Sorption of noble gases by solids, with reference to meteorites. II. Chromite and carbon

Abstract

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 + 4H 2O → (Fe,Cr) 3O 4 + 4H 2 (1) or Fe,Cr + 4CO → (Fe,Cr) 3O 4 + 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 Fe 3O 4, Cr 2O 3, carbides, and unreacted metal—were partially separated by selective solvents, such as HCl, H 2SO 4−H 3PO 4, or HClO 4. 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 m 2/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 HClO 4 (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. Ne Xe Ar Xe Kr Xe Geom. mean 0.006 0.035 0.15 Range 0.0004-0.03 0.01-0.2 0.06-0.4 These data largely support the suggestion of Lewis et al. (1977) that chromite and carbon in C2 and C3 chondrites were formed by reaction (2) in the solar nebula. Reaction (2) indeed is somewhat faster than (1); it yields ferrichromite (with part of the Cr replaced by Fe 3+); part of the carbon apparently is carbyne, judging from the thermal release of C 2-C 5 chains at 320 to 600°C; chromite and carbon trap substantial and essentially similar amounts of noble gases; and the gases show an elemental fractionation pattern resembling that for meteoritic chromite and carbon. However, in contrast to the meteoritic minerals, the synthetic materials show no isotopic fractionation of noble gases (⋍ 1%/dalton).