Time Differences in the Formation of Meteorites as Determined from the Ratio of Lead-207 to Lead-206

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Science  22 Jun 1973:
Vol. 180, Issue 4092, pp. 1279-1283
DOI: 10.1126/science.180.4092.1279


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.

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