An ion microprobe study of corundum in the Murchison meteorite: Implications for 26A1 and 16O in the early solar system

Abstract

Twenty-six A1 2O 3 grains from the Murchison CM2 chondrite have been analyzed by ion microprobe mass spectrometry for the isotopes of O, Mg, and Ti and the abundances of Mg, Ca, Sc, Ti, V, Sr, Y, Zr, La, and Ce. Being the most refractory major phase in solar matter, A1 2O 3 retains a particularly durable record of the early solar system. 26Mg 24Mg ranges up to 56× the solar-system ratio, owing to decay of extinct 26A1, but the initial 26Al 24Al ratios do not exceed the canonical maximum of 5 × 10 −5 established in earlier work. There is no evidence for fossil radiogenic 26Mg surviving from presolar times. Oxygen isotope compositions cluster mainly near δ 18 O = −50%. (lighter than bulk spinel), but range from −94 to −11%.. The grains divide into three groups on the basis of 26A1, 16O, Ti, and V content, and 26Al and O show distinctive correlations (in contrast to all previous studies), suggesting an origin from the following components. Group 1 (high 26A1, Ti, V): mixture of material with 26Al 24Al = 5 × 10 −5 and δ 18 O = −45%. with dead Al of δ 18 O ≈ −100%.. Group 2 (low 26A1, Ti, V): mixture of material with 26Al 27Al = 5 × 10 −6 with dead Al, with complex fractionation and exchange of O resembling that of FUN inclusions. Group 3 (no 26A1; high Ti, V): dead Al from various sources. In terms of this model, the corundum formed from two components with live 26Al and a mass fraction of 43% dead Al, but we do not know whether this figure is typical of carbonaceous chondrites in toto, let alone the entire solar nebula. Trace element abundances in corundum are generally at less than Cl levels relative to Al, and decline with increasing volatility, from Zr to Ca.