ISOTOPIC CONSTITUTION OF RADIOGENIC LEADS AND THE MEASUREMENT OF GEOLOGICAL TIME

Abstract

Isotopic abundances are reported for lead extracted from 1 galena and 96 uranium minerals. The radiogenic leads are mostly from the Athabasca province of the Canadian Shield. Chemical analyses are given for 9 of the uranium-bearing minerals. Ages have been calculated from the radiogenic Pb 207 /Pb 206 ratios for all samples with sufficiently low common lead contamination, and from the Pb 206 /U 238 ratios for the chemically analyzed samples. These have been used to establish that the Athabasca province was being actively mountain built from 1860 to 1630 million years ago, but not subsequently. It is suggested that the many younger ages determined for the pitchblende deposits in that region are correct and indicate solution and redeposition of the deposits at a later time. The method of extracting and preparing small quantities of lead as lead tetramethyl for analysis in a mass spectrometer is described.