Xenon and krypton in the Bruderheim meteorite

Abstract

In a series of heating experiments, xenon, radiogenic Xe129R, and krypton contents, and the xenon and krypton isotopic composition of the Bruderheim meteorite were studied for the separated minerals feldspar, pyroxene, olivine, and troilite and for numerous chondrule fractions. Important differences among the individual minerals and between minerals and chondrules were observed, and the following conclusions were reached: 1. There are wide variations in the Xe129/Xe132 ratios in xenon from the minerals and from chondrules, and variations of this quantity are observed as a function of heating temperature. These variations, which are not caused by atmospheric contamination, are further evidence that Xe129 anomalies in Bruderheim phases and chondrules result from in situ decay of I129. 2. Relative to the total meteorite, chondrules are depleted in primordial xenon and frequently enriched in Xe129R. The matrix material, in contrast, shows correlated xenon and Xe129R contents, the content of Xe129R varying nearly quadratically with the content of primordial xenon. 3. Because both xenon and Xe129R contents are highest in minerals of highest diffusion constants and lowest activation energies, there appears not to have been gross xenon diffusion since the decay of I129. The observed differences in xenon between chondrules and the matrix minerals argue against a simple mechanism of formation of chondrules directly from the matrix minerals, or the reverse. 4. Fractionation in the release of the four heaviest xenon isotopes from most minerals and chondrules occurs as a function of temperature, and correlations among the anomalies are consistent with the identification of this effect as resulting from the release of a fission-xenon component. If now-extinct Pu244 is the source by spontaneous fission, Pu244-Xe136 formation intervals from zero to 144 m.y. are deduced for chondrules; there is, however, 3 times too much of this suspected fission component in the bulk meteorite to be caused by Pu244 or U238 spontaneous fission after isolation of the solar nebula from the interstellar reservoir, based on elemental abundances calculated by Cameron. 5. Comparison of the release, from neutron-irradiated samples, of Xe129R versus pile-produced Xe128* from iodine or Xe131* from tellurium and barium favors an iodine-Xe129R correlation, but a tellurium-Xe129R correlation cannot be ruled out unequivocally. The galactic synthesis model for I129 yields an I-Xe129 formation interval of 58 m.y. for a single chondrule versus a questionable value of 34 m.y. for bulk Bruderheim. This discrepancy conflicts with the other evidence for an early origin for chondrules and raises some questions about the validity of the I-Xe129 dating method. 6. The unusually high spallation-type anomalies in krypton and xenon from chondrules in Bruderheim are probably due in part to recent cosmic rays. 7. The observed differences between chondrules and matrix material cannot be the result of processes acting after the meteorites formed. They reflect differences extant at the time of formation and argue for the presence of two or more types of primordial xenon. Possibly, a high-temperature component was associated with chondrules, and a lower-temperature component with the matrix minerals. If this identification is correct, it provides support for the hypothesis that chondrules represent a vapor-liquid precipitation from the primitive solar nebula and that matrix minerals represent a vapor-solid condensate. A possible explanation for differences between chondrules and matrix, consistent with the above hypothesis and permitting all solids to be equally irradiated during or after formation, is that during subsequent cooling chondrules were effectively isolated from the gas phase in the nebula by virtue of their larger size, whereas the fine-grained solids remained in equilibrium with the gases and I129 down to a relatively low temperature.