Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011apj...735l..37l&link_type=abstract
The Astrophysical Journal Letters, Volume 735, Issue 2, article id. L37 (2011).
Astronomy and Astrophysics
Astronomy
8
Meteorites, Meteors, Meteoroids, Protoplanetary Disks
Scientific paper
With a half-life of 0.73 Myr, the 26Al-to-26Mg decay system is the most widely used short-lived chronometer for understanding the formation and earliest evolution of the solar protoplanetary disk. However, the validity of 26Al-26Mg ages of meteorites and their components relies on the critical assumption that the canonical 26Al/27Al ratio of ~5 × 10-5 recorded by the oldest dated solids, calcium-aluminium-rich inclusions (CAIs), represents the initial abundance of 26Al for the solar system as a whole. Here, we report high-precision Mg-isotope measurements of inner solar system solids, asteroids, and planets demonstrating the existence of widespread heterogeneity in the mass-independent 26Mg composition (μ26Mg*) of bulk solar system reservoirs with solar or near-solar Al/Mg ratios. This variability may represent heterogeneity in the initial abundance of 26Al across the solar protoplanetary disk at the time of CAI formation and/or Mg-isotope heterogeneity. By comparing the U-Pb and 26Al-26Mg ages of pristine solar system materials, we infer that the bulk of the μ26Mg* variability reflects heterogeneity in the initial abundance of 26Al across the solar protoplanetary disk. We conclude that the canonical value of ~5 × 10-5 represents the average initial abundance of 26Al only in the CAI-forming region, and that large-scale heterogeneity—perhaps up to 80% of the canonical value—may have existed throughout the inner solar system. If correct, our interpretation of the Mg-isotope composition of inner solar system objects precludes the use of the 26Al-26Mg system as an accurate early solar system chronometer.
Bizzarro Martin
Connelly James N.
Ivanova Marina A.
Krot Alexander N.
Larsen Kirsten K.
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