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Published March 1, 1979 | Published
Journal Article Open

Calcium isotopic anomalies and the lack of aluminum-26 in an unusual Allende inclusion


We have studied the Mg and Ca isotopic compositions of an unusual Allende inclusion dominated by hibonite, which is the most refractory and possibly the most primitive major oxide mineral. No ^(26)Mg excess was found in spite of the high ^(27)Al/^(24)Mg (1 ≳ 10^3) of some samples, indicating an initial (^(26)Al/^(27)Al)_0 < 2 X 10^(-7), a factor of 250 less than found in some other Allende inclusions. The upper limit for Mg isotopic fractionation is 20%o per amu. Anomalous but uniform Ca isotopic compositions were found for bulk samples of coexisting phases and microscopic grains. The Ca anomaly is a superposition of a large mass-dependent fractionation effect of 7.5‰ per amu favoring the heavy isotopes and small (1‰-2‰) "nonlinear" effects of presumably nuclear origin. If the lack of ^(26)Al is due to a time delay of 6 X 10^6 yr for the formation of the hibonite inclusion, then condensation models require modification. The Ca effects suggest the alternative that ^(26)Al was not uniformly distributed in the solar system. These results accentuate the curious and unexplained association between large mass fractionation and nuclear effects. They also reinforce the scenario which envisages an early solar system consisting of isotopically and chemically distinct reservoirs resulting from the incomplete mixing of several nucleosynthetic components. It is not evident whether these components originated within the solar system or from another star.

Additional Information

© 1979 American Astronomical Society. Received 1978 October 31; accepted 1978 November 28. We thank E. Anders and R. Lewis for help in the excavation of HAL D. A. Papanastassiou's gracious aid with the mass spectrometry and his sagacious suggestions were essential to the completion of this work. T. L. gratefully acknowledges the warm hospitality from the inmates of the Lunatic Asylum while he was a Guest Investigator there and acknowledges the support from R. N. Clayton and T. K. Mayeda of the Fermi Institute. This work was supported in part by NSF grant 76-83685 and NASA grant NGL 05-002-188 at the California Institute of Technology and also by funds from the Robert R. McCormick Trust and NSF grant EAR 74-19038 at the University of Chicago.

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Published - 1979ApJ___228L__93L.pdf


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