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

Abstract

In order to investigate the distribution of ^(26)A1 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 ^(26)Mg (^(26)Mg^*) from the decay of ^(26)A1. The inferred initial ^(26)Al/^(27)Al ratios for these objects ((^(26)Al/^(27)Al)_0 ≅ 5 × 10^(−5)) are indistinguishable from the (^(26)Al/^(27)Al)_0 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 ^(26)Mg^* in two of eleven aluminum-rich chondrules. The (^(26)Al/^(27)Al)_0 ratio inferred for both of these chondrules is ∼1 × 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 (∼0.5 × 10^(−5) to ∼2 × 10^(−5)). The consistency of the (^(26)Al/^(27)Al)_0 ratios for CAIs and chondrules in primitive chondrites, independent of meteorite class, implies broad-scale nebular homogeneity with respect to ^(26)Al and indicates that the differences in initial ratios can be interpreted in terms of formation time. A timeline based on ^(26)Al 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 ^(26)Al as a principal heat source for melting and metamorphism.