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

摘要

Asuka 881394 is a unique basaltic meteorite that originated in the crust of a differentiated planetesimal in the early SolarSystem. We present high precision Pb, Mg, and Cr isotopic compositions of bulk samples and mineral separates from thisachondrite. A ~207pb-~206Pbinternal isochron obtained from the radiogenic pyroxene and whole-rock fractions of Asuka881394 yields an absolute age of 4566.5 ± 0.2 Ma, which we consider to be the best estimate for the crystallization age of thisbasaltic 4chondrite. The ~26M-~26Mgsystematics show some evidence of disturbance, but 5 the 6 analyzed whole-rock and mineral fractions define an isochron corresponding to a ~27A1/~26A1 ratio of (1.28 ± 0.07) x 10~-6.Comparison with the~26A1-26Mg Mgand Pb-Pb systematics in the D'Orbigny achondrite translates to a ~26A1-~26Mg 881394. The ~53Mn-~53Cr systematics in whole-rock, silicate and chromite fractions correspond to a ~53Mn/~55Mn ratio of(3.85 ± 0.23) x 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-~53Crage of 4565.5 ± 0.4 Ma, in agreement with the age obtained relative to D'Orbigny. While the ~26A1-~26Mg and ~53Mn-~53Cr agesappear to be concordant in Asuka 881394, these ages are -1 Ma younger than its ~207Pb-~206Pbage. This discordance mighthave been caused by one or more of several reasons, including differences in the closure temperatures for Pb versus Cr and Mgdiffusion in their host minerals combined with slow cooling of the parent body as well as differential resetting of isotopic sys-tems by a process other than volume diffusion, e.g., shock metamorphism. The ancient age of Asuka 881394 suggests thatbasaltic 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 accretedwithin 500,000 yrs of Solar System formation.