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1.
To simulate trapping of meteoritic noble gases by solids, 18 samples of Fe3O4 were synthesized in a noble gas atmosphere at 350–720 K by the reactions: 3Fe + 4H2O → Fe3O4 + 4H2 (Ne, Ar, Kr, Xe) 3Fe + 4CO → Fe3O3 + 4C + carbides (Xe only) Phases were separated by selective solvents (HgCl2, HCl). 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 and atm, but shows only a slight temperature dependence between 650 and 720 K (). The mean distribution coefficient KXe is cc STP/g atm for Fe3O4 and only a factor of greater for C; such similarity for two cogenetic phases was predicted by Lewis et al. (1977). Stepped heating and etching experiments show that 20–50% of the total Xe is physically adsorbed and about 20% is trapped in the solid. The rest is chemisorbed with . The desorption or exchange half-time for the last two components is >102 yr at room temperature.Etching experiments showed a possible analogy to “Phase ” in meteorites. A typical carbon + carbide sample, when etched with HNO3, lost 47% of its Xe but only 0.9% of its mass, corresponding to a ~0.6 Å layer. Though this etchable, surficial gas component was more thermolabile than (release below 1000°C, compared to 1200–1600°C), another experiment shows that the proportion of chemisorbed Xe increases upon moderate heating (1 hr at 450°C). Apparently adsorbed gases can become “fixed” to the crystal, by processes not involving volume diffusion (recrystallization, chemical reaction, migration to traps, etc.). Such mechanisms may have acted in the solar nebula, to strengthen the binding of adsorbed gases.Adsorbed atmospheric noble gases are present in all samples, and dominate whenever the noble gas partial pressure in the atmosphere is greater than that in the synthesis. Many of the results of Lancet and Anders (1973) seem to have been dominated by such an atmospheric component; others are suspect for other reasons, whereas still others seem reliable. When the doubtful samples of Lancet and Anders are eliminated or corrected, the fractionation pattern—as in our samples—no longer peaks at Ar, but rises monotonically from Ne to Xe. No clear evidence remains for the strong temperature dependence claimed by these authors. 相似文献
2.
We studied trapping of noble-gases by chromite and carbon: two putative carriers of primordial noble gases in meteorites. Nineteen samples were synthesized in a Ne-Ar-Kr-Xe atmosphere at 440 K to 720 K, by the following reactions: Fe,Cr + 4H2O → (Fe,Cr)3O4 + 4H2 (1) or Fe,Cr + 4CO → (Fe,Cr)3O4 + 4C + carbides (2)The reactant metal films were prepared either by vacuum evaporation of alloy or by thermal decomposition of Fe- and Cr-carbonyls. The products—including Fe3O4, Cr2O3, carbides, and unreacted metal—were partially separated by selective solvents, such as HCl, H2SO4?H3PO4, or HClO4. Samples were characterized by XRD, SEM, and atomic absorption; noble gases were measured by mass spectrometry. Surface areas, as measured by the BET method, were 2 to 100 m2/g.All samples are dominated by an adsorbed noble gas component that is largely released upon heating at ?400°C or slight etching. Elemental abundance patterns show that this component is derived from the highest-pressure noble gas reservoir seen by the sample—atmosphere or synthesis vessel—indicating that desorption or exchange rates at room T are slow on the time scale of our experiments (up to 1 year). Adsorptive capacity is reduced by up to 2 orders of magnitude upon light etching with HClO4 (though the surface area actually doubles in this treatment) and, less drastically, by heating. Apparently some active adsorption sites are destroyed by these treatments. A trapped component (typically 30% of the total) is readily detectable only in samples synthesized at partial pressures close to or greater than atmospheric.Noble gas contents roughly obey Henry's Law, but show only slight, if any, correlations with composition, surface area, or adsorption temperature. (Geometric) mean distribution coefficients for bulk samples and HCl-residues are, in 10?3 cc STP/g atm: Xe (100), Kr (15), Ar (3.5), Ne (0.62). Elemental fractionations are large and variable, but are essentially similar for the adsorbed and trapped components, or for chromite and carbon. They bracket the values for the corresponding meteoritic minerals.
Geom. mean | 0.006 | 0.035 | 0.15 |
Range | 0.0004-0.03 | 0.01-0.2 | 0.06-0.4 |