Abstract: Surface features on diamonds and water content of olivine from kimberlite as indicators of fluid systems in kimberlite magma
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Volatiles in magmas play an important role in the eruption style and the geology of volcanic landforms, determine presence of fluid phase, the depths of volatile exsolution, and the rates of magma accent. In kimberlites the original proportions of the two main volatiles, H2O and CO2, are obscured by complex origin of the groundmass minerals. Volatiles affect diamond preservation and determine the character of surface features produced on diamond faces during dissolution in kimberlites. Experiments demonstrated that oxidation of diamonds in magmas with H2Oor CO2-rich fluids and in the absence of fluid produce distinctively different types of surface features. In addition, water fugacity of kimberlitic magma can be estimated using water content in phenocrystic olivine measured by FTIR spectroscopy. We apply these two independent methods to several kimberlites in order to constrain the behavior of volatiles and their effect on diamond population and the geology of kimberlites. The study uses diamond parcels, olivine concentrates, and kimberlite core from six EKATI Mine kimberlites, Northwest Territories, Canada. These kimberlites have similar emplacement ages, erosion level, and country rocks, but different geology, composition, and diamond populations. The surface features on diamonds studied under optical and scanning electron microscopes were compared to the diamond surfaces produced experimentally in the presence and absence of fluid. Concentration and occurrence of hydroxyl in kimberlitic olivine were measured using FTIR spectroscopy. We found that Leslie and Grizzly kimberlites filled with hypabyssal facies, with low grade and quality of diamonds, show very sharp dissolution forms on diamond surfaces. Such features indicate absence of a free fluid phase during the last stages of kimberlite emplacement. Panda, Beartooth, Misery, and Koala kimberlites filled with volcaniclastic kimberlite, with higher grade and quality of diamonds have diamond surfaces with well-developed trigon pits, rounded edges with striation, and hillocks. Such features suggest emplacement in an H2O-fluidrich environment. However, diamond populations of Panda and Beartooth are dominated by octahedral unresorbed stones and IR spectra of their olivines give higher concentration of water in olivines <600 ppm and the depth of fluid separation greater than 2GPa. On the contrary, Misery and Koala diamonds are mostly rounded with high degree of resorption. Their olivines contain <450 ppm of H2O and fluid separated at more shallow depths. Group 2 OH IR absorption bands are absent in FTIR spectra of olivine from kimberlites filled with hypabyssal facies and present in olivine from all volcaniclasticfilled kimberlites. This can provide a possible link to an early loss of magmatic fluid. The excellent agreement between the two independent datasets suggests that both are linked to the activity of water in the system. We further apply these results to explain the differences between the geology of these kimberlite pipes and their diamond populations.Cite
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Kimberlites represent magmas derived from great mantle depths and are the principal source of diamonds. Kimberlites and their xenolith cargo have been extremely useful for determining the chemical composition, melting regime and evolution of the subcontinental mantle. The late-Devonian Udachnaya (means Fortuitous) pipe hosts the largest diamond deposit in Russia (> 60% diamond quantity and value) and one of the largest in the world, supplying gem-quality diamonds (~ 12% of world production). Since its discovery in 1956, the Udachnaya kimberlite pipe has become a "type locality" for geochemists and petrologists studying mantle rocks and mantle physical–chemical conditions. Apart from hosting a diverse suite of extremely well-preserved mantle xenoliths, the host kimberlite (East body) is the only known occurrence of fresh kimberlite, with secondary serpentine almost absent and uniquely high Na2O and Cl (up to 6.2 wt.%) and low H2O (< 1 wt.