Spatial heterogeneity of 26Al/27Al and stable oxygen isotopes in the solar nebula

The degree of isotopic spatial heterogeneity in the solar nebula has long been a puzzle, with different isotopic systems implying either large-scale initial spatial homogeneity (e.g., Al-26 chronometry) or a significant amount of preserved heterogeneity (e.g., ratios of the three stable oxygen isotopes, 160, 170, and 180). We show here that in a marginally gravitationally unstable (MGU) solar nebula, the efficiency of large-scale mixing and transport is sufficient to spatially homogenize an initially highly spatially heterogeneous nebula to dispersions of similar to 10% about the mean value of Al-26/Al-27 on time scales of thousands of years. A similar dispersion would be expected for O-17/O-16 and O-18/O-16 ratios produced by ultraviolet photolysis of self-shielded molecular CO gas at the surface of the outer solar nebula. In addition to preserving a chronological interpretation of initial Al-26/Al-27 ratios and the self-shielding explanation for the oxygen isotope ratios, these solar nebula models offer a self-consistent environment for achieving large-scale mixing and transport of thermally annealed dust grains, shock-wave processing of chondrules and refractory inclusions, and giant planet formation.