Variations in initial 26Al/27Al ratios among fluffy Type A Ca–Al-rich inclusions from reduced CV chondrites

Abstract

Al–Mg mineral isochrons of three Ca–Al-rich inclusions (CAIs) that formed primarily by condensation, one fine-grained, spinel-rich inclusion and two fluffy Type A CAIs, from the reduced CV chondrites Efremovka and Vigarano were obtained by in situ Al–Mg isotope measurements using secondary ion mass spectrometry. The slope of the isochron obtained for the fine-grained, spinel-rich inclusion gives an initial 26Al/27Al value, (26Al/27Al)0, of (5.19 ± 0.17) × 10−5. This is essentially identical to the Solar System initial 26Al/27Al determined by whole-rock Al–Mg isochron studies for CAIs in CV chondrites. In contrast, the isochron slopes for the two fluffy Type A CAIs from their Al–Mg mineral isochrons, (4.703 ± 0.082) × 10−5 and (4.393 ± 0.084) × 10−5, are significantly lower than the Solar System initial value. The range of (26Al/27Al)0 values of the three CAIs, from (5.19 ± 0.17) to (4.393 ± 0.084) × 10−5, corresponds to a formation age spread of 0.17 ± 0.04 Myr. This formation age spread is similar to that of igneous CAIs from CV chondrites. The data suggest that condensation and melting of minerals occurred in the hot nebular gas contemporaneously for ∼0.2 Myr at the very beginning of our Solar System, if 26Al was distributed homogeneously in the CAI forming region. Alternatively, the observed variations in (26Al/27Al)0 among fluffy Type A CAIs would also raise a possibility of heterogeneous distributions of 26Al in the forming region.