FLUID INCLUSIONS IN FIBROUS AND OCTAHEDRALLY-GROWN DIAMONDS by Evan Mathew Smith A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in The Faculty of Graduate and Postdoctoral Studies (Geological Sciences) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) March 2014 © Evan Mathew Smith, 2014 Abstract My thesis puts forth new models for diamond formation that explain the difference between octahedral and fibrous diamond growth, as well as the difference between octahedral diamond growth in the lithospheric and the sublithospheric mantle. Diamond growth in the mantle involves reactions between carbon-bearing fluid and the host rocks it infiltrates. This fluid is sometimes included in diamond. Fluids in dendritically-grown, fibrous diamonds from Wawa, Superior craton, were analysed in a novel way, using transmission X-ray diffraction. The technique allows bulk analysis of daughter minerals within fluid inclusions. The mineralogy, major and trace elements, Sr isotopes, volatiles, and nitrogen characteristics of the hydrous saline–high-Mg carbonatitic fluid in these Archean diamonds strongly resemble those of Phanerozoic fibrous diamonds. This implies that some mantle processes, including the formation of fibrous diamonds, can be extended unvaryingly back to 2.7 Ga. Fluid equilibrated with octahedrally-grown diamonds from the Siberian, Kaapvaal, and Congo cratons is trapped in healed fractures in the diamonds. They contain anhydrous CO2–N2 fluid inclusions with 40±4 mol% N2 and inclusions of former silicate melt that had an original N2 content of ~0.1 wt%, as shown by Raman, electron microprobe, and microthermometry analyses. The liberation of N2 from the convecting mantle is proposed to be controlled by increasing oxygen fugacity that destabilizes host phases. The observed distinct fluid compositions between hydrous fluids in fibrous and anhydrous fluids in octahedrally-grown diamond entail distinct processes of diamond formation that, ultimately, govern the growth habit. Water may trigger fibrous growth by inhibiting the expansion of {111} layers and lowering the interfacial energy between the diamond and fluid. Certain features in diamond fluids, such as Eu anomalies and potential carbonate–CO2 isotopic fractionation, show that several mantle processes can produce geochemical signatures that may be mistaken as input from subducted materials. The finding of N2 in diamond-forming fluids leads to an explanation for the characteristically low N content of sublithospheric diamonds. I propose this compositional trait is due to growth in a metal-saturated environment. Metallic Fe in the mantle below ~250 km should trap N and may be the largest mantle N reservoir. ii Preface The research program described herein began with the initial plan to analyse fibrous diamonds with X-ray diffraction, based on the preliminary work of my research supervisor, Dr. Kopylova. The subsequent directions taken in research were identified and designed principally by myself, with the help of Dr. Kopylova. I performed all parts of the primary research and led the analysis of data. Research results were written into 4 manuscripts, 2 of which are published, and the other 2 have been submitted. These 4 manuscripts form the chapters of this thesis. The published chapters are reproduced with permission of the copyright holders. Co-authorship details are summarized below along with related conference abstracts where this content was also presented. Chapter 2 is published as: Smith, E.M., Kopylova, M.G., Dubrovinsky, L., Navon, O., Ryder, J., and Tomlinson, E.L., 2011. Transmission X-ray diffraction as a new tool for diamond fluid inclusion studies. Mineralogical Magazine, 75(5): 2657-2675. Overall, I was responsible for 90% of the research and 80% of the writing. Kopylova provided the samples, the initial plan for this investigation and helped to identify implications of the work and also helped with the organization and editing of the written manuscript. Dubrovinsky helped oversee the majority of the analyses and verified the data interpretation. Navon and Tomlinson provided diamond samples and comments on the written manuscript. Ryder provided diamond samples. This work was also presented at 3 conferences: • Smith, E., Kopylova, M., Dubrovinsky, L., and Tomlinson, E., 2010. X-ray diffraction study of the mineral and fluid inclusions in fibrous diamond. Yellowknife Geoscience Forum 2010. (poster presentation) • Smith, E., Kopylova, M., and Dubrovinsky, L., 2010. X-ray diffraction study of the mineralogy of microinclusions in fibrous diamond. GAC MAC GeoCanada 2010. (oral presentation) • Smith, E., Kopylova, M., and Dubrovinsky, L., 2010. X-ray diffraction study of the mineralogy of microinclusions in fibrous diamond. Geophysical Research Abstracts. 