rather it has been interpreted as a result of some secondary calcite present in fractures of some perovskite grains. The δ 18 O values from Orapa perovskites show bimodal distribution with two distinct peaks (around +3.6hand -0.6h). Wesselton perovskites are clustered around δ 18 O values of +4h. One group of Orapa perovskites and the Wesselton perovskites are interpreted to reflect the δ 18 O of uncontaminated upper mantle derived kimberlite magma. The negative δ 18 O values from the second group of Orapa perovskites are attributed to crystallisation of perovskite after magma degassing, rather than to crustal assimilation, cooling or hydrothermal alteration. The Wesselton sills, however, did not experience significant degassing, at least to the extent to deplete the magma in
18 O.
Trace element and radiogenic isotope
(Sm-Nd and Rb-Sr) characteristics of the calculated kimberlite magma support their origin by a low degree of melting from a metasomatised SCLM source, which was already depleted.
To my wife Poushali, For being a patient endurer, for being a candle flame in dark times . . . for being there
v
Acknowledgements This thesis would not have been possible without the guidance and help of several individuals who in one way or another contributed and extended their valuable assistance in the preparation and completion of this study. First and foremost, my utmost gratitude to the supervisors Craig Storey and Chris Hawkesworth who guided me throughout this study with their expertise and knowledge whilst allowing me the room to work in my own way. My special thanks to Steve Sparks who not only took over the official supervisory duties on Chris and Craig’s departure but also provided a lot of encouragements. My review committee, Horst Marschall and Michael Walter helped me with their insightful comments to keep me organised and focused during the study period. In the lab many members of Bristol Isotope Group (BIG) helped with my understandings about analytical techniques and geochemical methods: Bruno Dhuime, Matthias Willbold, James Darling, Bob Steele and James Rae. Chris Coath, with his unique quality of explaining complex things in a simple way made mass spectrometry a fun to learn. Penelope Lancaster, Horst and Craig trained me in sample preparation and in-situ techniques of LA-ICP-MS. Derek Vance and Ann Osborne kindly helped me with Sm-Nd isotope chemistry and analysis in the ICP-MS. Sr isotope work would not have been possible without the invaluable help of Riccardo Avanzinelli and his support with the measurement. Stuart Kearns is thanked for his help with electron microprobe techniques. I am grateful to John Craven and Cees-Jan De Hoog who made available their support in a number of ways during the period of ion probe analyses in Edinburgh. Fieldwork was benefited greatly from the financial support from De Beers and the logistical support of Debswana Group Exploration and MRM, Botswana. Extensive supports from Peter Kesebonye, Fresh Thebe Tlhaodi and Benjamin Buse are strongly acknowledged. I am also grateful to Matthew Field for his help in the project, especially during early part of designing the original project. Beyond kimberlites and mass spectrometry (which sometimes seemed to be nothing more than a distant dream) I am thankful to my friends and colleagues for the stimulating discussions and all the fun we have had in the last three and half years. Academicals Cricket Club deserves a special mention as it made the British summer a well worth to wait for. Last but not the least, I would like to thank my parents, sister, family and wife, Poushali to be so encouraging, cheerful and supportive especially during hard times. My stay in Bristol was primarily funded by an Overseas Research Scholarship and University of Bristol Postgraduate Research Scholarship along with financial support from De Beers. Ion probe analyses at the University of Edinburgh was supported by NERC IMF grant 361/1008 to Craig Storey. vii
Author’s Declaration I declare that the work in this dissertation was carried out in accordance with the requirements of the University’s Regulations and Code of Practice for Research Degree Programmes and that it has not been submitted for any other academic award. Except where indicated by specific reference in the text, the work is the candidate’s own work. Work done in collaboration with, or with the assistance of, others, is indicated as such. Any views expressed in the dissertation are those of the author.
