Aluminum-26 in calcium-aluminum-rich inclusions and chondrules from unequilibrated ordinary chondrites

In order to investigate the distribution of Al-26 in chondrites, we measure d aluminum-magnesium systematics in four calcium-aluminum-rich inclusions ( CAIs) and eleven aluminum-rich chondrules from unequilibrated ordinary chon drites (UOCs). All four CAIs were found to contain radiogenic Mg-26 (Mg-26* ) from the decay of Al-26 The inferred initial Al-26/Al-27 ratios for these objects ((Al-26/Al-27)(o) approximate to 5 x 10(-5)) are indistinguishable from the (Al-26/Al-27)(o) ratios found in most CAIs from carbonaceous chon drites. These observations, together with the similarities in mineralogy an d oxygen isotopic compositions of the two sets of CAIs, imply that CAIs in UOCs and carbonaceous chondrites formed by similar processes from similar ( or the same) isotopic reservoirs, or perhaps in a single location in the so lar system. We also found Mg-26* in two of eleven aluminum-rich chondrules. The (Al-26/Al-27)(o) ratio inferred for both of these chondrules is simila r to1 x 10(-5), clearly distinct from most CAIs but consistent with the val ues found in chondrules from type 3.0-3.1 UOCs and for aluminum-rich chondr ules from lightly metamorphosed carbonaceous chondrites (similar to0.5 x 10 (-5) to similar to2 x 10(-5)). The consistency of the (Al-26/Al-27)(o) rati os for CAIs and chondrules in primitive chondrites, independent of meteorit e class, implies broad-scale nebular homogeneity with respect to Al-26 and indicates that the differences in initial ratios can be interpreted in term s of formation time. A timeline based on Al-26 indicates that chondrules be gan to form 1 to 2 Ma after most CAIs formed, that accretion of meteorite p arent bodies was essentially complete by 4 Ma after CAIs, and that metamorp hism was essentially over in type 4 chondrite parent bodies by 5 to 6 Ma af ter CAIs formed. Type 6 chondrites apparently did not cool until more than 7 Ma after CAIs formed. This timeline is consistent with Al-26 as a princip al heat source for melting and metamorphism.