Noble gases helium isotopes

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. 

Recall that fullerenes include spherical C6o carbon molecules ( buckyballs ) whose cavities can trap other atoms such as helium and argon. (See the accompanying figure.) The scientists postulate that the fullerenes originated in the collapsing gas clouds of stars where the noble gas atoms were trapped as the fullerenes formed. These fullerenes were then somehow incorporated into the object that eventually hit the earth. Based on the isotopic compositions, the geochemists estimate that the im-... 

The observed noble-gas abundances and isotopic ratios on Venus are summarized in Tables 3 and 4. The helium mixing ratio is a model-dependent extrapolation of the value measured in Venus upper atmosphere, where diffusive separation of gases occurs. The main differences between Venus and Earth are that Venus is apparently richer in He, Ar, and Kr than the Earth, and the low " Ar/ Ar ratio of — 1.1 on Venus, which is —270 times smaller than on Earth. The low " Ar/ Ar ratio may reflect more efficient solar-wind implantation of Ar in solid grains accreted by Venus and/or efficient early outgassing that then stopped due to the lack of plate tectonics. Wieler (2002) discusses the noble-gas data. Volkov and Frenkel (1993) and Kaula (1999) describe implications of the " Ar/ Ar ratio for outgassing of Venus. 

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). 

Note that storage of helium in the core remains only one component of a noble gas model that can describe the range of noble gas observations. The core has only been evaluated as a possible storage of He. The incorporation in the core of other noble gases, and their relative fractionations, cannot be clearly evaluated without more data. Also, the distribution of radiogenic nuclides such as "" Ar, Xe, and Xe that are produced within the mantle must be explained with a model that fully describes the mantle reservoirs. While these issues may be tractable, a comprehensive model that incorporates a core reservoir remains to be formulated. It should be emphasized that the core does not completely explain the distribution of helium isotopes, since the issue of the " He-heat imbalance is not addressed at all by this model. It appears that even if high He/ He ratios are the signature of involvement of core material in the source of mantle plumes, several mantle reservoirs are still required. 

In particular, the study of noble gas isotopes has provided a compelling case for mantle layering and the preservation of primitive mantle (see Chapter 2.06). For example, He/" He values for MORBs are nearly uniform, but large departures are seen for OIBs. Most oceanic hot spots have elevated He/" He values compared to MORBs. Helium-4 is generated by the decay of uranium... 

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 name comes from the Greek xenon, meaning stranger. Xenon was discovered by William Ramsay (1852-1916) and Morris W. Travers (1872-1961) in 1898 as part of their search for a noble gas between helium and argon. It is present as a trace element in atmospheric air. It is the heaviest of the noble gases. It is used commercially in specialty lamps and lasers, as well as in sophisticated laboratory equipment such as bubble chambers and as a radioactive isotope used as a tracer. 

CAS 7440-59-7. He. Noble element of atomic number 2, first element in the noble gas group of the periodic table, aw 4.00260, valence of 0. Helium nuclei are alpha particles. Most important isotope is helium-3. 

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]. 

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