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«By MOLATLHEGI LARTY LOSTMAN MOSEKI STUDENT NO. 208523856 Submitted in fulfillment of the academic requirements For the degree of Master of Science In ...»

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http://en.wikipedia.org/wiki/Sensitive_high-resolution_ion_microprobe Uranium converts slowly and steadily to lead by natural radioactive decay. All rocks take up small amounts of lead and uranium when they form, but some special minerals in rocks, such as zircon, take up only uranium. Any lead found in zircon crystals must therefore come from uranium decay. Uranium converts to lead so fast, so the ratio of lead to uranium in zircon tells how old it is. The difficulty with dating rocks using zircon is that many rocks contain zircon crystals of many different ages. Zircon is so tough that when new rocks form from older rocks, zircon crystals from the older rocks survive.

Even if a rock is melted, the old zircon crystals simply grow a new layer. Dating such mixed crystals by traditional methods, even one by one, gives meaningless average ages.

SHRIMP is able to measure the ages of layers within single zircon crystals as small as 10

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SHRIMP ANALYTICAL PROCEDURE

N.B: The following analytical procedure is an extract from a report from Dr. Richard Armstrong, Research School of Earth Sciences (RSES), Australian National University.

Firstly zircon samples were crushed into powder and then either passed over a Wilfley Table to concentrate heavy minerals or hand selected. The zircons were analysed using SHRIMP II (Sensitive High Resolution Ion Microprobe). Each analysis consisted of five scans through the mass range. The zircons were mounted in epoxy resin together with the zircon standard AS3 and RSES standard SL13. The grains were sectioned approximately in half and polished. The spot diameter was about 15-3 microns, and the primary beam intensity was about 2 nA. The data have been reduced as described by Williams and Claesson (1987, and references therein), using the SQUID Excel Macro of Ludwig (2000). Reflected and transmitted light photomicrographs and Cathodoluminescence CL) SEM images were prepared for all zircons to aid in the selection of target areas and to decipher the internal structure of zircons. The zircons were then analysed for U-Th-Pb.

The Pb/U ratios have been normalized relative to a value of 0.1859 (equivalent to an age of 1099 Ma) for AS3. Pb/Pb stepwise leaching experiments (PbSL) were applied. Mineral separates were obtained by standard techniques using jaw crusher and sieve analysis. The 150 to 250-ml sieve fractions were then purified by handpicking followed by repeated rinsing in deionized water; 200 mg of these materials were transferred to 7-ml Savillex1 screw-cap beakers for step-leaching. Successive 120°C acid leach steps (seven in total) involving various mixtures of HBr, HNO3, and HF were performed on each separate, to extract Pb selectively from the phases. The Ahrens-Wetherill Concordia plot (Wetherill

1956) has been prepared using Isoplot/EX (Ludwig 2003). Dates were calculated by use of radiometric ratios 207 Pb/ 206Pb ratios and corrections for common Pb were undertaken 204 by use of the measured Pb and the appropriate common Pb composition. Ages for