Signed:
Chiranjeeb Sarkar
Date: .............6th October, 2011...................
ix
Contents Abstract
iii
Acknowledgements
vii
Author’s Declaration
ix
List of Figures
xv
List of Tables
xix
Abbreviations
xxiii
1 Introduction 1.1 Background of the study . . . . . . . . . . . . . . . 1.2 Goals and objectives of this study . . . . . . . . . 1.3 Outline of the Chapters and author’s contribution 1.4 Regional geology of the study area . . . . . . . . . 1.4.1 Cratons . . . . . . . . . . . . . . . . . . . . 1.4.2 Mobile belts . . . . . . . . . . . . . . . . . . 1.5 Local geological setting . . . . . . . . . . . . . . . 1.5.1 Orapa A/K1 kimberlite . . . . . . . . . . . 1.5.1.1 Orapa Northern pipe . . . . . . . 1.5.1.2 Orapa South pipe . . . . . . . . . 1.5.1.3 Xenoliths and diamond inclusions 1.5.2 Wesselton kimberlite . . . . . . . . . . . . .
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2 Kimberlites: A review 2.1 Introduction . . . . . . . . . . . . . . 2.2 Definition of Kimberlites . . . . . . . 2.3 World wide occurrence of kimberlites 2.4 Kimberlite lithofacies . . . . . . . . . 2.5 Types of kimberlite deposits . . . . . 2.5.1 Extrusive kimberlite lava . . 2.5.2 Pyroclastic kimberlite . . . . 2.5.3 Epiclastic kimberlite . . . . .
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Contents 2.5.4 Massive volcaniclastic kimberlite . . . . . . . . . . . . . . . 2.5.5 Coherent kimberlite . . . . . . . . . . . . . . . . . . . . . . 2.6 Mineralogy of kimberlites . . . . . . . . . . . . . . . . . . . . . . . 2.6.1 Mantle xenoliths . . . . . . . . . . . . . . . . . . . . . . . . 2.6.2 Country rock fragments . . . . . . . . . . . . . . . . . . . . 2.6.3 Megacrysts and macrocrysts . . . . . . . . . . . . . . . . . . 2.6.4 Groundmass . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 Group I and Group II kimberlite . . . . . . . . . . . . . . . . . . . 2.7.1 Major differences between Group I and Group II kimberlite 2.8 Geochemistry and petrogenesis of kimberlites . . . . . . . . . . . . 2.9 Model of emplacement of kimberlite pipes . . . . . . . . . . . . . . 2.10 Kimberlite formation model . . . . . . . . . . . . . . . . . . . . . . 2.10.1 Magmatic models . . . . . . . . . . . . . . . . . . . . . . . . 2.10.2 Phreatomagmatic model . . . . . . . . . . . . . . . . . . . . 2.11 Erosion and alteration of kimberlite pipe . . . . . . . . . . . . . . .
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36 36 36 37 37 37 38 38 39 41 42 42 43 45 46
3 Methodology 3.1 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Imaging and textural analyses . . . . . . . . . . . . . . . . . . . . . . . 3.3 Major, minor and trace element analyses . . . . . . . . . . . . . . . . . 3.4 Trace element and REE analyses . . . . . . . . . . . . . . . . . . . . . 3.4.1 By LA-ICP-MS . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2 By SIMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Sm-Nd isotope analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Solution method . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1.1 Reagents used . . . . . . . . . . . . . . . . . . . . . . 3.5.1.2 Perovskite digestion . . . . . . . . . . . . . . . . . . . 3.5.1.3 Column chemistry . . . . . . . . . . . . . . . . . . . . 3.5.1.4 Mass Spectrometer instrumentation . . . . . . . . . . 3.5.1.5 Mass fractionation and interference correction . . . . 3.5.1.6 Calculation of Nd concentrations of the samples using spike . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2 Laser ablation method . . . . . . . . . . . . . . . . . . . . . . . 3.5.2.1 Instrumentation . . . . . . . . . . . . . . . . . . . . . 3.5.2.2 Interference and mass bias correction . . . . . . . . . 3.5.2.3 Standard summary . . . . . . . . . . . . . . . . . . . 3.6 Sr isotope analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.1 Column chemistry . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.2 Mass spectrometer instrumentation . . . . . . . . . . . . . . . . 3.6.3 Isotope fractionation during measurement . . . . . . . . . . . . 3.6.4 Static and dynamic mode measurement . . . . . . . . . . . . . 3.7 Stable Oxygen isotope analysis . . . . . . . . . . . . . . . . . . . . . . 3.7.1 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . 3.7.2 Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.3 Standard summary . . . . . . . . . . . . . . . . . . . . . . . . .
