Trapping of rare gases during the condensation of solids
Abstract
Rare gas trapping during crystallization from vapor phases of (1) CdTe, (2) Zn, (3) Mg and (4) Fe3O4 has been studied. Samples were deposited as very fine crystals (about several hundred angströms) in ambient Ar atmospheres of various pressures. It was found that the amount of Ar trapped in the samples was proportional to the ambient Ar pressure. Stepheating degassing of the crystals showed that Ar was rather loosely trapped and released at relatively low temperatures. However, on a simple mechanical compaction of the crystal powder the retentivity of Ar was considerably enhanced.
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Cited by (7)
Noble gas studies in vapor-growth diamonds: Comparison with shock-produced diamonds and the origin of diamonds in ureilites
1991, Geochimica et Cosmochimica ActaWe synthesized vapor-growth diamonds by two kinds of Chemical Vapor Deposition (CVD) using microwave (MWCVD) and hot filament (HFCVD) ionization of gases, and examined elemental abundances and isotopic compositions of the noble gases trapped in the diamonds. It is remarkable that strong differences existed in the noble gas concentrations in the two kinds of CVD diamonds: large amounts of noble gases were trapped in the MWCVD diamonds, but not in the HFCVD diamonds. The heavy noble gases (Ar to Xe) in the MWCVD diamonds were highly fractionated compared with those in the ambient atmosphere, and are in good agreement with the calculated fractionation patterns for plasma at an electron temperature of 7000–9000 K. These results strongly suggest that the trapping mechanism of noble gases in CVD diamonds is ion implantation during diamond growth. The degrees of fractionation of heavy noble gases were also in good agreement with those in ureilites. The vapor-growth hypothesis is discussed in comparison with the impact-shock hypothesis as a better model for the origin of diamonds in ureilites. The diamond (and graphite, amorphous carbon, too) may have been deposited on early condensates such as Re, Ir, W, etc. This model explains the chemical features of vein material in ureilites; the refractory siderophile elements are enriched in carbon and noble gases and low in normal siderophiles. The vapor-growth model is also compatible with the oxygen isotopic data of ureilites which suggests that nebular processes are primarily responsible for the composition of ureilites.
<sup>40</sup>Ar/<sup>39</sup>Ar laser dating of a single grain of magnetite
1990, TectonophysicsWe have determined an 40Ar/39Ar age spectrum for a single grain of magnetite using laser step-heating. A 300 μm magnetite grain from a Grenville Front gneiss was heated in 14 steps to 1600°C. Excess 40Ar∗ resulted in a saddle-shaped age spectrum with an integrated age of 1962 ± 39 Ma. However, the minimum of the saddle gave an age of 1144 ± 107 Ma, consistent with typical Grenvillian cooling ages of 1100–900 Ma. We have also dated a single grain of biotite from the same hand sample as the magnetite. The spectrum yielded an integrated age of 1261 ± 3.6 Ma. The thermal release patterns of 40Ar∗ and of Ca/K (37Ar/39Ar) were very different for the magnetite and biotite. Thus 40Ar∗ can be retained in measurable quantities in magnetite. Our results are a first step toward the attempted direct dating of different components of NRM in paleomagnetism.
Three-element correlation plots are used to assess the possible origin of C, N, and the noble gas elements in the atmosphere of Venus from various two-component mixtures and from elemental fractionation of solar gases during solar wind implantation and during low temperature adsorption. An origin of Venus' atmosphere by elemental fractionation of implanted solar wind gases into pre-Venus materials is approximately consistent wit Ar, Kr, Xe, and N, but does not readily explain the higher abundance of C and the lower abundance of Ne. Some simple two-component mixtures seem consistent with most of the data, especially a mixture of a Earth-like component with a lunar-like component that represents fractionated solar wind gases. Simple two-component mixtures with one component resembling volatiles in C1 chondrites are not consistent with the data and suggest that a significan fraction of these volatiles on Venus could not have been contributed by a C1-like veneer. None of these models can readily explain the low Ne/Ar ratio for Venus. Even lower Ne/Ar ratios observed in some meteorites suggest, however, that some mechanism existed in the inner Solar System to produce large Ne/Ar fractionation relative to the solar value. Preferential adsorption of Xe on C- and N-rich phases forming at low temperatures from the solar nebula may have caused the observed larger concentrations of C, N, and Xe relative to Ar, Kr, and Ne in carbonaceous meteorites compared to the atmospheres of the terrestrial planets. If so, the apparent deficiency of Xe relative to other noble gases for the Earth, Mars, and Venus compared to carbonaceous meteorites would not be due to hidden Xe in each planet, as has been previously suggested.
Sorption of noble gases by solids, with reference to meteorites. I. Magnetite and carbon
1982, Geochimica et Cosmochimica ActaTo 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.
Noble-gas-rich separates from the Allende meteorite
1981, Geochimica et Cosmochimica ActaPredominantly carbonaceous. HF/HCl-resistant residues from the Allende meteorite were studied before and after applying physical and chemical separation methods. Samples were characterized by SEM/EDXA. X-ray diffraction. INAA. C (including isotopic composition). S. H. N and noble gas analyses. Isotopic data for carbon showed little variation (≲ 5‰); isotopic data for noble gases confirmed previously established systematics (Frick, 1977; Alaertset al., 1980). In our samples, noble gas abundances correlated with those of C and N but not with those of S and metals; concomitant partial loss of C and ‘normal’ trapped gas occurred during treatments with oxidizing acids; and total gas loss accompanied combustion of carbon. In addition, HF/HCl demineralization of bulk meteorite resulted in similar fractional losses of C and trapped noble gases. These results lead us to conclude that various macromolecular carbonaceous substances serve as
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-the main host phase (‘Q’) for ‘normal’ trapped noble gases in acid-resistant residues from Allende.
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-the main host phase for ‘anomalous’ gas in the acid-resistant residues, and
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-the carrier of the major part of trapped noble gases lost during HF/HCl demineralization (‘solubles’).
At present little information exists concerning what fraction of the carbonaceous material in Allende consists of noble gas host phases; and the possibility is not yet ruled out that very minor amounts of dense non-carbonaceous minerals also could act as trapped gas carriers, especially if they were inseparable from the lighter carbonaceous matter by our methods. Limits on the possible abundances of dense mineralic host phases in the residues have been obtained, however; if Q were an Fe/Cr mineral or a sulfide. its abundance has to be more than an order of magnitude lower than inferred previously from etching experiments (Anderset al., 1975; gros and Anders, 1977). By analogy with C1 and C2 meteorites, where carbonaceous phases play host to noble gases with anomalies of nucleogenetic origin (Alaertset al., 1980). we also favor a nucleogenetic origin for CCF-XE. These considerations hold open the possibility that carbonaceous host phases and various forms of organic matter in carbonaceous meteorites may have had a presolar origin.
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Vapor-growth carbon and the origin of carbonaceous material in ureilites
1997, Geochemical Journal