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

Abstract

We evaluate initial (26Al/27Al)I, (53Mn/55Mn)I, and (182Hf/180Hf)I ratios, together with 207Pb/206Pb ages for igneous differentiated meteorites and chondrules 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 26Al, 53Mn, and 182Hf 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 × 10-6 and 1.07 ± 0.08 × 10-4, respectively, correspond to "canonical" I(Al)SS = 5.1 × 10-5. I(Hf)SS so determined is consistent with I(Hf)SS = 9.72 ± 0.44 × 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 26Al decay had already accreted. The 53Mn 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 207Pb/206Pb ages of the corresponding materials than with one another. The best consistency of short- and long-lived chronometers is obtained for (182Hf/180Hf)I and the 207Pb/206Pb ages of angrites. (182Hf/180Hf)I decreases with decreasing 207Pb/206Pb ages at the rate expected from the 8.90 ± 0.09 Ma half-life of 182Hf. The model solar system age thus determined is TSS,Hf-W = 4568.3 ± 0.7 Ma. (26Al/27Al)I and (53Mn/55Mn)I are less consistent with 207Pb/206Pb ages of the corresponding meteorites, but yield TSS,Mn-Cr = 4568.2 ± 0.5 Ma relative to I(Al)SS = 5.1 × 10-5 and a 207Pb/206Pb age of 4558.55 ± 0.15 Ma for the LEW86010 angrite. The Mn-Cr method with I(Mn)SS = 9.1 ± 1.7 × 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 × 10-6, parent body differentiation is found to extend at least to ∼5 Ma post-TSS, i.e., until differentiation of the angrite parent body ∼4563.5 Ma ago, or ∼4564.5 Ma ago using the directly measured 207Pb/206Pb ages of the D'Orbigny-clan angrites. The ∼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 207Pb/206Pb chronometer on the other. Differentiation 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 ∼10 Ma post-TSS.