POTASSIUM, STARDUST, AND THE LAST SUPERNOVA

Abstract

No isotopic anomalies have yet been reported for K, but the relevant published literature is sparse and error limits for the scarce (0.01% abundance) isotope 40K are relatively large, 0.5 to 1%. We have developed thermal ionization mass spectrometric procedures by which error limits on 40K are an order of magnitude lower and applied them to analysis of a series of sequential dissolution fractions of the carbonaceous chondrites Orgueil (CI) and Murchison (CM), a sampling procedure known to reveal pervasive isotopic anomalies in Cr. Most of the fractions analyzed have 40K abundances that are normal (i.e., consistent with terrestrial composition) within analytical error limits. Whole–rock 40K abundances of Orgueil and Murchison are normal within about 1 permil or less. However, some dissolution fractions do exhibit evident isotopic anomalies, excesses of 40K up to about 35 ϵ. For K, as for Cr, the most plausible interpretation is that the anomalies reflect the presence of presolar grains that have not been thoroughly mixed with other solar system materials. In detail, the K anomalies do not correlate with the Cr anomalies, and thus probably represent different mineral carriers. Neither carrier phase is yet identified, but they differ from known and well-studied forms of presolar grains in that they are not acid-resistant. The isotopes of K are likely co-synthesized with some short-lived radionuclides, notably 26Al, the presence of which in the early solar system demands a “late” nucleosynthetic injection into the interstellar molecular cloud from which the solar system formed, no more than about 1 Ma before its collapse. It has been suggested that the distribution of 26Al (and other short-lived radionuclides) in early solar system materials was radically heterogeneous, perhaps because of the late injection. Because of its relatively short half-life (1.25 Ga), not in the “extinct radionuclides” range but still short compared to the age of the galaxy, 40K provides a usefully sensitive measure of the distribution of late nucleosynthetic additions to the solar system or its antecedent cloud. As a specific quantitative illustration, if a model 25-solar-mass supernova is invoked to account for observed levels of 26Al it will also provide about 1% of nebular 39K and about 3% of nebular 40K; the difference in the proportions of the K isotopes simply reflects the circumstance that by the time of solar system formation most of the 40K ever added to the sun’s precursor materials over the history of the galaxy had already decayed. There would be a 24‰ 40K anomaly between materials that did or did not incorporate such a contribution. The absence of so large a difference between the earth and the carbonaceous chondrites implies that they incorporate nearly the same amounts of any freshly synthesized K component of this magnitude. Unless some efficient mechanism for nebular scale chemical separation is postulated, the same should be true for co-synthesized nuclides such as 26Al.