Pb-Pb DATING OF THE D’ORBIGNY AND ASUKA 881371 ANGRITES AND A SECOND ABSOLUTE TIME CALIBRATION OF THE Mn-Cr CHRONOMETER

A number of short-lived radioactive isotopes, which existed in the early solar system but are now extinct (e.g., Al (t1/2 = 0.74 Ma) Mg, Mn (t1/2 = 3.7 Ma) Cr, Hf (t1/2 = 9 Ma) W, Sm (t1/2 = 103 Ma) Nd), have proven useful for dating the relative ages of meteorites. Based on the ratio of the stable daughter isotope to a proxy isotope for the parent element, this method only provides relative ages, and then only if a number of conditions are met. For example, a direct temporal relationship between daughter and proxy parent ratios requires that the original isotopic composition of the parent element where the meteorites formed was homogeneous. The discovery of an initially variable Cr/Cr among meteorites could reflect a radial inhomogeneous distribution of Mn/Cr and Mn/Mn in the early solar system [1].