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Petrography and Mineral Chemistry of Ultramafic and Related Inclusions from the Orapa A/K-1 Kimberlite Pipe, Botswana by Richard P

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Petrography and Mineral Chemistry of Ultramafic and Related Inclusions from the Orapa A/K-1 Kimberlite Pipe, Botswana by Richard P PETROGRAPHY AND MINERAL CHEMISTRY OF ULTRAMAFIC AND RELATED INCLUSIONS FROM THE ORAPA A/K-1 KIMBERLITE PIPE, BOTSWANA BY RICHARD P. TOLLO '-- Lesotho (/) ~ Q,) Q,) -E 0 0 I{) 500 meters generalized cross-section of a kimberlite pipe modified after Hawthorne ( 197 5) CONTRIBUTION NO. 39 DEPARTMENT OF GEOLOGY AND GEOGRAPHY UNIVERSITY OF MASSACHUSETIS AMHERST, MASSACHUSETIS PETROGRAPHY AND MINERAL CHEMISTRY OF ULTRAMAFIC AND RELATED INCLUSIONS FROM THE ORAPA A/K-1 KIMBERLITE PIPE, BOTSWANA by RICHARD PAUL TOLLO Contribution No. 39 Department of GeoJogy and Geography University of Massachusetts Amherst, Massachusetts August, 1982 paAJaSaE S4q~lE TTV OtTOl tnad p~aqOJE TABLE OF CONTENTS ACKNOWLEDGEMENT xi ABSTRACT . xiii Chapter I. INTRODUCTION 1 The Orapa A/K-1 Kimberlite Pipe 1 Determinative Methods 3 II. OXIDE DISCRETE NODULES .. 6 Ilmenite . • • • • • . • • • 6 Rutile and Ilmenite-Rutile Intergrowths 33 III. SILICATE DISCRETE NODULES 65 Clinopyroxene • • • • 65 Ilmenite-Clinopyroxene Intergrowth 79 Garnet . 86 Knorringite Garnet-Clinopyroxene-Chromite Intergrowth • • • • • . • • . • . • • . • . 101 IV. ECLOGITE NODULES • 112 Introduction • • 112 Petrography 112 Phase Chemistry 121 Geothermometry and Geobarometry 150 V. DISCUSSION •.. 158 VI. SUMMARY A}ID CONCLUSIONS 177 REFERENCES 182 APPENDIX A: LISTING OF THE INDIVIDUAL ELECTRON MICROPROBE ANALYSES USED IN THE CONSTRUCTION OF FIGURE 7 196 APPENDIX B: SUMMARY OF THE EXPERIMENTAL CONDITIONS INVOLVED IN THE ILMENITE-RUTILE SYNTHESIS EXPERIMENTS . • • . 202 iii LIST OF TABLES Table lo Average chemical compositions of individual ilmenite discrete nodules from Orapa - nodule interiors 18 2o Chemical compositions of coexisting ilmenite and rutile in ilmenite-rutile intergrowth nodules from Orapa and Jagersfontein o o o o o 0 o o o o o • • o • • 0 42 3. Calculated bulk compositions for selected coarse lamellar ilmenite-rutile intergrowth nodules from Orapa and Jagersfontein 0 • o o o • • • • o o • 0 55 4. Comparison of X-ray diffraction data for niobian rutile discrete nodule 1/0R-406 from Orapa and "ilmenorutile" (JCPDS 1111-396) o • • • • • o • • o • • o • o • . • • 61 5. Chemical composition of niobian rutile discrete nodule 1/0R-406 from Orapa compared to lunar rutile o 63 6. Average chemical compositions of individual clino­ pyroxene discrete nodules from Orapa 0 0 • • 0 • 66 7o Average chemical compositions of coexisting clino­ pyroxene and ilmenite from Orapa nodule t/430B • • 82 8o Average chemical compositions of individual garnet discrete nodules from Orapa 0 o • 0 • • • • o 0 • 87 9. Average chemical compositions of coexisting garnet, clinopyroxene, and chromite from Orapa nodule 1/431 104 lOo Petrographic data for representative eclogite nodules from Orapa • • o • • o o • o o . • • 113 llo Average chemical compositions of garnet grains in individual eclogite nodules from Orapa o . o 122 12o Average chemical compositions of clinopyroxene grains in individual eclogite nodules from Orapa o o o 131 13. Average chemical compositions of rutile grains in individual eclogite nodules from Orapa • • • 0 • 143 14. Average chemical compositions of ilmenite grains in individual eclogite nodules from Orapa . • • 146 v Table 15. Representative analyses of individual amphibole grains in Orapa eclogite nodules #105 and #109 . • . • • • . 148 16. Comparative temperature estimates for garnet discrete nodules containing clinopyroxene inclusions from Orapa . 159 17. Comparative temperature estimates for ilmenite-bearing eclogites and ilmenite-clinopyroxene intergrowth nodule #430B from Orapa • • • • • • • • . • • • • • • • 162 18. Individual electron microprobe analyses of ilmenite discrete nodule #411A used in the construction of the chemical contour map presented in Figure 7 of the text . 198 19. Summary of experimental conditions for individual runs involved in the ilmenite-rutile intergrowth e-xperiments . 203 vi LIST OF ILLUSTRATIONS Figure 1. Generalized distribution of kimberlite clusters and major tectonic units in Botswana 2 2a. Typical inequigranular polycrystalline texture developed in the interior region of an ilmenite nodule . 9 2b. Well-developed, angular-polygonal polycrystalline texture viewed near the periphery of an ilmenite nodule . 