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49 51 52 52 54 54 56 58 58 58 59 60 62 65
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70 70 70 73 74 76 76 77 78 79 81 81 81 82
4 Characterisation of mineral standards
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85
Contents 4.1 4.2 4.3
4.4
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Analytical techniques . . . . . . . . . . . . . . . . . . . . . Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 BM 1909-381 . . . . . . . . . . . . . . . . . . . . . 4.3.1.1 Major and trace elements [BM 1909-381] 4.3.1.2 Nd isotope ratios [BM 1909-381] . . . . . 4.3.1.3 O isotope ratios [BM 1909-381] . . . . . . 4.3.2 BM 1963-732 . . . . . . . . . . . . . . . . . . . . . 4.3.2.1 Major and trace elements [BM 1963-732] 4.3.2.2 Nd isotopes [BM 1963-732] . . . . . . . . 4.3.2.3 O isotopes [BM 1963-732] . . . . . . . . . 4.3.3 BM 1978-402 . . . . . . . . . . . . . . . . . . . . . 4.3.3.1 Major and trace elements [BM 1978-402] 4.3.3.2 Nd isotopes [BM 1978-402] . . . . . . . . 4.3.3.3 O analysis [BM 1978-402] . . . . . . . . . 4.3.4 Ice River perovskite . . . . . . . . . . . . . . . . . 4.3.4.1 Major and trace elements [Ice River] . . . 4.3.4.2 Nd isotopes [Ice River] . . . . . . . . . . 4.3.5 O isotopes [Ice River] . . . . . . . . . . . . . . . . Evaluation of perovskite standards . . . . . . . . . . . . .
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85 87 91 91 91 92 92 93 94 95 97 97 97 98 99 104 105 109 109 110
5 Trace element and isotope studies of perovskite: An assessment of contamination in kimberlite to determine the primary mantle signature 115 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 5.2 Geology of the study area and samples . . . . . . . . . . . . . . . . . . . . 117 5.2.1 Orapa kimberlite . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 5.2.2 Wesselton kimberlite . . . . . . . . . . . . . . . . . . . . . . . . . . 118 5.3 Analytical procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 5.3.1 In-situ trace element and isotope analyses . . . . . . . . . . . . . . 120 5.3.2 Conventional solution isotope analyses . . . . . . . . . . . . . . . . 121 5.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 5.4.1 Petrography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 5.4.2 Major and trace element composition . . . . . . . . . . . . . . . . 128 5.4.3 Isotopic composition . . . . . . . . . . . . . . . . . . . . . . . . . . 133 5.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 5.5.1 Perovskite paragenesis . . . . . . . . . . . . . . . . . . . . . . . . . 135 5.5.2 Lithospheric and crustal contamination and alteration . . . . . . . 139 5.5.3 Laser vs TIMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 5.5.4 Pre-shallow level contamination magma characteristics and petrogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 5.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 6 Degassing in kimberlite: Oxygen isotope ratios in perovskite from plosive and hypabyssal kimberlites 6.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Background and samples . . . . . . . . . . . . . . . . . . . . . . . . . .
