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Search for 129Xe in Mineral Grains from Allende Inclusions: An Exercise in Miniaturized Rare Gas Analysis

Abstract A static mass spectrometer, modified by installation of a Baur-Signer (Zürich) ion source and an ion-counting system, was used to detect xenon from single mineral grains from inclusions in the C3 meteorite Allende. The grains were melted in small conical heaters wound from tungsten wire. In almost all cases excess 129Xe from 129I decay was detected, in concentrations varying from < 1000 to 600,000 atoms per microgram. Inferred iodine concentrations increased on the average from pentlandite (0.6 ppb), to hedenbergite (21 ppb), to olivine (92 ppb), to enstatite (203 ppb) to melilite (422 ppb), but vary widely from one grain to another of the same mineral, indicating that the iodine resides in some minor phase which is included “spottily” in the bulk phases over which we had good mineralogical control. About 23,000 atoms of fissiogenic 132Xe, presumably from 244Pu decay, was detected in the largest melilite sample analyzed, but we cannot determine from this study whether the 244Pu is “spotty” like 129I or is uniformly distributed in the melilite. One can foresee that if the samples could be loaded into previously outgassed heater cones, a system such as the one described in this paper would have 132Xe blanks of ~ 50,000 atoms. It is likely that in such a system the detectability for excess Xe achieved in this paper (~ 20,000 atoms) could be substantially improved. We also ran St. Severin troilite, which contains less than ~ 33 atoms of excess 129Xe per microgram, and a single chondrule from Alllegan, wich contains higher concentrations of radiogenic xenon than were previously reported by Podosek from runs on a pile-irradiated sample.