Noble gases in the howardites Bholghati and Kapoeta

Abstract

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 ¹²⁹Xe, ¹³¹Xe, ¹³²Xe, ¹³⁴Xe, and ¹³⁶Xe 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 (¹²⁹I, ²⁴⁴Pu, and ²³⁸U) 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 244Pu. Calculated fission Xe retention ages are 30-70 Ma after the formation of the solar system, consistent with the apparent presence of ¹⁴⁶Sm 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.