Noble gases neon isotopes

When an alkali metal atom loses its valence electron, the ion formed is isoelec-tronic with a noble gas atom. (The prefix, iso-, means same, as in isotope.) For example, the electron configuration of sodium is ls 2s 2p 3s or [Ne]35 If the 3 electron is removed, s 2s 2p is left. This is the configuration of the noble gas neon. In each case the alkali metal ion reaches the same configuration as the noble gas just before it in the periodic table. Its highest-energy octet is complete all electron orbitals are filled. This is a highly stable electron distribution. The chemical properties of many... 

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. 

At least three exotic noble gas components have been detected in carbonaceous chondrites. The first component is so-called carbonaceous chondrite fission xenon, which is enriched in the heavy and light isotopes of this remarkable element with nine stable isotopes. The carrier carbon phase is characterised by a 513C = —38%0, and is called carbon-5. The second component is s-process xenon, which is enriched in even-numbered middle isotopes. The carrier carbon phase is characterised by a 8 = +1100%o, and is called carbon-0. The third component is neon-E(L),... 

Niedermann, S., Bach, W., Erzinger, J. (1997) Noble gas evidence for a lower mantle in MORBs from the southern East Pacific Rise Decoupling of helium and neon isotope systematics. Geochim. Cosmochim. Acta, 61, 2697-715. 

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... 

With the exception of Davies, who favored whole-mantle convection all along, the above authors concluded that it was only the upper mantle above the 660 km seismic discontinuity that was needed to balance the continental crust. The corollary conclusion was that the deeper mantle must be in an essentially primitive, nearly undepleted state, and consequently convection in the mantle had to occur in two layers with only little exchange between these layers. These conclusions were strongly reinforced by noble gas data, especially He/ He ratios and, more recently, neon isotope data. These indicated that hotspots such as Hawaii are derived from a deep-mantle source with a more primordial, high He/" He ratio, whereas MORBs are derived from a more degassed, upper-mantle reservoir with lower He/ He ratios. The noble-gas aspects are treated in Chapter 2.06. In the present context, two points must be mentioned. Essentially all quantitative evolution models dealing with the noble gas evidence concluded that, although plumes carry... 

Specific nuclear reactions capable of producing noticeable quantities of noble gas daughters in the Earth ( He and Ne in particular) are initiated by alpha and fission activities of the natural radioelements. Helium-3 is produced through a neutron capture reaction involving Li (HUl, 1941), whereas Ne production occurs through a number of a-induced reactions (Wetherill, 1954). In the case of helium, the He/ He ratio produced is of the order 10 and primarily reflects the lithium abundance at the site of production (Mamyrin and Tolstikhin, 1984). Eor neon, the only conspicuous isotope produced is Ne due to its low natural abundance. The present-day Ne/ He production ratio in the mantle has been calculated at 4.5 X 10 (Yatsevich and Honda, 1997) (see Ballentine and Bumard, 2002 for discussion regarding calculation of this parameter). 

Marty B. and Alle P. (1994) Neon and argon isotopic constraints on Earth-atmosphere evolution. In Noble Gas Geochemistry and Cosmochemistry (ed. J.-l. Matsuda). Terra Scientihc Publishing Company, Tokyo, pp. 191-204. 

An outstanding question is how much of the mantle still maintains high volatile concentrations. This involves resolution of the nature of the high He/" He OIB-source region. Most models equate this with undepleted, undegassed mantle, although some models invoke depletion mechanisms. However, none of these has matched the end-member components seen in OIB lithophile isotope correlations. It remains to be demonstrated that a primitive component is present and so can dominate the helium and neon isotope signatures in OIB. The heavy-noble-gas characteristics in OIB must still be documented. It is not known to what extent major volatiles are stored in the deep Earth and associated with these noble gas components. 

The noble gas geochemistry of natural waters, including formation waters in sedimentary basins, has been used to determine paleotemperatures in the recharge areas, to evaluate water washing of hydrocarbons, and to identify mantle-derived volatiles (Pinti and Marty, 2000). The dissolved noble gases, helium, neon, argon, krypton, and xenon in sedimentary waters, have four principal sources the atmosphere, in situ radiogenic production, the deep crust, and the mantle. These sources have characteristic chemical and isotopic compositions (Ozima and Podosek, 1983 Kennedy et al., 1997). 

The source of the Earth s volatiles In detail there were two sources for the Earth s volatiles. Neon isotopes show that neon at least, and probably all the noble gas inventory, was derived from a solar nebular gas. The second source was chondritic, and most of the COa and HaO were derived through the accretion of chondritic material. However, this is not the full story, for the Earth also experienced volatile loss - the topic of the next section. 

The noble gases krypton, argon and neon have essentially a similar behaviour as observed for xenon. The growth of the lightest noble gas atom helium with the isotopes He and He is different and behaves more like a fermion gas, as has been extensively discussed by Bj0rnholm [97]. 

In this section, we review noble gas systematics of arc-related volcanism worldwide. Helium isotope studies dominate because most arc products are erupted subaerially, and air contamination is a relatively minor (correctable) problem for helium this is not the case for Ne-Ar-Kr-Xe isotope systematics. Consequently, this section is weighted towards reporting observations of helium isotope variations in arc-related minerals and fluids. However, we summarize also the available database for neon, argon and xenon isotopes (todateKr shows only air-like isotopic compositions). Finally, we consider the limited database of the relative abundances of the noble gases in arc-related products. 

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