Present status of the lead method of age determination

The available results on the age of radioactive minerals as determined by the various isotopic ratios Pb/sup 206/U/sup 238/, Pb/sup 207//U/sup 235/, Pb/ sup 207// Pb/sup 206/Pb/sup 210/, and Pb/sup 208/ Th/sup 232/ are summarized. The probable sources of error are radon leakage, leaching and alteration, uncertainty in the isotopic composition of any common lead, chemical analysis, and instrumental techniques. With sufficient isotopic analyses on the common lead of an area, it is usually possible to make a precise correction for the common lead contribution to the isotopic composition of a radioactive mineral. Radon leakage can be measured under laboratory conditions and extrapolated to the integrated thermal history of the mineral. Chemical and instrumental errors are generally of second-order importance. The occurrence of leaching and alteration can be estimated from the comparison of the lead-lead with the lead-uranium ages. In the most ideal conditions, the uncertainty in the half life of U/sup 235/ significant error. It is concluded that the 207/235 and 206/210 ages are the most reliable over the greater range of gelogic time. The 206/238 age is generally correct to 5 to 10 percent and supersedes the 207/235 age in accuracy for young minerals. The Pb/sup 208/ Th/supmore » 232/ age is considered usable for minerals high in thorium content. The 207/206 age is the least reliable of all. Nomographs are presented which simplify the calculation of the age of a mineral from the isotopic ratio. (auth)« less