Extremely Brief Formation Interval for Refractory Inclusions and Uniform Distribution of 26 Al in the Early Solar System

Abstract

Calcium-aluminum-rich inclusions (CAIs) are millimeter-sized refractory objects commonly found in chondritic meteorites and are the oldest solids formed in our solar system. Primary CAI formation may have occurred through condensation and/or evaporation processes near the proto-Sun or, alternatively, during localized events in the asteroid belt. As such, these objects provide us with a unique window into the earliest development of the Sun and into the evolution of the protoplanetary disk. Here we report a 26 Al- 26 Mg isochron for bulk CAIs from four CV carbonaceous chondrites, which yields an initial 26 Al/ 27 Al of (5.85 ± 0.05) × 10 -5 , suggesting that primary formation of the CV CAIs may have occurred within an interval as brief as 20,000 years. This timescale is inconsistent with the secular evolution of T Tauri stars but may be consistent with CAI formation during the infall stage of the protostellar evolution of the Sun. High-precision Mg isotope measurements of samples from the Earth, Moon, Mars, and bulk chondrite meteorites show that these have identically elevated 26 Mg abundances compared to the initial 26 Mg abundance (δ 26 Mg* = -0.0317‰ ± 0.0038‰) defined by the CAI isochron. This observation unequivocally demonstrates the homogeneous distribution of 26 Al within the accretion region of the terrestrial planets. However, the initial 26 Mg abundance of CAIs implies a brief history of elevated Al/Mg in CAI precursor material, which may represent primary condensation of refractory silicates and oxides from the solar nebula.