Variations in initial Al-26/Al-27 ratios among fine-grained Ca-Al-rich inclusions from reduced CV chondrites

Fine-grained Ca-Al-rich inclusions (FGIs) in CV chondrites are suggested to be condensates formed directly from the solar nebular gas. Al-Mg mineral isochrons of seven FGIs from reduced CV chondrites Efremovka, Vigarano, Thiel Mountains 07007, and Northwest Africa 8613 were obtained via in situ Al-Mg isotope measurements using secondary ion mass spectrometry. The slopes of the mineral isochrons for seven FGIs exhibit statistically significant variations in initial Al-26/Al-27 ratios, (Al-26/Al-27)(0), ranging from (5.19 +/- 0.17) to (3.35 +/- 0.21) x 10(-5), which correspond to a relative age spread of 0.44 +/- 0.07 Myr. Inferred upper limit of (Al-26/Al-27)(0) for the FGIs is identical to the Solar System (Al-26/Al-27)(0) of similar to 5.2 x 10(-5) as determined by whole-rock Al-Mg isochron studies for CAIs in CV chondrites. The intercepts of the mineral isochrons, the initial Mg-26/Mg-24 ratios the FGIs formed with, are consistent with Mg-isotope evolution path of a solar-composition nebular gas. The observed variations in (Al-26/Al-27)(0) for FGIs are essentially similar to those (similar to 5.2 to similar to 4.2 x 10(-5)) for coarse-grained, igneous CAIs of CV chondrites that are formed by melting and solidification. If Al-26 was distributed homogeneously in the forming region, then our data indicate that thermal processes of condensation and melting for CAI formation occurred contemporaneously and continued for at least similar to 0.4 Myr at the very beginning of the Solar System. Alternatively, the observed variations in (Al-26/Al-27)(0) also indicate the possibility of heterogeneous distributions of Al-26 in the forming region, corresponding to a range of over at least 3.4 x 10(-5) < (Al-26/Al-27)(0) < 5.2 x 10(-5). (C) 2020 Elsevier Ltd. All rights reserved