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Komatiites of the Weltevreden Formation, Barberton Greenstone Louisiana State University LSU Digital Commons LSU Doctoral Dissertations Graduate School 2005 Komatiites of the Weltevreden Formation, Barberton Greenstone Belt, South Africa: implications for the chemistry and temperature of the Archean mantle Keena Kareem Louisiana State University and Agricultural and Mechanical College, [email protected] Follow this and additional works at: https://digitalcommons.lsu.edu/gradschool_dissertations Part of the Earth Sciences Commons Recommended Citation Kareem, Keena, "Komatiites of the Weltevreden Formation, Barberton Greenstone Belt, South Africa: implications for the chemistry and temperature of the Archean mantle" (2005). LSU Doctoral Dissertations. 3249. https://digitalcommons.lsu.edu/gradschool_dissertations/3249 This Dissertation is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Doctoral Dissertations by an authorized graduate school editor of LSU Digital Commons. For more information, please [email protected]. KOMATIITES OF THE WELTEVREDEN FORMATION, BARBERTON GREENSTONE BELT, SOUTH AFRICA: IMPLICATIONS FOR THE CHEMISTRY AND TEMPERATURE OF THE ARCHEAN MANTLE A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy in The Department of Geology and Geophysics by Keena Kareem B.S., Tulane University, 1999 May 2005 ACKNOWLEDGMENTS The author wishes to thank, Dr. Gary Byerly, the author’s major advisor for guidance and support in defining the project, technical assistance with instrumentation, and practical advice concerning non-dissertation related issues. Appreciation goes out to the other members of the advisory committee: Drs. Huiming Bao, Lui-Heung Chan, Ray Ferrell, Darrell Henry, of the Department of Geology and Geophysics, and Dr. David Constant of the Department of Civil and Environmental Engineering at Louisiana State University. This research was supported by NSF grant to Dr. Gary Byerly, a student grant from GSA, and a Huel Perkins Fellowship. The author wishes to thank others in the Department of Geology who offered technical assistance, Dr. Xiaogang Xie in microanalysis, Wanda Leblanc in x-ray diffraction data collection and whole rock chemical determinations, Rick Young in rock sample preparation, and Tommy Blanchard in sample preparation for lithium concentration and isotopes. The author also wishes to extend appreciation to the office staff for all the assistance they provided. The author wishes to thank her family members for support during her years in graduate school, her mother, Lily Kareem, her father, Majid Kareem, and her siblings, Rashad Kareem, Najla Kareem, and Angela Ross. The author wishes to extend appreciation to a few of her friends in Baton Rouge, Juan Chow, Swati Ghoshal, Matt Hackworth, Maria Antonieta Pacheco, Melanie Thompson, and Paul White, for feedback with presentations, excursions such as coffee breaks, and enjoyable conversations. ii TABLE OF CONTENTS ACKNOWLEDGMENTS………...…………...…………………………………..…….ii ABSTRACT………….……………………………………………………………..….…v CHAPTER 1. INTRODUCTION…………………..………………………………...1 CHAPTER 2. PETROLOGY AND GEOCHEMISTRY OF KOMATIITES OF THE WELTEVREDEN FORMATION, BARBERTON GREENSTONE BELT, SOUTH AFRICA…………………..…..…..4 Introduction……………………………………………........................4 Geologic Background…………………….……………………......….5 Weltevreden Formation………………………………………..….8 Analytical Techniques……………………..…………………….....…9 Electron Microprobe……………………….……………….…..…9 Inductively Coupled Plasma Spectrometer ………..…………….10 X-ray Fluorescence and Inductively Coupled Plasma Source Mass Spectrometer……..………………………..10 Results……………………………………….………………….……11 Petrography………………………………...…………………….11 Mineral Chemistry……….………………….………...…………18 Rock Chemistry……………………………………………….…62 Melt Inclusion Chemistry………………………….…..………...64 MELTS…………………………………...……….……..........…73 Discussion………………………………………………….…….…..82 Alteration and Metamorphism……………………………..…….82 Partition Coefficients……………...…………………………..…84 Model for the Petrogenesis of the Weltevreden Formation Komatiites……………………………………………….……….90 Conclusions………………………………………….………...……104 References……………………………………………………..…....105 CHAPTER 3. ALTERATION OF KOMATIITES FROM THE WELTEVREDEN FORMATION, BARBERTON GREENSTONE BELT, SOUTH AFRICA...…………..……...…..113 Introduction………………………………………………………....113 Geologic Background……………………………………......….….115 Analytical Techniques……………………………………...…...….116 X-ray Diffraction…………………...…………………..……....116 Electron Microprobe………………………………..…….….....117 Results………………………………………………………..…..…117 Petrography…………………………………………..……....…117 X-ray Diffraction………………………………...……………..122 Mineral Chemistry………………….……………………..…....123 iii Discussion…………………………………………...………….