Evidence for Very Early Planetesimal Formation and 26Al/27Al Heterogeneity in the Protoplanetary Disk

Abstract

We present a U-corrected Pb–Pb age of 4566.19 ± 0.20 Ma (1.11 ± 0.26 Myr after t _0 ) for the moderately volatile element rich, andesitic meteorite Erg Chech 002 (EC002). Our Al–Mg isochron defines a ^26 Al/ ^27 Al initial ratio of (8.65 ± 0.09) × 10 ^−6 that corresponds to a ^26 Al/ ^27 Al ratio of ${2.48}_{-0.56}^{+0.67}$ × 10 ^−5 for the parent body precursor at the time of solar system formation. Whereas the published bulk chemistry and our high-precision Ca isotope measurement correspond to those for inner solar system materials, the ^26 Al/ ^27 Al ratio overlaps that for outer solar system CI chondrites. This indicates that the carriers and/or processes responsible for the nucleosynthetic isotope compositions for inner and outer disk materials are different than those controlling the heterogeneous distribution of ^26 Al. A low μ ^26 Mg* initial value of −6.1 ± 1.7 ppm infers a source region with a subchondritic Al/Mg ratio until 1.1 Myr after t _0 such that melt generation must have immediately preceded its crystallization. With ^26 Al as the main heating source, a modeled temperature–time path for a 100 km radius parent body with our inferred ^26 Al abundance suggests that accretion must have occurred before 0.5 Myr after t _0 to reach melting temperatures at appropriate depths within 1.1 Myr. This requires that the parent body formed very early within the protoplanetary disk, consistent with predictions of rapid formation of planetesimals by streaming instabilities within high-density dust filaments during the earliest phase of the protoplanetary disk. Finally, an absence of initial Pb in this otherwise moderately volatile-rich achondrite implies Pb was effectively sequestered to the Fe–Ni core.

Keywords

• Planet formation
• Cosmochronology
• Cosmochemistry