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The precise intercomparison of lead isotope ratios Kollar, Francis

Abstract

The isotopic constitution of lead became important in geophysics as the lead-uranium and lead-thorium methods of absolute geological age determination were established. Interest in the isotopic abundances of common lead in lead minerals was a natural development. The interpretations of observed variations in lead isotope ratios has been more simple than had been expected twenty years ago when the first measurements of this kind were made by A. 0. Nier, and results have been obtained that are of very great importance to geophysics. Consequently this field is now expanding very rapidly. Many interpretations now being made are limited by the available precision of the measurements, which is of the order of several tenths of a per cent. A mass spectrometer laboratory was set up in the Department of Physics at The University of British Columbia, and a mass spectrometer capable of measuring heavy elements with high precision was designed and constructed. It is a 90 degree sector, 12 inch radius, direction focusing instrument with a copper tube and using a modified Nier-type gas source. It is essentially of orthodox design, but special attention was given to try to eliminate small sources of error. To establish more stable source conditions, an exceptionally stable filament emission control was constructed and purified lead tetramethyl samples were used. The ion beam was measured with a servo-voltmeter of original design that is capable of a high precision. Readings can be made from a calibrated dial on the voltmeter eliminating sources of error in the usual chart recorder. Tests have shown that the improvement in precision warrants this step. This mass spectrometer is believed to be the first to use extensively transistorized circuits. Error was reduced further by comparing each sample measured with a standard within reasonably short intervals of time (about twenty minutes). Three comparisons were made to loop two samples with the standard, and the looping error was determined and distributed around the loop. The analyses obtained in this way were compared with existing analyses from other laboratories, and an improvement in precision between a factor of five and a factor of ten seems to have been obtained. To demonstrate the precision obtainable with this mass spectrometer and with the improvement in operating techniques, analyses were made on the isotopic composition of leads from Broken Hill and Mount Isa, Australia. It was previously known that the leads are very similar in composition at both localities. From these new measurements it was established that the Broken Hill and Mount Isa deposits contain lead of distinctly different isotopic composition. In both deposits the isotope ratios were found less variable than could be inferred from previous measurements. A fine structure was found in the isotope ratios in both localities. These small variations indicate contaminations due to radiogenic leads. Values of the thorium/uranium ratios of the source of contamination were estimated. The precision of analyses made it possible to determine an age difference between the Mount Isa and Broken Hill deposits of 40 million years.

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