title: Early accretion of protoplanets inferred from a reduced inner solar system26Al inventory.
abstract: The mechanisms and timescales of accretion of 10-1000 km sized planetesimals, the building blocks of planets, are not yet well understood. With planetesimal melting predominantly driven by the decay of the short-lived radionuclide26Al (26Al→26Mg;t1/2= 0.73 Ma), its initial abundance determines the permissible timeframe of planetesimal-scale melting and its subsequent cooling history. Currently, precise knowledge about the initial26Al abundance [(26Al/27Al)0] exists only for the oldest known solids, calcium aluminum-rich inclusions (CAIs) - the so-called canonical value. We have determined the26Al/27Al of three angrite meteorites, D'Orbigny, Sahara 99555 and NWA 1670, at their time of crystallization, which corresponds to (3.98 ± 0.15)×10-7, (3.64 ± 0.18)×10-7, and (5.92 ± 0.59)×10-7, respectively. Combined with a newly determined absolute U-corrected Pb-Pb age for NWA 1670 of 4564.39 ± 0.24 Ma and published U-corrected Pb-Pb ages for the other two angrites, this allows us to calculate an initial (26Al/27Al)0of [Formula: see text] for the angrite parent body (APB) precursor material at the time of CAI formation, a value four times lower than the accepted canonical value of 5.25 × 10-5. Based on their similar54Cr/52Cr ratios, most inner solar system materials likely accreted from material containing a similar26Al/27Al ratio as the APB precursor at the time of CAI formation. To satisfy the abundant evidence for widespread planetesimal differentiation, the subcanonical26Al budget requires that differentiated planetesimals, and hence protoplanets, accreted rapidly within 0.25 ± 0.15 Ma of the formation of canonical CAIs.
authors.affiliation: Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Denmark.
authors.forename: Martin
authors.initials: M
authors.lastname: Schiller
authors.affiliation: Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Denmark.
authors.forename: James N
authors.initials: JN
authors.lastname: Connelly
authors.affiliation: Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Denmark.
authors.forename: Aslaug C
authors.initials: AC
authors.lastname: Glad
authors.affiliation: Department of Earth & Planetary Science, University of Tokyo, Tokyo, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Mikouchi
authors.affiliation: Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Denmark.
authors.forename: Martin
authors.initials: M
authors.lastname: Bizzarro
keywords: Pb–Pb dating; angrite meteorites; initial 26Al abundance; planetesimal accretion