Mathematics – Logic
Scientific paper
Mar 1960
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1960gecoa..19...41r&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 19, Issue 1, pp.41-52
Mathematics
Logic
3
Scientific paper
Recently a rather large number of analyses of common lead samples has become available, and these in turn have led to a better understanding of the precise nature of the physical and geological processes resulting in the observed variations of lead isotope ratios. Specifically, there now appear to be three classes of leads, each of which has lead isotope ratios that vary in a remarkably simple pattern. These are meteoritic leads, anomalous leads, and leads from certain conformable lead ore deposits. The meteoritic leads and conformable leads both appear to have developed in surroundings where thorium and uranium had very similar properties, and in both cases the thorium to uranium ratio corresponds to a present value of 3 · 73 ± 0·03. This is taken to be evidence that both classes developed under predominantly reducing conditions. In contrast, the presence of thorium to uranium ratio present during the production of anomalous leads is extremely variable, suggesting an oxidizing environment. It has been suggested that all vein leads may be anomalous to some degree and therefore should be interpreted with this possibility in mind. The methods for dating galenas proposed previously by the present writers are re-examined in view of these developments and it is shown that they may lead to more explicit information on the age and history of lead ores than has usually been obtained.
Farquhar Ronald M.
Russell David R.
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