together with the combined effect of the slightly
smaller errors on the age due to the greater spread
in Pb-Pb data to define the 207PbR/206PbR ratio,
greater number of points defining the isochron,
acceptable sample/blank ratios for all measure-
ments used to define the array (table S4), and the
low amount of terrestrial Pb contamination (21).
Allende meteorite (Fig. 1), we used the well-constrained solar 238U/235U value of 137.786 T
0.013 to calculate the absolute Pb-Pb ages of our
subset of chondrules. In contrast to the narrow
age span defined by CAIs, the chondrule ages
Fig. 2. Pb-Pb isochron diagrams for Efremovka CAI 22E (A), Allende chon-
drule C30 (B), and NWA 5697 chondrule C2 (C).
Fig. 3. Initial Pb isotopic compositions of individual
chondrules. The initial Pb isotope compositions are defined
by the intersection of the individual isochrons and a Pb
evolution array anchored on the solar system initial Pb
isotope composition defined by the Nantan iron meteorite
(14). The m values (238U/204Pb) of chondrules are indicated
in parentheses. Chondrule C30 was displaced to the right-
hand side of the solar system initial Pb array for clarity. The
uncertainty of the solar system initial Pb value is smaller
than the symbol.