John F. Wacker

Noble gases in the diamond-free ureilite, ALHA 78019: The roles of shock and nebular processes

Primordial noble gases were measured in the unshocked ureilite ALHA #78019 (hereafter 78019) and in the moderately shocked ureilite Kenna. Much of the gas (> 25%) in 78019 is contained in fine-grained, amorphous carbon, where it is enriched 60-fold relative to that in the more abundant crystalline graphite. Isotopically, the gas in the gas-rich C resembles the planetary component (“phase Q”) in chondrites, but it is not lost upon etching with HNO3 and hence is not surface sited, unlike the chondritic gas. In most other respects, the noble gases are in the normal range for ureilites: bulk 132Xe = 0.30 × 10−8 ccSTP/g; 36Ar132Xe ~ 600 (this value is at the high end for ureilites, however); and trapped 20Ne22Ne and 21Ne22Ne are 10.5 ± 0.7 and 0.032 ± 0.003. These results suggest that the gases—and carbon—predate the shock event that produced the diamond, hence the gas-rich C in 78019 appears to be a good candidate for the hypothetical pre-shock, noble-gas-bearing C in diamondiferous ureilites. Furthermore, the results suggest that the shock event has no bearing on the origin of carbon in these meteorites. More importantly, though, the gas-rich C in 78019 has a 132XeC = 4.2 × 10−10, which falls in the chondritic range, within 30% of C30s and unequilibrated ordinary chondrites. Thus, the gas-rich C in 78019 appears to have acquired its gases while exposed to the solar nebula and the similarity between the 78019 and chondritic 132XeC ratios implies fairly similar P and T at the times and places of trapping. The problem then reduces to explaining how the carbon and its noble gases survived incorporation into the ureilites.

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 ΔH s 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.

Trapping of xenon in ice - Implications for the origin of the earth's noble gases

Although the Earths atmosphere contains Ne, Ar, and Kr in about C1,2-chondrite proportions, Xe is depleted about 20-fold. To test the suggestion that the “missing” Xe is trapped in Antarctic ice, we have measured distribution coefficients for Xe in artifically formed frost at −20 to −60°C, using Xe127 tracer. The values are 0.098 ± 0.004 cc STP/g atm for trapping and < 5 cc STP/g atm for trapping plus adsorption. If these results are representative of natural ice, then the Antarctic ice cap contains less than 1% of the atmospheric Xe inventory, or ≤ 10−3 the amount needed for a C1,2-chondrite pattern. Two possibilities remain for the “missing” Xe, both on the premise that the Earths noble gases, along with other volatiles, came from chondritic material. 1. (1) Xenon is preferentially retained in the mantle and lower crust, due to the strong affinity of Xe for clean silicate surfaces and amorphous carbon. 2. (2)The source material of the Earths volatiles had high, relatively unfractionated, ArXe and KrXe ratios, like the non-carbonaceous noble gas carriers in C30 and E-chondrites.

Accelerator Mass Spectrometry (AMS)

Accelerator mass spectrometry (AMS) is a mass spectrometry technique making use of tandem accelerators and high-charge states for determination of very small isotopic ratios (10−10 to 10−15). The technique takes advantage of the high ion energies achieved and negative ion interferential instabilities to enable extremely low backgrounds to be achieved. AMS developments, instrumentation, and applications are reviewed.

Mass Spectrometric Radionuclide Analyses

Measurement of ionized atoms by mass spectrometry is an alternative to radiation detection for measuring radioactive isotopes. These systems are large and complex; they require trained operators and extensive maintenance. They began as research systems but have been developed commercially for measuring amounts of radioactive isotopes and their atom ratios to other isotopes. Several types of mass spectrometer systems are in use. This chapter covers the basics of mass spectrometry and surveys the application of these instruments for radionuclide detection and discusses the circumstances under which use of mass spectrometers is advantageous, the type of mass spectrometer used for each purpose, and the conditions of sample preparation, introduction and analysis.