Noble gases cosmic abundance

Figure 7.1 shows a noble gas elemental abundance relative to 36Ar for the Earth atmosphere, Q, SW, and lunar soils [cf. Table 3.2,3.3(a), and 3.3(b)]. We also included the supposed Martian atmospheric noble gas (e.g., Pepin, 1991). The abundances are normalized to the solar (cosmic) abundance. 

Weigel A., Eugster O., Koeberl C., Michel R., Krahenbiihl U., and Neumann S. (1999) Relationships among lodranites and acapulcoites Noble gas isotopic abundances, chemical composition, cosmic-ray exposure ages and solar cosmic ray effects. Geochim. Cosmochim. Acta 63, 175-192. 

Noble gas isotopes are also produced through irradiation by cosmic rays. These rays are mostly high-energy protons that produce a cascade of secondary particles when they bombard other target nuclei, in a process called spallation. Neon produced by spallation reactions has similar abundances of all three isotopes (Fig. 10.8). Cosmic-ray irradiation occurs on the surfaces of airless bodies like the Moon and asteroids, as well as on small chunks of rock orbiting in space. Using these isotopes, it is possible to calculate cosmic-ray exposure ages, as described in Chapter 9. 

This scarcity is probably the most important single feature to remember about noble-gas cosmo-chemistry. As illustration of the absolute quantities, for example, a meteorite that contains xenon at a concentration of order 10 cm STP g (4 X 10 mol g ) would be considered relatively rich in xenon. Yet this is only 0.6 ppt (part per trillion, fractional abundance 10 ) by mass. In most circumstances, an element would be considered efficiently excluded from some sample if its abundance, relative to cosmic proportions to some convenient reference element, were depleted by several orders of magnitude. But a noble gas would be considered to be present in quite high concentration if it were depleted by only four or five orders of magnitude (in the example above, 10 ° cm STP g of xenon corresponds to depletion by seven orders of magnitude), and one not uncommonly encounters noble-gas depletion of more than 10 orders of magnitude. 

DOS, Argon. Ar at. wt 39,948 at- no. 18. Three stable isotopes 36 (0.337%) 38 (0.063%) 40 (99.600%) artificial, radioactive isotopesr 33 35. 37 39 4] 42. Abundance in earth s crust 4 X 10 % concentration in the atmosphere 0.93% by vol cosmic abundance 1.5 X 10 atom M0 atoms of Si. Elemental, monoatomic, gaseous constituent Of air, discovered by Rayleigh and Ramsay in 1894. Although molecular ions, hydrates and cl at h rates of argon have been observed, it should be considered a noble , chemically inert gas, due to its electronic structure. The outer p subshell is entirely filled ls22s42p63s23p6. Obtained commercially... 

Variations in isotopic abundances that are caused by nuclear reactions induced by cosmic rays are most commonly utilized in cosmic ray exposure dating, but this employs isotopes that are measured by either accelerator or noble gas mass spectrometry [28, 29]. In fact, there are only a very limited number of elements that are suitable for the study of cosmogenic isotopic variations, which can be readily analyzed by either TIMS or MC-ICP-MS [28]. The most important application of these techniques are studies of the secondary neutron fluxes that are generated by (primary) cosmic rays. Such measurements aim to detect anomalies in Sm, Gd, and Cd isotopic abundances that are produced by (n,y) reactions, for example " Cd(n, y) Cd. Many of these investigations were conducted by TIMS [137-139], but some cosmogenic Cd isotope variations of lunar rocks and soUs were evaluated based on MC-ICP-MS isotope ratio data that were originally acquired as part of a stable isotope study [134]. 

Figure 6 Elemental abundance patterns for trapped noble gases in various planetary materials. For each gas identified on the abscissa, the ordinate shows the depletion factor in a given sample, i.e., the gas concentration in the sample divided by what the concentration would be if the gas were present in undepleted cosmic proportion (normalized for a nominal rock with 17% Si). The relative elemental abundances in the left panel illustrate the solar pattern, those in the right panel the planetary pattern. The vertical broken lines for each gas illustrate typical in situ gas concentrations (the radiogenic component for " He, spallation for the others), below which it becomes progressively more difficult to characterize or even identify trapped components (source Ozima and Podosek, 2002).

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