9 2c. Polycrystalline texture developed at the periphery of an otherwise homogeneous ilmenite nodule . • ••• 11 2d. Prominent twinning lamellae oriented along probable crystallographically equivalent directions in an ilmenite grain at a nodule periphery • • . • • • 11 2e. Anisotropic shear fabric developed in the interior region of an otherwise homogeneous ilmenite nodule • 13 2f. Heavy surface pitting developed in the interior region of an ilmenite nodule • • • • • • . • • • . • • • • 13 3. Generalized sequence of events believed to result in the range of textural features observed in the ilmenite discrete nodule suite from Orapa 15 4a. Compositions of the ilmenite discrete nodule suite from Orapa plotted in the ternary system ilmenite­ geikielite-hematite • . • • . • • • . 22 4b. Compositions of homogeneous and completely poly­ crystalline discrete nodules plotted in the ternary system ilmenite-geikielite-hematite • . • •• 22 5. Plot of MgO and cr2o3 content for the ilmenite discrete nodule suite from Orapa . • • . • . • 24 6. Compositional variation along core-to-periphery microprobe traverses of three ilmenite discrete nodules • • • • . • • • . • • . • • . • . • . 26 vii Figure 7. Chemical contour map of the MgO content (~xpressed as weight percent) of ilmenite discrete nodule specimen ti411A • • • • • • • • • • • • . • • • 28 8. Plot of MgO versus FeO content for the ilmenite discrete nodule suite from Orapa • • • • . 31 9. Plot of Cr versus Ni for the ilmenite discrete nodule suite from Orapa • • • • • • • . 32 10. Generalized classification of ilmenite-rutile intergrowth textures • • • • • • • • • • . 35 lla. Typical lamellar-type ilmenite lamellae oriented along nearly orthogonal planes within the host rutile . 38 llb. Rim ilmenite situated along the nodule periphery and branching inward to form typical ilmenite lamellae characteristic of the lamellar type of ilmenite­ rutile intergrowth nodules at Orapa • . • • • 38 12. Compositions of rutiles from kimberlitic nodules, meteorites, lunar surface materials, and terres­ trial rocks plotted in terms of atomic percent Ti versus (Nb5++er3++Ta5+) • • • . • • • . 46 13. Compositions of niobian rutiles from kimberlitic nodules and various alkalic igneous rocks plotted in terms of atomic percent Nb-Fe-Ta • . • . • 47 14. Compositions of coarse lamellar, patchy, and atoll­ type ilmenite intergrowths within rutile plotted in the ternary system ilmenite-geikielite- hemati te . 49 15. Calculated average bulk composition of coarse l~ellar ilmenite-rutile nodules from Orapa and Jagersfontein plotted in the hypothetical armalcolite compositional space Ti4+-(Fe2++Mg2+)­ cr3+, assuming a ± 5 percent relative error inherent in the modal analyses • • • • • . • . 57 16. Summary of ilmenite-rutile experimental results plotted in terms of temperature and -log f02 . 59 viii Figure 17. Compositions of the clinopyroxene discrete nodule suite from Orapa plotted in the conventional pyroxene quadrilateral system diopside-enstatite­ ferrosilite-hedenbergite . • • • . • • • . • 70 18. Compositions of selected clinopyroxene discrete nodules from Orapa plotted in terms of the "Others" components VIriJ12Na-IVA1 • • • 72 19a. Compositions of the clinopyroxene discrete nodule suite from Orapa plotted as a function of chrome content versus Mg/(Mg+Fe) ratio ••••.••• 74 19b. Compositions of the clinopyroxene discrete nodule suite from Orapa plotted as a function of chrome content versus Ca/(Ca+Mg) ratio ••••..•• 74 19c. Compositions of the clinopyroxene discrete nodule suite from Orapa plotted as a function of chrome content versus Na20 • • • • • • • • • • • • • . 75 19d. Compositions of the clinopyroxene discrete nodule suite from Orapa plotted as a function of chrome content versus Al2D3 • • • . • • • • • • . 75 20. Detailed drawing of a polished section through Orapa nodule #430B comprised of ilmenite inter­ grown with clinopyroxene and polymineralic alteration products 80 21. Compositions of the garnet discrete nodule suite from Orapa plotted in terms of mole percent Ca-Mg-Fe • • . • • • • . • • . • . 94· 22. Compositions of the garnet discrete nodule suite from. Orapa plotted as function of chrome content versus Mg/(Mg+Fe) ratio . • • . • . • • . • • • • • . • 96 23. Plot of Mg/(Mg+Fe) ratio versus Ti02 content for the garnet discrete nodule suite from Orapa . 97 24. Detailed drawing of a thin section through Orapa nodule #431 comprised of green garnet intergrown with clinopyroxene and chrornite • • • . • . • 102 ix Figure 25. Summary plot of (CaO + MgO) content versus cr 2o3 content for kimberlitic green garnets, garnet inclusions in diamonds, garnets from peridotite xenoliths in kimberlite, and low-chrome garnet discrete nodules . • • . • . 107 26. Summary plot of garnet compositions taken from the literature for eclogitic inclusions in kimber- li tes • • • • • • . • • • . 129 27. Summary plot of clinopyroxene compositions taken from the literature for eclogitic inclusions in kimberli tes . • • • • . 138 28. Average compositions of garnets and clinopyroxenes coexisting in eclogite xenoliths from Orapa plotted in terms of Ca-Mg-Fe components . 140 29a. Compositions of garnet discrete nodules and garnets in eclogites from Orapa plotted in terms of MgO/(MgO+FeO) ratio and Cr203 content 141 29b. Compositions of clinopyroxene
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