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Geochemical Characteristics of Carbonatite-Related Volcanism and Sub-Volcanic Metasomatism at Oldoinyo Lengai, Tanzania

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Geochemical Characteristics of Carbonatite-Related Volcanism and Sub-Volcanic Metasomatism at Oldoinyo Lengai, Tanzania Geochemical characteristics of carbonatite-related volcanism and sub-volcanic metasomatism at Oldoinyo Lengai, Tanzania Laura Carmody Department of Earth Sciences A thesis submitted to University College London for the degree of Doctor of Philosophy 2012 1 I, Laura Carmody, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. Laura Carmody Date 2 As a change I have included two opposing quotes for my thesis; the first is a statement by which many scientists live by known as Occam’s razor. The second is a quote from one of my favourite authors H.G Wells from his science fiction work The Time Machine. “Simpler explanations are, other things being equal, generally better than more complex ones” Paraphrased from William of Ockham “Very simple was my explanation, and plausible enough—as most wrong theories are!” H.G. Wells, The Time Machine 3 Abstract The eruption of xenolithic material during large explosive eruptions, at any volcano, supply vital samples of the sub-surface lithologies upon which it is built, which in turn provides an indication of the evolution of the volcanic complex, in particular the volcanic conduit, magma storage zones and crustal / mantle lithologies. This is particularly important at alkaline- carbonatite complexes which are known to have “exotic” chemistries and also cause extensive zones of alteration through fenitisation processes. As the only active carbonatite volcano on Earth and also the unusual nature of Oldoinyo Lengai, Tanzania, it is an excellent study site to better understand the generation and chemical influence of carbonate-rich melts and fluids from source to surface. This study has attempted to better constrain the sub-volcanic environment, the source of the carbon within the material and the processes which lead to the formation of such unique rocks. Using geochemistry, isotopic studies and fluid inclusions, this thesis highlights the importance of fluid circulation within the volcanic system, both at the surface but also within the sub- volcanic mantle, leading to metasomatised material rich in carbon and alkali elements from which natrocarbonatite and potentially kimberlitic material could be derived. Almost all of the geochemical evidence and composition of fluid inclusions trapped within fenitised aureoles indicates a mantle derivation of carbonatitic material with isotopic signatures typical of the pre-defined “mantle-box”. The nature of the fluids is also investigated using trace element modelling and argued to be both carbonatitic and siliceous in origin, which have been circulating within the mantle beneath the Gregory Rift since before the establishment of Oldoinyo Lengai. These themes of research are discussed in terms of the genesis of natrocarbonatite, focussing upon the notion that it may be an evolutionary feature of Oldoinyo Lengai rather than a constant eruptive product. 4 Acknowledgments I would first like to thank my supervisors Dr Adrian Jones and Dr Christopher Kilburn for their continual guidance throughout my time as their student. Both challenged me to achieve the best that I could out of my PhD and allowed me the freedom to work truly independently, devising my own research projects and collaborations. Dr Jones is also thanked for providing me with a number of opportunities during my time here at UCL, which have helped enhance my skill set. Thanks must also be given to Dr Matthew Genge (Imperial College) for his encouragement in following this PhD in the first place. This research project would not have been possible without the financial support of NERC, the Graduate School at UCL and The Geochemistry Group. Thank you to Dr Andrew Steele and Dr Dina Bower (Geophysical Laboratory, Carnegie Institute) for their extensive help with micro-Raman analysis. Thanks are also given to all of Andrew’s post docs for making me feel welcome during my visits to the GL. Dr Ian Millar and the staff at British Geological Survey, Keyworth, for taking the time to help me with data collection and interpretation. Prof Peter J Treloar and Dr David Lawrence at Kingston for their time for fluid inclusion work at the very last minute. Dr Stanislav Strekopytov at NHM for helping with the geochemistry data and David Smith (curator of the NHM) for lending me thin sections for almost all of my PhD. Dr Sasha Verchovsky (OU) and Alexei Buikin (Verdansky Institute) are also thanked. Dr Dorinda Ostermann (Bloomsbury Environmental Isotope Facility) and Dr Stuart Robinson (UCL) for their help with stable isotope work from such difficult material to work with and for the patience to try new methods of preparation when the first set of results were strange. Jim Davy (UCL) for his fantastic guidance in rock preparation, masterful skill of the temperamental SEM and putting up with me covering