⁵³Mn-⁵³Cr CHRONOMETRY OF CB CHONDRITE: EVIDENCE FOR UNIFORM DISTRIBUTION OF⁵³Mn IN THE EARLY SOLAR SYSTEM

Abstract

High-precision Cr isotope ratios for chondrules and metal grain separated from CB chondrite Gujba were determined. The ε54Cr values (εiCr = [(iCr/52Cr)sample/(iCr/52Cr)standard − 1] × 104) for all samples were identical within the analytical uncertainty, with a mean value of +1.29 ± 0.02. Uniform ε54Cr signatures of both chondrules and metal grains imply that the Cr isotope systematics of the meteorite was once completely equilibrated. The ε53Cr values of the chondrules and metal grain, on the other hand, display a strong correlation with the 55Mn/52Cr ratio. The 53Mn/55Mn calculated from the slope of the isochron is (3.18 ± 0.52) × 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 53Mn in the early solar nebula.