AN ION MICROPROBE STUDY OF CORUNDUM IN THE MURCHISON METEORITE - IMPLICATIONS FOR AL-26 AND O-16 IN THE EARLY SOLAR-SYSTEM

Twenty-six Al2O3 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, Al2O3 retains a particularly durable record of the early solar system. Mg-26/Mg-24 ranges up to 56 x the solar-system ratio, owing to decay of extinct Al-26, but the initial Al-26/Al-27 ratios do not exceed the canonical maximum of 5 x 10(-5) established in earlier work. There is no evidence for fossil radiogenic Mg-26 surviving from presolar times. Oxygen isotope compositions cluster mainly near delta-O-18 = -50 parts per thousand (lighter than bulk spinel), but range from -94 to -11 parts per thousand. The grains divide into three groups on the basis of Al-26, O-16, Ti, and V content, and Al-26 and O show distinctive correlations (in contrast to all previous studies), suggesting an origin from the following components. Group 1 (high Al-26, Ti, V): mixture of material with Al-26/Al-27 = 5 x 10(-5) and delta-O-18 = -45 parts per thousand with dead Al of delta-O-18 almost-equal-to -100 parts per thousand. Group 2 (low Al-26, Ti, V): mixture of material with Al-26/Al-27 = 5 x 10(-6) with dead Al, with complex fractionation and exchange of O resembling that of FUN inclusions. Group 3 (no Al-26; high Ti, V): dead Al from various sources. In terms of this model, the corundum formed from two components with live Al-26 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.