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12 13 14 Carbon has two stable isotopes, C and C, and one radioactive isotope, C. Carbon isotope ratios are measured against Vienna Pee Dee Belemnite (VPDB). They have been used to track ocean circulation, among other things. Oxygen has three stable isotopes, 16 O, 17O, and 18O. Oxygen ratios are measured relative to Vienna Standard Mean Ocean Water (VSMOW) or Vienna Pee Dee Belemnite (VPDB). Variations in oxygen isotope ratios are used to track water movement, paleoclimate and atmospheric gases such as ozone and carbon dioxide. Typically, the VPDB oxygen reference is used for paleoclimate, while VSMOW is used for most other applications. Oxygen isotopes appear in anomalous ratios in atmospheric ozone, resulting from mass-independent fractionation. Isotope ratios in fossilized foraminifera have been used to deduce the temperature of ancient seas (http://en.wikipedia.org/wiki/Isotope_geochemistry, 27 June 2013) N.B: The following analytical procedure is an extract from a report from Dr. Andrey Bekker, who did the isotope analysis at the University of Manitoba In the analyses calibration was performed by analyzing two internal calcite standards (CHI and USC-1) for data in Table 5.2 and three international and internal calcite standards (NBS18, NBS19 and CHI) for data in Table 5.3. During the analysis, calibration was done at the beginning, middle and end of each run. Calibration lines were calculated by least squares linear regression using the known and measured isotope values of the calibration standards. To check the quality of analysis, calibrated international or internal standards were analysed together with unknown samples. For dolomites minerals in Table 5.2, four calibrated internal calcite and dolomite standards (LiPO#3, δ13C = +1.02‰ VPDB and δ18O = -7.98‰ VPDB; Exp50, δ13C = -3.80 ± 0.10‰ and δ18O = -10.50 ± 0.10‰; MD, δ13C = +1.00 ± 0.10‰ and δ18O = -8.70 ± 0.10‰; Tytyri, δ13C = +0.78‰ and δ18O = -7.07‰) were analysed together with the unknown samples. Replicate analyses of internal standards yielded the results of δ13C = +0.97 ± 0.08‰ and δ18O = -8.01 ± 0.10‰ (n = 45) for LiPO#3, δ13C = -3.93 ± 0.10‰ 174 and δ18O = -10.68 ± 0.11‰ (n = 48) for Exp50, δ13C = +0.90 ± 0.07‰ and δ18O = -8.47 ± 0.11‰ (n = 36) for MD, and δ13C = +0.73 ± 0.10‰ and δ18O = -5.92 ± 0.12‰ (n = 41) for Tytyri. For the dolomite and calcite minerals in Table 5.3, quality check was performed analyzing 2 calibrated internal calcite and dolomite standards (Exp50, δ13C = ± 0.10‰ VPDB and δ18O = -10.50 ± 0.10‰ VPDB; Tytyri, δ13C = +0.78 ± 0.01‰ and δ18O = -7.07 ± 0.04‰) together with unknown samples. Replicate analyses of internal standards yielded the results of δ13C = -3.90 ± 0.10‰ and δ18O = -10.74 ± 0.14‰ (n = 66) for Exp50, and δ13C = +0.73 ± 0.09‰ and δ18O = -5.96 ± 0.14‰ (n = 70) for Tytyri.





175 ACKNOWLEDGEMENTS

I would like to express my special appreciation and thanks to my supervisor Professor Stephen McCourt for finding time for supervision and editorial work, you have been a tremendous mentor for me. I thank you for encouraging my research and for allowing me to grow as a research scientist. Without your supervision and constant help, this dissertation would not have been possible. Thanks are also due to anonymous examiner for the constructive and brilliant comments and suggestions. I would especially like to thank Dr. A. Bekker (Department of Geological Sciences, University of Manitoba, Canada) for the stable isotope analysis on carbonate samples. I am also grateful to the following; Dr. Richard Armstrong (RSES, The Australian National University, Canberra) for the SHRIMP work (zircon separation, CL imaging, dating of samples, data reduction and interpretation), Dr. Hoffman for the field visit, Peter Arone (GS field assistant), Alfred Pule (GS field assistant). I greatly acknowledge the Director of Geological Survey, Mr. Ngwisanyi and all the supporting staff of Geological Survey who provided assistance when I collected data for my research work. Special thanks to my family.

Words cannot express how grateful I am to my Mother; your prayer for me was what sustained me thus far. I also thank my friends and everyone who incited me to strive towards my goal, the list is endless. Many thanks go to the local people in the STF area who were at all times showing willingness to assist. At the end, I would like to thank Kaone and Itumeleng Senatla who were always my support in the moments when there was no one to answer my queries. What kept me going was my unwillingness to give in. I have learnt to forge ahead and never to give up even during the tough and testing times.

Most of all, I have learnt to be specific with directions. I will never again loose my bearings.

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Bennett, J.D. (1968 a). A provisional description of the Mosetse-Matsitama area.

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