ex165 . . 165 . . 166 . . 168
Contents
6.4
6.5
6.6
6.7
xiv
6.3.1 Orapa kimberlite . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 Wesselton kimberlite . . . . . . . . . . . . . . . . . . . . . . . . . Analytical procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Major and minor element analysis . . . . . . . . . . . . . . . . . 6.4.2 Stable oxygen and trace element analysis . . . . . . . . . . . . . 6.4.3 Sr isotope analysis . . . . . . . . . . . . . . . . . . . . . . . . . . Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Petrography of perovskite from Orapa and Wesselton kimberlites 6.5.2 Sr and O isotope ratios from perovskite . . . . . . . . . . . . . . Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.1 Robustness of perovskite . . . . . . . . . . . . . . . . . . . . . . . 6.6.2 δ 18 O of the uncontaminated kimberlite magma in equilibrium with perovskite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.3 Petrography and textures of the high and low δ 18 O perovskite . 6.6.4 The cause of the variation of O isotope ratios in perovskite . . . 6.6.4.1 Assimilation and water-rock interaction . . . . . . . . . 6.6.4.2 Effect of fractional crystallisation . . . . . . . . . . . . 6.6.4.3 Effect of degassing . . . . . . . . . . . . . . . . . . . . . 6.6.4.4 Equilibrium vs kinetic fractionation . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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7 Conclusions and future research
A Appendix A: Data tables A.1 EPMA data . . . . . . . . . . . . . . . A.2 Trace element data from LA-ICP-MS . A.3 Sm-Nd isotope data from LA-ICP-MS A.4 Sr isotope data from TIMS . . . . . . A.5 O isotope data from SIMS . . . . . . .
Bibliography
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List of Figures 1.1
1.2 1.3 1.4 1.5 1.6
1.7
2.1
2.2
2.3 3.1
3.2 3.3 3.4 4.1
Initial 87 Sr/86 Sr ratio for selected Indian kimberlites from in-situ analysis for A) bulk rock analysis and B) in-situ perovskite analysis, after Paton et al. (2007a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Map of South Africa showing major Cratons. The black dots represent kimberlite intrusions. Taken from Jelsma et al. (2009) . . . . . . . . . . Tectonic map of Kalahari Craton in southern Africa . . . . . . . . . . . Geological settings of Orapa A/K1 and other clusters of kimberlite in the adjacent area in Botswana. Taken from Gernon et al. (2009b) . . . . . . Geology of Orapa A/K1 kimberlite pipe showing different lithounits from North and South pipe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sketch map of the Kimberley cluster of kimberlites, showing location of the six major kimberlite pipes including Wesselton, after Shee (1985); Le Roex et al. (2003). . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross section through Wesselton Kimberlite Pipe. Distribution of some of the different phases of kimberlite have been shown between 435 and 930 m mining levels (depths below surface). Modified from Mitchell et al. (2009) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
4
. 10 . 12 . 13 . 16
. 20
. 22
Global distribution of diamond bearing kimberlites. Major cratons of Archean and Proterozoic ages are also shown along with the craton boundary, after Kjarsgaard (2007) . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Emplacement ages of kimberlite hosted diamond mines (past producers and active mines) and significant deposits from he world. Only phanerozoic kimberlites have been plotted here, after Heaman et al. (2003); Kjarsgaard (2007) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Model of typical kimberlite pipe. Modified from Hawthorne (1975) . . . . 34 Comparison of ion-microprobe (SIMS) and electron microprobe (EMP) analysis of Nb content in perovskite from Orapa kimberlites and secondary standard material (Ice River perovskite). . . . . . . . . . . . . . . . . . . Record of 143 Nd/144 Nd isotope ratio from La Jolla Nd standard solution Record of 143 Nd/144 Nd isotope ratio from NIST 610 standard glass, SP HUL and SP REN standard titanites . . . . . . . . . . . . . . . . . . . . Record of variation of δ 18 O within BM 1909-381 perovskite. . . . . . . .
. 57 . 69 . 75 . 82
Comparison of Nb concentration in Ice River perovskite, measured by EPMA and LA-ICPMS. Concentrations are in ppm. The error bar represents the maximum 1sigma for both techniques. . . . . . . . . . . . . . . 89
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List of Figures 4.2
4.3 4.4
4.5 4.6 4.7
4.8
4.9 4.10
4.11 4.12 4.13
4.14 4.15 4.16
5.1 5.2 5.3 5.4 5.5
xvi
Comparison of Sr concentration in Ice River perovskite, measured by EPMA and LA-ICPMS. Concentrations are in ppm. The error bar represents the maximum 1sigma for both techniques. . . . . . . . . . . . . . . 89 Back scattered electron image (BSE) of BM 1909-381 . . . . . . . . . . . 91 Variation of the concentrations of Nd, Sr and Sm in BM 1909-381 perovskite along four different profiles shown in 4.3. Analytical uncertainties are