his lab in oil and methylated spirit. Evans Munanga and William Masaai for their help organising the logistical nightmare that is field work at Oldoinyo Lengai, Tanzania. William’s expertise proved vital during field work. The staff at Moivaro Lodge, Natron must also be thanked for their hospitality and friendly nature during our visit. The help of Abigail Church and all the staff at James Robertson Safari’s for field work in Kenya and continual advice and support regarding field work and sample collection 5 and honey badger incidents. Thanks is also given to Jörg Keller (Uni Freiburg) for initial carbonatite samples. Dr Sami Mikhail is thanked for being the first person who spoke to me when I first arrived at UCL and for every conversation since, which have always proven to be useful in one form or another. His passion for science is infectious and he has been the sounding block for a number of theories within this thesis. I would like to acknowledge all of the academics I have spoken with during workshops and conferences for taking the time to discuss ideas which has always proved beneficial. All of my PhD colleagues (present and past) – I wish I could thank you all individually. Collectively you have all helped me through the good times and the bad, offering advice, a break from writing and shoulders to cry on. A particular thanks to Lottie Davis for an amazing first set of field work in Kenya which we survived together. Also to Rick Wall for helping me keep my sanity via endless random conversations over coffee. For those of you who are still going, there is light at the end of the tunnel and for those who have already submitted, I finally see what you are all smiling about. My house mates, Lucy Woodbine and Elinor Savage, also require a massive thank you for putting up with my ups and downs during this time in my life and the endless amount of field gear which fills the flat from time to time during field work seasons. My friends outside of my academic life must also be acknowledged for their support – I know my choice to study rocks seemed odd to most of you. To my boyfriend Barry, we met in the most unlikely of circumstances and you have gotten to know me at the most stressful time of my life and yours, but have been a rock to me thus far. Thank you for keeping me sane and putting up with me when I am grouchy. I promise to do the same when you write up your thesis. My final and biggest thank you goes to my parents, Lynn and Noel, and also to my sister Gemma. This thesis is for you. You have always supported me in whatever I decide to do without question and only the smallest amount of taunting. You have kept me focussed on the task at hand, supported me during the rough times and teary phone calls, helping to make my ambitions come true. In particular my mum who has made every attempt to learn all of the technical stuff, my dad for all the early morning airport runs and Gem for all the phone calls when things were not working out. I hope I have made you all proud. 6 Table of Contents Abstract 4 Acknowledgements 5 List of Figures 10 List of Tables 13 Chapter 1 –An introduction to terrestrial carbon reservoirs and the carbonatites of 14 the East African Rift and Oldoinyo Lengai 1.1. Carbon-rich igneous melts: Carbonatites 14 1.1.1. Subdivisions of Carbonatites 14 1.1.2. Tectonic Setting of Carbonatites 15 1.1.3. Temporal Distribution of Carbonatite 16 1.1.4. Genesis of Carbonatites 18 1.1.5. What is meant by fenitisation and metasomatism? 19 1.2. The carbonatites of Oldoinyo Lengai, Tanzania 21 1.2.1. Geological context of the Gregory Rift and Oldoinyo Lengai 21 1.2.2. Stratigraphy of Oldoinyo Lengai 22 1.2.3. Geometry of Oldoinyo Lengai from 1960 to 2007 23 1.2.4. Eruption Style – effusive and explosive 24 1.2.5. Debris Deposits on Oldoinyo Lengai 26 1.2.6. Mineralogy of Natrocarbonatite from Oldoinyo Lengai 29 1.2.7. Xenolith suites from Oldoinyo Lengai 32 1.3. Uniqueness of Oldoinyo Lengai 35 1.4. Formation of natrocarbonatite lava at Oldoinyo Lengai 35 1.5. Aims of this thesis 39 Chapter 2: Methodology for all Analyses 41 2.1. Sample preparation for thin sections 41 2.2. Scanning Electron Microscopy and Electron Microprobe 43 2.3. Whole rock elemental analysis 45 2.4. Radio and stable isotope analysis 47 2.5. Micro Raman Spectroscopy 51 2.6. Microthermometry analysis of fluid inclusions 55 2.7. Permeability measurement 58 Chapter 3: New Field Observations of Oldoinyo Lengai (post eruption) and 61 surrounding rift region 3.1 New observations of Oldoinyo Lengai 61 3.1.1 Crater Geometry and Morphology 61 3.1.2 Estimation of solid carbonatite flux 68 3.1.3 Observations of 2006 lava flow on Western Flank (morphology and 71 features) 3.1.4 Interpretation of 2006 lava flow 74 3.2 Field observations of hummocks 76 3.2.1 Volcanic activity during collapse 81 3.3 Field observations of Kerimasi units 82 3.3.1 Interpretation of Lapilli Unit 85 3.4 Field observations of explosion craters and tuff cones 86 3.4.1 Loolmurwak explosion crater 86 7 3.4.2 Tuff cones (Ubehebe type) – Loluni, Pello Hill and Deeti.
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