Time differences in the formation of meteorites as determined from the ratio of lead-207 to lead-206

Measurements of the lead isotopic composition and the uranium, thorium, and lead concentrations in meteorites were made in order to obtain more precise radiometric ages of these members of the solar system. The newly determined value of the lead isotopic composition of Canyon Diablo troilite is as follows: ²⁰⁶Pb/²⁰⁴Pb = 9.307, ²⁰⁷Pb/^2O4Pb = 10.294, and ²⁰⁸Pb/²⁰⁴Pb = 29.476. The leads of Angra dos Reis, Sioux County, and Nuevo Laredo achondrites are very radiogenic, the ²⁰⁶Pb/²⁰⁴Pb values are about 200, and the uranium-thorium-lead systems are nearly concordant. The ages of the meteorites as calculated from a single-stage ²⁰⁷Pb/²⁰⁶Pb isochron based on the newly determined primordial lead value and the newly reported ²³⁵U and ⁸³⁸U decay constants, are 4.528 x 10⁹ years for Sioux County and Nuevo Laredo and 4.555 x 10⁹ years for Angra dos Reis. When calculated with the uranium decay constants used by Patterson, these ages are 4.593 x 10⁹ years and 4.620 x 10⁹ years, respectively, and are therefore 40 to 70 x 10⁶ years older than the 4.55 x 10⁹ years age Patterson reported. The age difference of 27 x 10⁶ years between Angra dos Reis and the other two meteorites is compatible with the difference between the initial ⁸⁷Sr/⁸⁶Sr ratio of Angra dos Reis and that of seven basaltic achondrites observed by Papanastassiou and Wasserburg. The time difference is also comparable to that determined by ¹²⁹1-¹²⁹Xe chronology. The ages of ordinary chondrites (H5 and L6) range from 4.52 to 4.57 x 10⁹ years, and, here too, time differences in the formation of the parent bodies or later metamorphic events are indicated. Carbonaceous chondrites(C2 and C3) appear to contain younger lead components.