Evolutionary model for lead isotopes in conformable ores and in ocean volcanics

Recently, absolute isotopic ratios have been published for lead standards. This makes possible for the first time a meaningful comparison of published isotopic ratios of lead from conformable ores with those from oceanic rocks. Such a comparison is interesting because these classes of material exhibit isotopic patterns that may be of worldwide significance. A preliminary study has been made of the adjusted ratios of 18 conformable ores and 22 oceanic volcanic‐rock leads. Each group shows a recognizable pattern such that leads from one regime cannot be obtained by any simple mixture of leads from the other. One widely recognized difference between the patterns is the considerable variation of isotopic ratios for oceanic volcanic samples of similar age, indicative of a heterogeneous source. This contrasts sharply with conformable ores for which isotopic ratios are similar for samples of the same age. Another characteristic, considered to be of equal importance, is the significant deficiency of 207 Pb in the oceanic volcanic leads. This paper shows that both characteristics can be reproduced by a model in which uranium, thorium, and lead are transferred between two reservoirs. The transport of uranium and thorium must occur more quickly than for lead, and for all elements transport must be sufficiently rapid to reach quasi‐steady state in a time much shorter than the age of the earth. The close relationship between this model and bidirectional transport between the systems is discussed. Two forms of the model are presented. In each, the time‐varying concentrations of 204 Pb approach asymptotic values exponentially with time. Model A, in which the transport of uranium and thorium is very rapid, provides the most recent beginning of differentiation, at a time 2500 m.y. ago. Model B, in which the transport of uranium and thorium is also exponentially related to time, corresponds to a differentiation beginning 3000 m.y. ago. These seem to be extreme times for the form of the model used. It is suggested that lead‐isotope measurements might be used to identify characteristically oceanic materials now in continental areas as the result of tectonic plate movement.