Sorption of noble gases by solids, with reference to meteorites

To simulate trapping of meteoritic noble gases by solids, 18 samples of Fe3O4 were synthesized in a noble gas atmosphere at 350 to 720 K. Phases were separated by selective solvents. Noble gas contents were analyzed by mass spectrometry, or, in runs where 36 d Xe127 tracer was used, by γ-counting. Surface areas, as measured by the BET method, ranged from 1 to 400 m2/g. Isotopic fractionations were below the detection limit of 0.5%/m.u. Sorption of Xe on Fe3O4 and C obeys Henry's Law between 1 x 10-8 and 4 x 10-5 atm, but shows only a slight temperature dependence between 650 and 720 K. The mean distribution coefficient Ksub(Xe) is 0.28 +- 0.09 cc STP/g atm for Fe3O4 and only a factor of 1.2 +- 0.4 greater for C. Stepped heating and etching experiments show that 20 to 50% of the total Xe is physically adsorbed and about 20% is trapped in the solid. The rest is chemisorbed with ΔHsub(s) approximately = -13 kcal/mole. The desorption or exchange half-time for the last two components is > 102 yr at room temperature. The results are discussed. (author)