%) contents. The discovery of such compositional features in the only unaltered kimberlite has profound implications for models of parental kimberlite magma compositions, and the significance of the high Na and Cl abundances in the Udachnaya-East pipe has therefore been subjected to vigorous criticism. The main argument against a primary magmatic origin of high Na-Cl levels involves the possibility of contamination by salt-rich sedimentary rocks known in the subsurface of the Siberian platform, either by assimilation into the parental magma or by post-intrusion reaction with saline groundwaters. In this paper we review evidence against crustal contamination of Udachnaya-East kimberlite magma. This evidence indicates that the kimberlitic magma was not contaminated in the crust, and the serpentine-free varieties of this kimberlite owe their petrochemical and mineralogical characteristics to a lack of interaction with syn- and post-magmatic aqueous fluids. The groundmass assemblage of this kimberlite, as well as earlier-formed melt inclusions, contains alkali carbonate, chloride and other Na- and Cl-bearing minerals. This mineralogy reflects enrichment of the parental melt in carbonate, chlorine and sodium. The combination of low H2O, high alkali-Cl abundances, lack of serpentine, and the presence of alteration-free mantle xenoliths all indicate that the Udachnaya-East kimberlite preserves pristine compositions in both kimberlite and mantle xenoliths. Evidence for broadly similar chemical signatures is found in melt inclusions from kimberlites in other cratons (South Africa, Canada and Greenland in our study). We demonstrate that two supposedly "classic" characteristics of kimberlitic magmas – low sodium and high water contents – relate to postmagmatic alteration. A "salty" carbonate composition of the kimberlite parental melt can account for trace element signatures consistent with low degrees of partial melting, low temperatures of crystallisation and exceptional rheological properties that enable kimberlite magmas to rise with high ascent velocities, while carrying a large cargo of entrained xenoliths and crystals. Our empirical studies are now supported by experimental data which suggest that carbonate-chloride fluids and melts derived by liquid immiscibility are a crucial factor of diamond formation.
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ix List of Abbreviations and Symbols Used x Acknowledgements xi CHAPTER 1: INTRODUCTION 1 1.1 Volatiles and Diamonds in Kimberlites 1 1.2 Types and Origins of Olivine in Kimberlites 2 1.3 Hydrogen in Olivine 4 1.3.1 Infrared Spectra of Kimberlitic Olivine 5 1.3.2 Assignment and Interpretation of OH Bands 8 1.3.3 Modification of H Defects during Kimberlite Magmatism 11 1.4 This Study 13 1.4.1 Statement of Problem and Objectives 13 1.4.2 Scope 14 1.4.3 Claim 14 1.4.4 Agenda 15 CHAPTER 2: MATERIALS AND METHODS 17 2.1 Sample Selection and Preparation 17 2.1.1 Xenoliths and In Situ Macrocrysts 17 2.1.2 Macrocrysts from Mineral Separates 20 2.1.3 Characteristics of the Host Kimberlites 20 2.2 Electron Microprobe Analysis 21 2.3 Fourier Transform Infrared Spectroscopy 22 2.3.1 Data Collection 22 2.3.2 Processing of Infrared Spectra 23 2.3.3 Uncertainties of Integrated Absorbance Intensities 24 CHAPTER 3: RESULTS 26 3.1 Petrography 26 3.1.1 Jericho Xenoliths and Kimberlite 26 3.1.2 Matsoku Xenolith 28 3.1.3 Pipe 200 Xenolith 28
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P-T- Oxygen fugacity (fO2) conditions and fluid compositions were estimated for the formation conditions of pyrope garnet inclusions in diamonds and xenocrysts from diamond-bearing and diamond-free kimberlites using their total chemical analyses and single oxythermobarometry. Our data indicate that optimal conditions for diamond growth and preservation occur in the presumed water-rich mantle fluids containing the lowest abundance of free atomic carbon. The majority of the calculated C-H-O fluid compositions for diamond formation in peridotite xenoliths from high diamond grade kimberlites correspond to a high hydrogen and low carbon and oxygen atomic fluid percents, while those from the majority of peridotite xenoliths in the low grade diamond kimberlites corresponds to the low hydrogen, high carbon and oxygen atomic percent fluids. This new approach defines the conditions of diamond formation for kimberlitic deposits. It better characterizes diamond grades in kimberlites in comparison to the previous empirical mineralogical Ca-Cr methods and can be used as a more precise mineralogical-petrological method for prospecting for kimberlitic diamond deposits.
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