THE 4.56 GA U-PB AGE OF THE MET 78008 UREILITE

Abstract

The U-Th-Pb and Sm-Nd isotopic systematics of mineral separates from the Ca-rich ureilite MET 78008 are reported. Extensive acid leaching procedures were used to eliminate terrestrial contamination because the amounts of U, Th, Pb, Sm, and Nd are extremely low in the meteorite. The three residues without terrestrial Pb contamination yield a Pb-Pb age of 4.563 +/- 0.021 Ga. On a U-Pb concordia diagram, the residue data corrected for an initial lead isotopic composition calculated with μ = 11.7 for 3 Ma from starting Canon Diablo troilite Pb in indicate a 207Pb*206Pb* model age of 4.563 Ga. The Sm-Nd isotopic data are scattered around the 4.56-Ga chondritic isochron. However, the Sm-Nd model age for handpicked augite is 4.573 +/- 0.029 Ga, assuming an initial CHUR composition. Two separates show a positive 142Nd anomaly similar to those found in other meteorites of the same age (4.5-4.6 Ga). These Sm-Nd isotopic results support the Pb-Pb and U-Pb ages of this meteorite. Therefore, it is very likely that the formation age of ureilites is consistent with the so-called ''4.56-Ga canonical age of meteorites.'' It is also possible that extensive igneous processes for generating the ultramafic mineralogy of ureilites took place in the parent body within a few million years after formation of the solar nebula.