Variations in initial Al-26/Al-27 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 Al-26/Al-27 value, (Al-26/Al-27)(0), of (5.19 +/- 0.17) x 10(-5). This is essentially identical to the Solar System initial Al-26/Al-27 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) x 10(-5) and (4.393 +/- 0.084) x 10(-5), are significantly lower than the Solar System initial value. The range of (Al-26/Al-27)(0) values of the three CAIs, from (5.19 +/- 0.17) to (4.393 +/- 0.084) x 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 similar to 0.2 Myr at the very beginning of our Solar System, if Al-26 was distributed homogeneously in the CAI forming region. Alternatively, the observed variations in (Al-26/Al-27)(0) among fluffy Type A CAIs would also raise a possibility of heterogeneous distributions of Al-26 in the forming region. (C) 2019 Elsevier B.V. All rights reserved.