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Trace lead isotope studies with gas source mass spectrometry Whittles, Arthur Brice Leroy

Abstract

On the basis of data published prior to 1960, two basic lead isotope models - the isochron model and the primary lead model - were postulated to fit those crustal leads that have had the simplest history. The present writer's analyses of samples from Southern Finland, and the results of other research workers, now indicate that the primary lead model more adequately describes these leads, and that primary "isochrons" are either very short, or else non-existent. Lead in conformable deposits and in ultrabasic sulfides were postulated by R.L. Stanton to be examples of primary lead. Analyses of samples from several massive lead-zinc conformable deposits have indicated that the leads of many of these deposits possess primary lead characteristics. The present thesis describes a technique devised to prepare tetramethyllead from the conformable chalcopyrite deposits and ultrabasic sulfides and describes some initial results. This is a free radical technique, involving the reaction of methyl radicals with a lead mirror evaporated out of a sulfide. It appears to be capable of preparing tetramethyllead from samples containing one to two orders of magnitude less lead than previously reported for such a technique. Unfortunately, the full extent of the method cannot be utilized with present gas source mass spectrometer facilities, and only fair precision (0.4%) is possible at 10 ppm. lead. Samples from the conformable chalcopyrite deposits of Mt. Isa were analysed. The lead isotope ratios were found to be significantly different from those of the massive lead-zinc deposits. The results, interpreted with other lead and sulfur isotope data, suggest that these deposits could be the result of metal-bearing solutions entering a partially enclosed bay. Initial results for two ultrabasic deposits suggest that the lead in the sulfides is not necessarily primary; for example, that in two Stillwater ultrabasic complex samples appears to be the result of three stages of development. Hence, both the present lead isotope analyses, and those reported earlier by Ostic (1963) indicate that Stanton's geological criteria, while good, are not completely adequate for identifying primary leads. It is here suggested that sulfur isotope analyses might indicate which of these samples have been contaminated by crustal leads. During the investigation of the gas source mass spectrometric analyses of small tetramethyllead samples, lead isotope fractionation was observed. This occured with molecular flow into the mass spectrometer. Lead isotope fractionation has not been reported previously, and has some interesting geophysical implications. A preliminary study suggests that such fractionation in nature is likely to be rare, except possibly in biological processes in the sea.

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