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Gas source mass spectrometry of trace leads from Sudbury, Ontario Ulrych, Tadeusz Jan

Abstract

The measurement of lead isotope abundances with a gas source mass spectrometer has been limited to lead ores and minerals. This thesis describes a technique by means of which the sample size necessary for a precise gas source analysis has been decreased by more than two orders of magnitude. In this way, the range of minerals which may be studied by means of a gas source mass spectrometer has been greatly extended. No particular effort has been made to analyze very small samples. The technique which has been developed begins with the evaporation of lead from a mineral to form a mirror. The synthesis of tetramethyllead is completed by the reaction of free methyl radicals with the lead mirror followed by gas chromatographic purification. The free radical method has been applied to the study of lead isotope abundance variations in the mining district of Sudbury, Ontario. A satisfactory chronological history for this region has not been obtained from the many conventional age determinations that exist. The analysis of lead from various sulphides has yielded lead isotope ratios which are linearly related on a plot of Pb²⁰⁷/Pb²⁰⁴ vs. Pb²⁰⁶/Pb²⁰⁴ with a slope of 0.131 ± 0.003. The standard deviation of points from their best straight line is 0.37% of the average Pb²⁰⁷/Pb²⁰⁴ value. From these results it is concluded that the linear relationship is the result of just two geological events, which fact simplifies the possible interpretations. The maximum ages for these two events are 2150 ± 50 million years and 1280 ± 50 million years. Two interpretations of the results are suggested. The first gives ages of 1950 million years and 350 million years for the primary and secondary events. The two events in the second interpretation are considered to have occurred 1600 million years and 950 million years ago. The second interpretation is more easily defended on geological grounds. The simple relationship of lead isotope ratios which has been observed in the Sudbury district is particularly significant in view of the geological complexity of this region. Thus the technique developed may be expected to have wide application.

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