Al-Mg Isotopic Constraints on the Condensation Age of Fine-Grained Refractory Inclusions in Reduced Cv3 Chondrites

Fine-grained Ca-Al-rich inclusions (FGIs) in carbonaceous chondrites are interpreted as aggregates of direct condensates from the solar nebula that escaped extensive melting. Thus far, a few high-precision SIMS Al-Mg chronology studies of FGIs from CV3 chondrites have been reported. [2,3] argued that (26Al/27Al)0 of all FGIs are close to the bulk refractory inclusion value of ~5.2×10−5, which may define a time zero for the FGI condensation. A similar (26Al/27Al)0 value was also inferred from an amoeboid olivine aggregate (AOA), which also represents a direct condensate from the solar nebula. In contrast, [4] reported significant variations in (26Al/27Al)0 of FGIs, ranging from ~5.2×10−5 to ~3.4×10−5, and concluded that their formation continued over at least ~0.4 Ma after the canonical value. These contrasting results suggest that the chronological significance for the formation age spread of FGIs in CV3 chondrites is still poorly constrained.

Building on our preliminary SIMS study, we report Al-Mg isotopic data of seven FGIs from reduced CV3 chondrites to better establish the distribution of (26Al/27Al)0 of FGIs relative to coarse-grained, igneous CAIs (CGIs) in CV3 chondrites. We also analyzed two fluffy Type A CAIs (FTAs), which are widely believed to be the product of nebular condensation. The overall objective of our isotopic study was to provide additional chronological constraints on condensation processes in the early solar nebula.