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.