EVIDENCE FOR MAGNESIUM ISOTOPE HETEROGENEITY IN THE SOLAR PROTOPLANETARY DISK

Abstract

With a half-life of 0.73 Myr, the 26 Al-to- 26 Mg 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 26 Al– 26 Mg ages of meteorites and their components relies on the critical assumption that the canonical 26 Al/ 27 Al ratio of ∼5 × 10 −5 recorded by the oldest dated solids, calcium–aluminium-rich inclusions (CAIs), represents the initial abundance of 26 Al 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 26 Mg composition (μ 26 Mg*) of bulk solar system reservoirs with solar or near-solar Al/Mg ratios. This variability may represent heterogeneity in the initial abundance of 26 Al across the solar protoplanetary disk at the time of CAI formation and/or Mg-isotope heterogeneity. By comparing the U–Pb and 26 Al– 26 Mg ages of pristine solar system materials, we infer that the bulk of the μ 26 Mg* variability reflects heterogeneity in the initial abundance of 26 Al across the solar protoplanetary disk. We conclude that the canonical value of ∼5 × 10 −5 represents the average initial abundance of 26 Al 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 26 Al– 26 Mg system as an accurate early solar system chronometer.