Ancient relative and absolute ages for a basaltic meteorite: Implications for timescales of planetesimal accretion and differentiation

Asuka 881394 is a unique basaltic meteorite that originated in the crust of a differentiated planetesimal in the early Solar System. We present high precision Pb, Mg, and Cr isotopic compositions of bulk samples and mineral separates from this achondrite. A ²⁰⁷Pb-²⁰⁶Pb internal isochron obtained from the radiogenic pyroxene and whole-rock fractions of Asuka 881394 yields an absolute age of 4566.5 ± 0.2 Ma, which we consider to be the best estimate for the crystallization age of this basaltic achondrite. The ²⁶Al-²⁶Mg systematics show some evidence of disturbance, but 5 of the 6 analyzed whole-rock and mineral fractions define an isochron corresponding to a ²⁷Al/²⁶Al ratio of (1.28 ± 0.07) × 10^-6. Comparison with the ²⁶Al-²⁶Mg and Pb-Pb systematics in the D'Orbigny achondrite translates to a ²⁶Al-²⁶Mg age of 4565.4 ± 0.2 Ma for Asuka 881394. The ⁵³Mn-⁵³Cr systematics in whole-rock, silicate and chromite fractions correspond to a ⁵³Mn/⁵⁵Mn ratio of (3.85 ± 0.23) × 10^-6. Compared to the most precise ⁵³Mn-⁵³Cr and Pb-Pb systematics available for the D'Orbigny angrite, this translates to a ⁵³Mn-⁵³Cr age of 4565.3 ± 0.4 Ma; similarly, a comparison with the NWA 4801 angrite yields a ⁵³Mn-⁵³Cr age of 4565.5 ± 0.4 Ma, in agreement with the age obtained relative to D'Orbigny. While the ²⁶Al-²⁶Mg and ⁵³Mn-⁵³Cr ages appear to be concordant in Asuka 881394, these ages are ∼1 Ma younger than its ²⁰⁷Pb-²⁰⁶Pb age. This discordance might have been caused by one or more of several reasons, including differences in the closure temperatures for Pb versus Cr and Mg diffusion in their host minerals combined with slow cooling of the parent body as well as differential resetting of isotopic systems by a process other than volume diffusion, e.g., shock metamorphism. The ancient age of Asuka 881394 suggests that basaltic volcanism on its parent planetesimal occurred within ∼3 Ma of the formation of earliest solids in the Solar System, essentially contemporaneously with chondrule formation. This requires that the Asuka 881394 parent body was fully accreted within ∼500,000 yrs of Solar System formation.