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

  • Thrane K
  • Bizzarro M
  • Baker J
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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 26Al-26Mg isochron for bulk CAIs from four CV carbonaceous chondrites, which yields an initial 26Al/27Al 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 26Mg abundances compared to the initial 26Mg abundance (delta26Mg* = -0.03170/00 +/- 0.00380/00) defined by the CAI isochron. This observation unequivocally demonstrates the homogeneous distribution of 26Al within the accretion region of the terrestrial planets. However, the initial 26Mg 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.

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Thrane, K., Bizzarro, M., & Baker, J. A. (2006). Extremely Brief Formation Interval for Refractory Inclusions and Uniform Distribution of 26 Al in the Early Solar System. The Astrophysical Journal, 646(2), L159–L162. https://doi.org/10.1086/506910

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