U-TH-PB SYSTEMATICS OF INDIVIDUAL PEROVSKITE GRAINS FROM THE ALLENDE AND MURCHISON CARBONACEOUS CHONDRITES

Ion microprobe U-Th-Pb isotopic measurements have been made of individual perovskite grains from the Allende (CV3) and Murchison (CM2) chondrites. These are the first chronological data reported for Murchison refractory inclusions. Several generations of perovskite were identified in polished thin sections of "fluffy" Type A inclusions 3529,44 and 4691 from Allende of which only the largest anhedral perovskites (> 20-mu-m) were analysed with the ion microprobe. The Allende perovskites are characterised by low U (typically < 3 ppm, but ranging up to 12 ppm in grain 7-1 from 3529,44), and Th/U of around 4 in 3529,44 and 10 in 4691. The Pb-204 concentrations in the Allende perovskites range from 6 to 140 ppb. A precise age measurement could be obtained on grain 7-1 only; the U-Pb systematics of this grain are concordant with a Pb-207/Pb-206 age of 4565 +/- 34 (2-sigma-) Ma. Two of the Murchison perovskites occurred as individual 20-mu-m inclusions in spinel grains (7-309, 7-A52), while the third perovskite-bearing inclusion was a spherule dominated by perovskite with interstitial spinel (7-396). U concentrations range from 3.6 to 15 ppm with Th/U less than 0.5, and Pb-204 concentrations ranging from 61 to 369 ppb. All analyses of the Murchison perovskites are consistent with a single formation age of 4569 +/- 26 Ma. The ages determined by ion microprobe from these perovskites show good agreement with previous age estimates of Allende refractory inclusions measured by conventional mass-spectrometric techniques. There is no evidence in the U-Pb systematics of Allende and Murchison refractory inclusions for material that substantially predates the formation of the solar system.