XENON ISOTOPES IN IRRADIATED AND UNIRRADIATED SAMPLES OF ALLAN HILLS 84001.  J. D. Gilmour, J. A. Whitby, R. D. Ash, and G. Turner, Department of Earth Sciences, Manchester University, Manchester M13 9PL, UK.

Published in Meteoritics, 30, pp. 510-511.

Interior samples .110, .111, .112, altered sample .127, and fusion crust sample .136 from the SNC meteorite ALH 84001 [1] have been studied. Approximately 3 mg of each sample was degassed stepwise in 5-min heating steps on a filament. Temperature was monitored with an optical pyrometer while the chamber pressure was measured with a capacitance manometer. An aliquot (~1% ) of the evolved gas was characterized using a quadrupole mass spectrometer; the remainder was gettered and admitted to the RELAX mass spectrometer [2] for Xe isotopic analysis.

The data obtained from five unirradiated samples are shown in Fig. 1. These data are consistent with a mixing among terrestrial (air) Xe, SPB-Xe [3], and Chassigny-like Xe [4], with a maximum 129Xe/132Xe of 1.92 and a minimum 136Xe/132Xe of 0.313. There is no evidence of the excess (i.e., exceeding that required to be on the SPB-Chassigny mixing line) 129Xe previously attributed to a heterogeneously distributed 129Xe-rich component [5]. This absence could be explained by the mixture of air Xe into even the most 129Xe rich of our extractions or by our smaller samples (3-4 mg vs. 142 mg) having failed to sample this phase.

Samples of .127, .111, and .112 were irradiated with a thermal neutron fluence of 7.22 × 1018 n cm-2. Each showed a marked peak release of excesses of 129Xe and reactor-derived isotopes (128Xe, 131Xe, 136Xe) at high temperature (1500°-1800°C), while the 129Xe/128Xe showed an increase with temperature and was consistent in the largest releases from each sample. (Excesses are calculated assuming 130Xe was derived from a Chassigny component.) Excess 128Xe always accompanied excess 129Xe and the high temperature points (containing 50% of the excess 129Xe evolved) from the three samples define a mixing line with Chassigny. However, more data are needed to establish a true correlation. The fission isotope ratios in the irradiated samples are consistent with neutron-induced fission of 235U, and indicate an abundance of around 20 ppb (total U). Assuming Xe has a 4-b.y. closure age as Ar does [6], the 238U spontaneous fission contribution to a typical high-temperature, SPB Xe-rich extraction is ~1%.

The major release of active gases in each sample (unirradiated and irradiated) was between 600° and 800°C and was composed of CO2 and CO. Evolved gas pressures were converted to equivalent masses of carbonate assuming published Mg-Ca-Fe abundances for this phase [1]; altered sample .127 contained 5× as much carbonate as the mean of the other samples (0.4% vs. 0.08%). The inferred Xe content of the carbonate was significantly above blank in only one unirradiated sample and yielded a carbonate 130Xe content of 6 × 10-10 cc STP/g and 129Xe/132Xe =1.03 ± 0.03, 136Xe/132Xe = 0.33 ± 0.02, indicating that carbonate is not the major carrier of SPB-Xe in these samples. Significant excesses of 128Xe and 131Xe were associated with the CO/CO2 release in the irradiated sample of .127.

References:  [1] Mittlefehldt D. W. (1994) Meteoritics, 29, 214-221. [2] Gilmour J. D. et al. (1994) Rev. Sci. Instrum., 65, 617-625. [3] Swindle T. D. et al. (1986) GCA, 52, 1001-1015. [4] Ott U. (1988) GCA, 52, 1937-1948. [5] Swindle T. D. et al. (1995) GCA, 59, 793-801. [6] Ash R. D. et al., this volume.