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Abstract

A review is given of the physical concepts involved in the interpretation of lead isotope evolutionary patterns. A new approach is suggested based on the observation that each of the integral decay equations can be considered to be a weighted average. Thus, variations from sample to sample in the Pb206/Pb204 abundance represent precisely variations in the weighted average of the U/Pb ratio in the rocks with which the leads have been associated throughout their lifetimes. A second weighted average of the same geochemical ratio may be obtained from the Pb207/Pb204 ratio, and a weighted average of the Th/Pb ratio, from the Pb208/Pb204 ratio.

The pre-depositional history of a lead sample can be described in terms of the U/Pb and Th/Pb ratios of the environment in which it has existed. One lead ratio gives comparatively little information about this environment, since many histories will give the same mean U/Pb. However, the requirement that two different weighted means be mutually consistent greatly restricts the possible choice. This provides an important constraint that must be satisfied by theories for the pre-depositional history of ore samples.

To illustrate the use of this isotopic constraint, published analyses by Bate et al (1) for Mississippi-valley-type leads which are thought to occur over a single Precambrian province, assumed to be 1,350 m.y. old, are examined. It is shown why the observed abundances require a multi-stage history to satisfy the conditions outlined above. Possible histories are described which involve the evolution of lead in subcrustal, as well as in crustal environments.

It is emphasized that the studies of lead isotope abundances give information about time intervals and not events, so that ages are only an indirect product of such studies.

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