Early accretion of protoplanets inferred from a reduced inner solar system 26Al inventory

- We report Al-Mg and U-corrected Pb-Pb ages of volcanic angrites.
- Al-Mg and Pb-Pb age intervals between angrites and CAI are inconsistent by ∼1.5 Myr.
- The 26Al/27Al of the angrite parent body at the time of CAI formation is 1.33−0.18+0.21×10−5.
- Thermal models require parent body accretion <0.25−0.19+0.17 of CAIs to ensure differentiation.
- 26Al was heterogeneously distributed in the solar protoplanetary disk when CAIs formed.

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 radionuclide 26Al (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 initial 26Al abundance [(26Al/27Al)0] exists only for the oldest known solids, calcium aluminum-rich inclusions (CAIs) - the so-called canonical value. We have determined the 26Al/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)0 of (1.33−0.18+0.21)×10−5 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 similar 54Cr/52Cr ratios, most inner solar system materials likely accreted from material containing a similar 26Al/27Al ratio as the APB precursor at the time of CAI formation. To satisfy the abundant evidence for widespread planetesimal differentiation, the subcanonical 26Al budget requires that differentiated planetesimals, and hence protoplanets, accreted rapidly within 0.25±0.15 Ma of the formation of canonical CAIs.