Xenon solar abundance

For krypton and xenon abundances were derived from computer hts of aN (neutron capture cross-section times abundance) versus mass number. Nuclei that are shielded from the r-process, so-called s-only nuclei, were used for the ht and the abundances of Kr and Xe were calculated. From these data, and the isotopic composition of the solar wind, the krypton and xenon elemental abundances were calculated (Palme and Beer, 1993) and are listed in Table 1. The meteorite data given in Table 1 will be discussed in a later section. 

Noble gases and nitrogen in martian meteorites reveal several interior components having isotopic compositions different from those of the atmosphere. Xenon, krypton, and probably argon in the mantle components have solar isotopic compositions, rather than those measured in chondrites. However, ratios of these noble gas abundances are strongly fractionated relative to solar abundances. This decoupling of elemental and isotopic fractionation is not understood. The interior ratio in martian meteorites is similar to chondrites. 

FIGURE 5.6 Major volatile element and noble gas abundances in the outer Earth reservoir of Kramers (2003) and in carbonaceous chondrites relative to Al and solar abundances. The data show that apart from xenon the Earth and chondritic meteorites have similar element distribution patterns and that both are strongly depleted in the noble gases and in H, C, and N relative to solar abundances. 

The Zag meteorite fell in the western Sahara of Morocco in August 1998. This meteorite was unusual in that it contained small crystals of halite (table salt), which experts believe formed by the evaporation of brine (salt water). It is one of the few indications that liquid water, which is essential for the development of life, may have existed in the early solar system. The halite crystals in the meteorite had a remarkably high abundance of 128Xe, a decay product of a short-lived iodine isotope that has long been absent from the solar system. Scientists believe that the iodine existed when the halite crystals formed. The xenon formed when this iodine decayed. For this reason, the Zag meteorite is believed to be one of the oldest artifacts in the solar system. In this lab, you will use potassium-argon radiochemical dating to estimate the age of the Zag meteorite and the solar system. 

Swindle TD, Podosek FA (1988) Iodine-Xenon dating. In Meteorites and the Early Solar System. Kerridge JF and Matthews MS (eds) University of Arizona Press, Tucson, p 1114-1146 Tang M, Lewis RS, Anders E (1988) Isotopic anomalies of Ne, Xe, and C in meteorites. I. Separation of carriers by density and chemical resistance. Geochim Cosmochim Acta 52 1221-1234 Tera F, Eugster O, Burnett DS, Wasserburg GJ (1970) Comparative study of Li, Na, K, Rb, Cs, Ca, Sr and Ba abundances in achondrites and in Apollo 11 lunar samples. Geochim Cosmochim Acta Suppl 1 1637-1657... 

Noble gases may provide a constraint on the source of water and other volatiles. The abundance pattern of noble gases in planetary atmospheres resembles that of chondrites, perhaps arguing against comets. However, there are some differences, especially in the abundance of xenon. Relative to solar system abundances, krypton is more depleted than xenon in chondrites, but in the planets, krypton and xenon are present in essentially solar relative abundances (Fig. 10.11). This observation has been used to support comets as the preferred source of volatiles (even though measurements of xenon and krypton in comets are lacking). A counter-argument is that the Ar/H20 ratio in comets (if the few available measurements are accurate and representative) limits the cometary addition of volatiles to the Earth to only about 1%. 

For most of the chemical elements, the relative abundances of their stable isotopes in the Sun and solar nebula are well known, so that any departures from those values that may be found in meteorites and planetary materials can then be interpreted in terms of planet-forming processes. This is best illustrated for the noble gases neon, argon, krypton, and xenon. The solar isotopic abundances are known through laboratory mass-spectrometric analysis of solar wind extracted from lunar soils (Eberhardt et al., 1970) and gas-rich meteorites. Noble gases in other meteorites and in the atmospheres of Earth and Mars show many substantial differences from the solar composition, due to a variety of nonsolar processes, e.g., excesses of " Ar and... 

Wider R., Kehm K., Meshik A. P., and Hohenberg C. M. (1996) Secular changes in the xenon and krypton abundances in the solar wind recorded in single lunar grains. Nature 384, 46-49. 

Figure 5 The relative abundances of noble gases in gas-rich MORE (Moreira et al, 1998), as calculated for the upper mantle (see text), air (Tables 2 and 3), and in the solar composition (see Ozima and Podosek, 2001). The upper mantle is enriched in neon and xenon, relative to argon, compared to the air composition.

Hagee et al., 1990) calculated values of (4-7) X 10 2, with the higher number considered more likely to represent the solar value, although a lower value remains a possibility. The amount produced by U in the bulk silicate Earth (7.5 X 10 2 atoms Xe) is much less, and so bulk silicate Earth (and atmospheric) 2 Xe is dominantly plutonium-derived even if xenon was lost over the first 10 a of Earth history (see Chapter 4.12) so that half of the plutonium-derived xenon was lost, plutonium-derived Xe is still 13 times more abundant in the Earth. 

The martian mantle has high xenon concentrations and distinct abundance patterns. Martian meteorites contain components other than those derived directly from the atmosphere (see detailed discussion by Swindle, 2002). In particular, noble gases in the dunite meteorite Chassigny appear to represent a distinct interior reservoir. The " Kr/i Xe ratio of 1.2 (Ott, 1988) is lower than both the martian atmosphere (20) and solar (16.9) values, but is similar to that of Cl chondrites. If this is truly a source feature, it indicates that heavy noble gases trapped within the planet suffered substantially different elemental fractionation than the atmosphere. The interior " Kr/ Ar ratio of 0.06 is much higher than the solar value of 2.8 X 10 ", but it is close to the atmospheric value of 0.02 and so does not display the same contrast as the Kr/Xe ratio. Unfortunately, it is not possible to determine if the measured elemental abundance ratios were modified by planetary processing or transport and incorporation into the samples. 

The isotopic composition of the martian interior is only available for xenon. Data for the dunite Chassigny indicate that there is a mantle reservoir with nonradiogenic isotope ratios that appear to be indistinguishable from solar values (Ott, 1988 Mathew and Marti, 2001), and so does not exhibit the strong isotopic fractionation seen in the atmosphere. The relative abundances of Xe and Xe are also close to solar, indicating that this reservoir had a high Xe/Pu and Xe/I ratios, at least during the lifetime of " Pu. Data from other meteorites indicate that there are other interior martian reservoirs that contain solar xenon but with... 

The principal difficulty encountered by these mixing models is their inability to account for differences in nonradiogenic noble-gas isotopic distributions between Earth and Mars, and between both of these and solar compositions. Experiments specifically designed to investigate isotopic fractionation in the gas-trapping process showed maximum heavy-isotope enrichments which are too small to explain the observed offsets of martian Ar/ Ar and of xenon on both Mars and Earth from solar ratios (Notesco et al., 1999). Also, models that rely on supply of noble gases from material trapped in the outer parts of the solar system cannot explain the abundances of... 

Xenon in chondritic metal. Marti et al. (1989) have identified a xenon component (FVM-Xe) in a metal separate of the Forest Vale (H4) chondrite which appears to be distinct from xenon identified in other solar system reservoirs. It is characterized by relative abundances of the heaviest isotopes with unusually high " Xe. A possible e mlanation is recoil of fission fragments into the metal grains, possibly from Pu, " Tm or neutron induced fission of (Marti et al. 1989). This suggestion is consistent with the observed grain size dependence of FVM which favors a near-surface location. 

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