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 measured aluminum-magnesium systematics in four calcium-aluminum-rich inclusions (CAIs) and eleven aluminum-rich chondrules from unequilibrated ordinary chondrites (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 chondrites. These observations, together with the similarities in mineralogy and 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 solar 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 similar to1 x 10(-5), clearly distinct from most CAIs but consistent with the values found in chondrules from type 3.0-3.1 UOCs and for aluminum-rich chondrules 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) ratios for CAIs and chondrules in primitive chondrites, independent of meteorite class, implies broad-scale nebular homogeneity with respect to Al-26 and indicates that the differences in initial ratios can be interpreted in terms of formation time. A timeline based on Al-26 indicates that chondrules began to form 1 to 2 Ma after most CAIs formed, that accretion of meteorite parent bodies was essentially complete by 4 Ma after CAIs, and that metamorphism was essentially over in type 4 chondrite parent bodies by 5 to 6 Ma after 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 principal heat source for melting and metamorphism.