The distribution of short-lived radioisotopes in the early solar system and the chronology of asteroid accretion, differentiation, and secondary mineralization

We evaluate initial (Al-26/Al-27)(0), (Mn-53/Mn-55)(0), and (Hf-182/Hf-180)(I), ratios, together with Pb-207/Pb-206 ages for igneous differentiated meteorites and chondrulles from ordinary chondrites for consistency with radioactive decay of the parent nuclides within a common, closed isotopic system, i.e., the early solar nebula. The relative initial isotopic abundances of Al-26, Mn-53, and Hf-182 in differentiated meteorites and chondrules are consistent with decay from common solar system initial values, here denoted by I(Al)(SS), I(Mn)(SS), and I(Hf)(SS), respectively. I(Mn)(SS) and I(Hf)(SS) = 9.1 +/- 1.7 x 10(-6) and 1.07 +/- 0.08 x 10(-4), respectively, correspond to canonical I(Al)(SS) = 5.1 x 10(-5). I(Hf)(SS) so determined is consistent with I(Hf)(SS) = 9.72 +/- 0.44 x 10(-5) directly determined from an internal Hf-W isochron for CAI minerals. I(Mn)(SS) is within error of the lowest value directly measured for CAIs. We suggest that erratically higher values measured for CAIs in carbonaceous chondrites may reflect proton irradiation of unaccreted CAIs by the early Sun after other asteroids destined for melting by Al-26 decay had already accreted. The Mn-53 incorporated within such asteroids would have been shielded from further local spallogenic contributions from within the solar system. The relative initial isotopic abundances of the short-lived nuclides are less consistent with the Pb-207/Pb-206 ages of the corresponding materials than with one another. The best consistency of short- and long-lived chronometers is obtained for (Hf-182/Hf-180)(I), and the Pb-201/Pb-206 ages of angrites. (Hf-182/Hf-180)(I), decreases with decreasing Pb-207/Pb-206 ages at the rate expected from the 8.90 +/- 0.09 Ma half-life of Hf-182. The model solar system age thus determined is T-SS,T-Hf-W = 4568.3 +/- 0.7 Ma. (Al-26/Al-27)(I) and (Mn-53/Mn-55)(I) are less consistent with Pb-207/Pb-206 ages of the corresponding meteorites, but yield T-SS,T-Mn-Cr = 4568.2 +/- 0.5 Ma relative to I(Al)(SS) = 5.1 x 10(-5) and a Pb-207/Pb-206 age of 4558.55 +/- 0.15 Ma for the LEW86010 angrite. The Mn-Cr method with I(Mn)(SS) = 9.1 +/- 1.7 x 10(-6) is useful for dating accretion (if identified with chondrule formation), primary igneous events, and secondary mineralization on asteroid parent bodies. All of these events appear to have occurred approximately contemporaneously on different asteroid parent bodies. For I(Mn)(SS) = 9.1 +/- 1.7 x 10(-6), parent body differentiation is found to extend at least to similar to 5 Ma post-T-SS, i.e., until differentiation of the angrite parent body similar to 4563.5 Ma ago, or similar to 4564.5 Ma ago using the directly measured Pb-201/Pb-206 ages of the D'Orbigny-clan angrites. The similar to 1 Ma difference is characteristic of a remaining inconsistency for the D'Orbigny-clan between the Al-Mg and Mn-Cr chronometers on one hand, and the Pb-207/Pb-206 chronometer on the other. Differentiatio of the IIIAB iron meteorite and ureilite parent bodies probably occurred slightly later than for the angrite parent body, and at nearly the same time as one another as shown by the Mn-Cr ages of IIIAB irons and ureilites, respectively. The latest recorded episodes of secondary mineralization are for carbonates on the CI carbonaceous chondrite parent body and fayalites on the CV carbonaceous chondrite parent body, both extending to similar to 10 Ma post-T-SS. Published by Elsevier Ltd.