26Al in CAIs and chondrules from unequilibrated ordinary chondrites

Details 26Al in CAIs and chondrules from unequilibrated ordinary chondrites 26Al in CAIs and chondrules from unequilibrated ordinary chondrites

Jul 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001m%26ps...36..975h&link_type=abstract

Meteoritics & Planetary Science, vol. 36, no. 6, p. 975-997

Computer Science

65

Scientific paper

In order to investigate the distribution of 26Al 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 26Mg (26Mg*) from the decay of 26Al. The inferred initial 26Al/27Al ratios for these objects ((26Al/27Al)o 5x10-5) are indistinguishable from the (26Al/27Al)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 26Mg* in two of eleven aluminum-rich chondrules. The (26Al/27Al)o ratio inferred for both of these chondrules is ~1x10-5, 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 (~0.5x10-5 to ~2x10-5). The consistency of the (26Al/27Al)o ratios for CAIs and chondrules in primitive chondrites, independent of meteorite class, implies broad-scale nebular homogeneity with respect to 26Al and indicates that the differences in initial ratios can be interpreted in terms of formation time. A timeline based on 26Al indicates that chondrules began to form one to two million years after most CAIs formed, that accretion of meteorite parent bodies was essentially complete by four million years after CAIs, and that metamorphism was essentially over in type 4 chondrite parent bodies by five to six million years after CAIs formed. Type 6 chondrites apparently did not cool until more than seven million years after CAIs formed. This timeline is consistent with 26Al as a principal heat source for melting and metamorphism.

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