Robert Hill Nichols

Cosmogenic neon from individual grains of CM meteorites - Extremely long pre-compaction exposure histories or an enhanced early particle flux

Abstract Meteoritic grains which contain solar flare VH ion tracks have clearly been individually exposed to energetic particles prior to assembly. In order to observe the effects of irradiation during the precompaction era, spallation-produced neon has been measured in individual grains, selected by the presence of solar flare VH tracks, from the CM regolith breccias Murchison, Murray, and Cold Bokkeveld. The presence of pre-compaction spallation neon correlates well with the presence of solar flare VH tracks ( Z > 20) and, in this study, detection of SF tracks is the critical parameter used to identify those grains where pre-compaction spallation effects are likely to be present. Only a few percent of the grains (at most) that do not contain solar flare VH tracks contain amounts of cosmogenic Ne larger than would be produced during the conventional cosmic-ray exposure age (and for them the excess is only marginal), whereas most of the grains with solar flare VH tracks contain spallation-produced Ne in significant excess of that due to the nominal cosmic-ray exposure. The magnitude of this excess, which clearly must have been produced prior to compaction, provides evidence for extensive energetic particle exposure during the pre-compaction era. If a contemporary energetic particle complex is assumed (galactic and solar cosmic rays: GCR and SCR), and if production is taken at the maximum present rates, minimum GCR pre-compaction exposure times can be found. The most heavily irradiated grains from Murray and Murchison would require a minimum GCR regolith exposure time of 145 Ma to accumulate the observed cosmogenic Ne. This is the lower limit because it is computed using the peak production rates from the GCR cascade, which occur at roughly 60 g/cm 2 and it requires that the grain spent its entire regolith residence time at that optimum depth. Studies of compaction constraints for CI and CM meteorites suggest that such long regolith residence times may be unlikely. The alternative to such long periods of parent body regolith activity is increased production rates in the early solar system from an enhanced energetic particle environment.

Noble gases in the howardites Bholghati and Kapoeta

Analyses of noble gases in whole rock samples of the howardites Bholghati and Kapoeta and grain-size separates of Kapoeta yield evidence for excesses of the Xe isotopes {sup 129}Xe, {sup 131}Xe, {sup 132}Xe, {sup 134}Xe, and {sup 136}Xe in a low-temperature component, similar to lunar excess fission Xe. Such a component may be able to provide chronometric information if the relative abundances of radioactive progenitors ({sup 129}I, {sup 244}Pu, and {sup 238}U) can be determined, but the isotopic spectra we obtain are not sufficiently precise to do so. Eucritic clast BH-5 in Bholghati contains Xe produced in situ by the decay of {sup 244}Pu. Calculated fission Xe retention ages are 30-70 Ma after the formation of the solar system, consistent with the apparent presence of {sup 146}Sm decay products. Both the clast and the matrix of Bholghati have K-Ar ages of about 2 Ga, suggesting a common thermal event at least that recently.

Xenon and neon from acid-resistant residues of Inman and Tieschitz

Abstract Xenon and neon from highly oxidized acid residues of the unequilibrated ordinary chondrites Inman and Tieschitz are dominated by Xe-HL and Ne-A2, although Xe-S is also present in the oxidized Inman residue. The isotopic composition of Xe released at lower temperatures from the Inman residues most likely represents mixtures of trapped Xe and Xe-HL at all isotopes except 129Xe and 128Xe where significant and correlated excesses are observed. These are clearly both derived from I, the former from decay of extinct 129I and the latter from neutron capture by 127I, requiring a thermal equivalent fluence of ~3 × 1017 n/cm2, or 2–3 × 1016 n/cm2 in the resonance region, confirmed by artificial irradiation of an aliquot. The 129 I 127 I ratio at Xe closure is about 1 × 10−4, similar to the canonical meteoritic value which, given the short half-life of 129I, suggests closure times similar to other meteorites and subsequent irradiation within the solar system. In Tieschitz there may be some evidence for separation of Xe-L from Xe-H as would be expected if the neutron-deficient end of the Xe-HL spectrum is not produced by the same processes as the neutronrich end and is not completely homogenized with it. Ne-E(H) is measured in oxidized residues from both of these meteorites, but no evidence is found for Ne-E(L). Correlations are observed between cosmogenic 21Ne and Ne-E(H) in the higher temperature fractions of both Inman and Tieschitz. These may not be due to a common host phase, but could instead be due to high temperature reactions between spinel and SiC.

Potassium, stardust and the last supernova

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.