12: EGU 2010-4741-1 (European Geosciences Union – oral presentation) iii Chapter 3 is published as: Smith, E.M., Kopylova, M.G., Nowell, G.M., Pearson, D.G. and Ryder, J., 2012. Archean mantle fluids preserved in fibrous diamonds from Wawa, Superior craton. Geology, 40(12): 1071-1074. Overall, I was responsible for 60% of the research and 80% of the writing. Kopylova helped with sample acquisition, data interpretation, as well as manuscript editing and restructuring. Nowell oversaw the extensive procedures for data collection. Pearson provided help with data quality and interpretation. Ryder provided diamond samples. This work was also presented at 3 conferences: • Smith, E.M. and Kopylova, M.G., 2013. A fresh look at Eu anomalies: The effect of Cl- rich fluids. GAC MAC Winnipeg 2013. Abstract No. 142 (oral presentation) • Smith, E.M. et al., 2012. The contrast in trace element chemistry and volatile composition between fluid inclusions in fibrous and octahedral diamonds. 10th International Kimberlite Conference, Extended Abstract No. 10IKC-102. (poster presentation) • Smith, E.M., Kopylova, M.G. and Ryder, J., 2011. Fluid inclusions in Archean diamonds from Wawa, Ontario. GAC MAC Ottawa 2011, Abstract No. 95. (oral presentation) Chapter 4 is published as: Smith, E.M., Kopylova, M.G., Frezzotti, M.L. and Afanasiev, V.P., 2014. N-rich fluid inclusions in octahedrally-grown diamond. Earth and Planetary Science Letters, 393(0): 39-48. Overall, I was responsible for 70% of the research and 80% of the writing. Kopylova helped with sample acquisition and the organization and editing of the manuscript. Frezzotti oversaw the data collection and interpretation. Afanasiev provided diamond samples. This work was also presented at 3 conferences: • Smith, E.M., Kopylova, M.G., Frezzotti, M.L. and Afanasiev, V.P., 2013. Diamond inclusions reveal fugitive mantle nitrogen. Goldschmidt 2013, Abstract ID 1601. (oral presentation - invited speaker) • Smith, E.M., Kopylova, M.G., Frezzotti, M.L. and Afanasiev, V.P., 2013. “Vapour” vs. melt inclusions in Siberian placer diamonds. GEM (Geoscience for Energy and Minerals) Geological Survey of Canada, Diamond Project Workshop, Vancouver. (oral presentation) iv • Smith, E.M., Kopylova, M.G., Frezzotti, M.L. and Afanasiev, V.P., 2013. Nitrogen bubbles in the mantle: Evidence from diamond inclusions. GAC MAC Winnipeg 2013. Abstract No. 141 (oral presentation) Chapter 5 has been accepted (March 2014) for publication in the Canadian Journal of Earth Sciences as: Smith, E.M., and Kopylova, M.G., Implications of metallic iron for diamonds and nitrogen in the sublithospheric mantle. Overall, I was responsible for 90% of the research and 70% of the writing. Kopylova provided significant help with the structure and presentation of the text and figures, as well as help with editing. v Table of contents Abstract ........................................................................................................................................... ii Preface............................................................................................................................................ iii Table of contents ............................................................................................................................ vi List of tables ................................................................................................................................... ix List of figures .................................................................................................................................. x Acknowledgments......................................................................................................................... xii 1 Introduction ............................................................................................................................. 1 1.1 The importance of diamond research ............................................................................... 1 1.2 What is known about diamond and related fluids ............................................................ 2 1.2.1 Metasomatic fluid as an agent of diamond growth ................................................... 2 1.2.2 Diamond growth habits ............................................................................................. 3 1.2.3 Mantle host rocks for diamond ................................................................................. 7 1.2.4 Fluid inclusions in fibrous diamond ......................................................................... 9 1.2.5 Fluid inclusions in octahedrally-grown diamond ................................................... 11 1.2.6 Temporal evolution of diamond-forming