53Mn-53Cr CHRONOMETRY OF CB CHONDRITE: EVIDENCE FOR UNIFORM DISTRIBUTION OF 53Mn IN THE EARLY SOLAR SYSTEM

High-precision Cr isotope ratios for chondrules and metal grain separated from CB chondrite Gujba were determined. The epsilon Cr-54 values (epsilon Cr-i = [(Cr-i/Cr-52)(sample)/(Cr-i/Cr-52)(standard) - 1] x 104) for all samples were identical within the analytical uncertainty, with a mean value of +1.29 +/- 0.02. Uniform epsilon Cr-54 signatures of both chondrules and metal grains imply that the Cr isotope systematics of the meteorite was once completely equilibrated. The epsilon Cr-53 values of the chondrules and metal grain, on the other hand, display a strong correlation with the Mn-55/Cr-52 ratio. The Mn-53/Mn-55 calculated from the slope of the isochron is (3.18 +/- 0.52) x 10(-6). This corresponds to absolute ages of 4563.7 +/- 1.2 Ma and 4563.5 +/- 1.1 Ma using angrites D'Orbigny and LEW 86010, respectively, as time anchors. These ages are consistent with the ages obtained using other short-and long-lived radio nuclides, supporting the uniform distribution of Mn-53 in the early solar nebula.