Aluminum-26 chronology of dust coagulation and early solar system evolution

Abstract

Dust condensation and coagulation in the early solar system are the first steps toward forming the terrestrial planets, but the time scales of these processes remain poorly constrained. Through isotopic analysis of small Ca-Al–rich inclusions (CAIs) (30 to 100 mm in size) found in one of the most pristine chondrites, Allan Hills A77307 (CO3.0), for the short-lived 26Al-26Mg [t1/2 = 0.72 million years (Ma)] system, we have identified two main populations of samples characterized by well-defined 26Al/27Al = 5.40 (±0.13) × 10−5 and 4.89 (±0.10) × 10−5. The result of the first population suggests a 50,000-year time scale between the condensation of micrometer-sized dust and formation of inclusions tens of micrometers in size. The 100,000-year time gap calculated from the above two 26Al/27Al ratios could also represent the duration for the Sun being a class I source.