Laboratory simulation of meteoritic noble gases. III. Sorption of neon, argon, krypton, and xenon on carbon: Elemental fractionation

Abstract

In a continuing effort to understand the origin of trapped noble gases in meteorites, the sorption of Ne, Ar, Kr, and Xe was studied in carbon black, acridine carbon, and diamond. Twenty-seven samples were exposed to a mixture of Ne, Ar, Kr, and Xe for 24 hours, at temperatures between 23 and 410°C and total pressures from 0.00088 to 0.078 atm. Loosely sorbed noble gases were pumped away, and the remaining tightly bound gases were measured mass spectrometrically. All samples trapped large amounts of gas. Desorption times for the tightly bound gases were 35 and 20 hours for Ar, Kr, and Xe at 23 and 105°C, respectively. Apparent enthalpies ( ΔH) of adsorption for gases sorbed on carbon black were −2 to − 8 kcal/mol, consistent with physical adsorption. Both the ΔHs and elemental fractionation patterns match those obtained from experiments where the sorbed phase is analyzed during gas/solid equilibrium. Surprisingly, Ne concentrations in most samples equalled or exceeded those for Ar, Kr, or Xe, indicating significant solubility. The Ne diffusion coefficient at 23°C in carbon black was 5 · 10 −17 cm 2/sec. The sorption behavior strongly depended on the type of carbon, and, for acridine carbon, upon heat treatment T. The new results corroborate a model in which gases are physically adsorbed on interior surfaces formed by a pore labyrinth within amorphous carbons. The new data show that 1) the adsorption/desorption times are controlled by choke points that restrict the movement of noble gas atoms within the pore labyrinth, and 2) physical adsorption controls the temperature behavior and elemental fractionation patterns. Ne and, by analogy, He are trapped by solubility. These results strongly support the hypothesis that surface-sited planetary components ( e.g., phase Q) are trapped by physical adsorption on interior surfaces of carbon grains, at temperatures between 300 and 400 K in the solar nebula. The observation that Ne diffuses at temperatures relevant to the solar nebula implies that diffusive fractionation produced some of the isotopic variation in trapped Ne and He.