Abundance and isotopic composition of noble gases in metal and graphite of the Bohumilitz IAB iron meteorite

Abstract

Abstract-Abundances and isotopic compositions of noble gases in metal and graphite of the Bohumilitz IAB iron meteorite were measured. The abundance ratios of spallogenic components in metal reveal a 3He deficiency which is due to the difhsive loss of parent isotopes, that is, tritium (Tilles, 1963;Schultz, 1967). The diffusive loss likely has been induced by thermal heating by the Sun during cosmic-ray exposure (-1 60 Ma; Lavielle et al., 1999). Thermal process such as impact-induced partial loss may have affected the isotopic composition of spallogenic Ne. The 129Xe/l31Xe ratio of cosmogenic components in the metal indicates an enhanced production of epi-thermal neutrons.The abundance ratios of spallogenic components in the graphite reveal that it contained small amounts of metal and silicates. The isotopic composition of heavy noble gases in graphite itself was obtained from graphite treated with HF/HCl. The isotopic composition of the etched graphite shows that it contains two types of primordial Xe (i.e., Q-Xe and El Taco Xe). The isotopic heterogeneity preserved in the Bohumilitz graphite indicates that the Bohumilitz graphite did not experience any high-temperature event and, consequently, must have been emplaced into the metal at subsolidus temperatures. This situation is incompatible with an igneous model as well as the impact melting models for the IAB-IIICD iron meteorites as proposed by Choi et al. (1995) and Wasson et al. (1980).

I‐Xe ages of Campo del Cielo silicates as a record of the complex early history of theIABparent body

“…The virtual absence of trapped Xe in the minerals studied here suggests that the trapped gases may be concentrated in the graphite inclusions or graphite rims—similar to what was found by Maruoka et al. () and Matsuda et al. ().…”

Noble gas content and isotope abundances in phases of the Saint‐Aubin (UNGR) iron meteorite

“…4. The trend of the excesses in Saint-Aubin troilite is similar to the trend of Great Namaqualand troilite , Cape York troilite (Murty and Marti 1987), and Bohumilitz metal (Maruoka et al 2001). It is likely that the excesses of these fractions are due to epi-thermal neutron capture on 128 Te and 130 Te (E > 0.4 eV; 129 Xe/ 131 Xe = 3.9 ± 0.5; Browne and Berman 1973).…”

The 3He/4He ratio of the new internal He Standard of Japan (HESJ).

“…The 3 He/ 4 He ratio of HESJ is now sufficiently well-determined to be used as an internal standard for measurement of He isotopic ratios among different laboratories. HESJ has already been used as an internal standard in a few investigations (Matsumoto et al, 2001;Maruoka et al, 2001).…”