…..136 Serpentine Chemistry………………………...………...……….136 Correlation Between Serpentine and Host Mineral and Rock ....139 Alteration History…………………………………...……….....145 Conclusions…………………………………………...……….……148 References……………………………………...…….……….….…149 CHAPTER 4. LITHIUM AND OXYGEN ISOTOPIC COMPOSITIONS OF KOMATIITES FROM THE BARBERTON GREENSTONE BELT, SOUTH AFRICA: IMPLICATIONS FOR THE COMPOSITION OF THE ARCHEAN MANTLE AND SURFACE ALTERATION PROCESSES………………..…152 Introduction……………………………………………...……………152 Sample Description…………………………...………………………154 Analytical Techniques………………………………………..………156 Lithium……………………………………………...…..………156 Oxygen………………………………………………………….157 Results……………………………………………………...…………157 Lithium Concentration……………………...………...……..….157 Lithium Isotopic Composition……………...……...….…….….160 Oxygen Isotopic Composition………….……………………....165 Discussion………………………………………………………….....165 Lithium and Oxygen in the Mantle………...……….…………..165 Behavior of Lithium and Oxygen During Serpentinization........179 Model – Recycled Crust and Viscously Coupled Hydrated Mantle…………...……………………………………………...188 Conclusions……………………………………………………...……192 References……………………………………………….....................194 CHAPTER 5. CONCLUSIONS…………………………….……………………….201 APPENDIX A: STATISTICS…………………………………………..…………....205 APPENDIX B: MELTS RESULTS AND CIPW NORM……………..…………...211 APPENDIX C: STANDARDS FOR SERPENTINE ………………………......….226 VITA……………………………………………………………...……………………227 iv ABSTRACT The komatiites of the 3.29 Ga Weltevreden Formation, Barberton Greenstone Belt, South Africa represent extremely magnesian lavas derived from the Archean mantle. Three related papers are presented on aspects of the temperature and composition of the Archean mantle, and surface alteration processes. Petrographical and geochemical analyses confirm that olivines from the komatiites retain much of their original igneous character, containing fresh olivines, pyroxenes, and chromites. The olivines from the Weltevreden Formation are extremely magnesian, with Fo contents up to 95.6. Primitive mantle normalized Gd/Yb values near 1, and Al2O3/TiO3 values of 26-33 for these rocks are consistent with derivation from deep primitive mantle, leaving a melt residue of only olivine. Models based on the geochemistry of these rocks indicate eruption temperatures of at least 1660˚C, higher than previously reported for komatiitic eruption temperatures. X-ray diffraction and electron microprobe analyses show that the dominant alteration mineral in these rocks is serpentine. The dominant serpentine mineral present, lizardite, shows a wide range of compositional variation. Observed textures and geochemical variation of the alteration products indicate these rocks were altered at a lower metamorphic grade than previously reported for the Barberton Greenstone Belt. The δ7Li and δ18O values of the fresh olivine separates from these rocks range from 0.5 to 10.0‰ and 3.8 to 4.2‰, respectively. The variation of δ7Li values and the low δ18O values of olivine separates indicates the incorporation of a subduction component in the mantle source region, which suggests the Archean mantle was v heterogeneous. Temperatures of serpentinization ranging from 142 to 310˚C were determined using the oxygen isotopic compositions of the whole rocks assuming fluids with δ18O values ranging from -1 to 2‰. The estimated temperatures and the petrographical and geochemical data of these rocks indicate alteration at temperatures lower than previously estimated for the Barberton Greenstone Belt. vi CHAPTER 1. INTRODUCTION The 3.55-3.22 Ga Barberton Greenstone Belt, South Africa, represents one of the best preserved greenstone terrains available for studies of the early history of the Earth. It represents a natural laboratory for examination of the early Earth including evolution of life, ancient meteorite impacts, and volcanism. Unique rocks that erupted predominantly during the Archean, the komatiites are important because they are a significant component of the Archean crust. Komatiites are ultramafic volcanic rocks, usually characterized by their spinifex textures. The Barberton Greenstone Belt contains some of the best exposures of komatiites on Earth, therefore analyses of these rocks and their fresh primary igneous minerals may aid in constraining the thermal and compositional nature of the mantle source region. For years, the origin and occurrence of komatiites have been the subject of debate. Researchers have disagreed on the nature and composition of the mantle source for komatiites, origin of spinifex texture, possible dissolved volatiles in komatiitic liquids, and eruption temperatures and tectonic setting for komatiitic lavas. The objectives of
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