Neon, isotopes

Although Keesom and van Dljk were led to the successful separation of the neon Isotopes through predictions made by Professor Otto Stern based on Eq. (1), Keesom and Haantjes (jS) Interpreted their measurements of the vapor pressure ratio of Ne/ Ne In terms of a 6X/T rather than a temperature 

Herzfeld and Teller (15) showed that a more general form of Eq. (1) could be derived through the use of the Wlgner distribution function. For the vapor pressure isotope effect between a condensed phase and a monatomic vapor they obtained 

Although Equations (1) and (3) can provide significant insight into the behavior of polyatomic molecules, they are Inadequate approximations to account for the role of molecular structure on the isotope chemistry of condensed phases (16). 

ACS Symposium Series American Chemical Society Washington, DC. 1975. 

After he had succeeded In enriching deuterium by the Raleigh distillation of liquid hydrogen, Urey undertook both theoretical and experimental investigations of the differences in the chemistry of protium and deuterium compounds. On the theoretical side Urey and Rittenberg (17) utilized the methods developed for the calculation of the partition function and the free energy of a diatomic molecule from spectroscopic data. For an ideal gas 

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The efficiency of a helium—neon laser is improved by substituting helium-3 for helium-4, and its maximum gain curve can be shifted by varying the neon isotopic concentrations (4). More than 80 wavelengths have been reported for pulsed lasers and 24 for continuous-wave lasers using argon, krypton, and xenon lasing media (111) (see Lasers). 

As detailed earlier, Aston had been faced with this problem for the case of neon where he was not convinced by Thomsons s conclusion that there are two neon isotopes of masses 20 and 22. He attempted to show that indeed neon consists of two isotopes by trying to separate the two isotopes using thermal diffusion. The result proved unsatisfactory and he then proceeded to invent the mass spectrograph. 

Chemical techniques of analysis deal with a very large number of atoms and yield averages over the sample. Once the concept of isotopes was accepted, a search for different isotopes of every element was pursued. The key to the success of this search was the development of a precision instrument that sampled the atoms one at a time. It had been known since the development of the cathode ray tube that positive ions were also produced, and early experiments with these particles revealed singly and doubly charged species of the atoms and molectrles that were contained in the tube. Sir J.J. Thomson observed in 1912 that when neon was the background gas, particles of mass ntrmber 20 and 22 were observed. Attempts to obtain pure samples of the two different atoms by fractionation techniques were unsuccessful, but in retrospect this was because they were both neon isotopes. 

Figure 5.9 shows a neon three-isotope plot (Kennedy et al., 1990) where data were obtained from natural gases and brines in North America representing a broad geographical distribution. A fairly well-defined linear correlation on which air Ne is situated at the upper-left end suggests that the observed neon isotopic data resulted from a mixing between air Ne and another end member Ne characteristic to the crust. It is then reasonable to attribute the latter component to the nucleogenic component produced in the crust. 

Figure 5.9 Neon three-isotope plot for data set from brines in North America. The solid line represents the least-squares fit to the data set, and the solid triangle is the neon isotopic composition in air. After Kennedy et al.

Among the five noble gases, Ne and Xe deserve special attention because their isotopic compositions are unique (as far as we know) to the Earth, suggesting that their evolution processes are fundamentally related to some specific processes of Earth evolution. In Section 7.4, we will discuss Ne in that a key issue is to understand the distinct difference between mantle neon and atmospheric neon isotopic compositions. In Section 7.5, we discuss a long-standing missing Xe problem. 

From Figure 7.10, we can see that if intense EUV radiation had lasted more than 70 Ma, this would account for the observed Ne isotopic fractionation in the atmosphere relative to solar (SW) Ne, regardless of the amount of the C02 content. Although not shown in Figure 7.10, a fractionation in a pure steam atmosphere is little different from those with the second constituent. Figure 7.10 also shows that the existence of the second major constituent C02 in the steam atmosphere would result in a much larger fractionation effect in the argon isotopic ratio than in the neon isotopic ratio. This calculation is made for an assumed steam atmosphere of 270 bars, which corresponds to the mass of the present ocean. Note that the amount of the prim-... 

Dodson, A., Kennedy, B. M., DePaolo, D. J. (1997) Helium and neon isotopes in the Imnaha basalt, Columbia River Basalt Group Evidence for a Yellowstone plume source. Earth Planet. Sci. Lett., 150, 443-51. 

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. 

Nier, A. O., Schlutter, D. J., Brownlee, D. E. (1990) Helium and neon isotopes in deep Pacific Ocean sediments. Geochim. Cosmochim. Acta, 54, 173-82. 

Poreda, R., Radicati di Brozolo, F. (1984) Neon isotope variations in Mid-Atlantic Ridge basalts. Earth Planet. Sci. Lett., 69, 277-89. 

Theory shows that the vapour pressures of ortho- and para- states of diatomic molecules ( 23.IV) should be diflferent, and this has been confirmed experimentally for hydrogen.3 According to Herzfeld and Teller, the heavier isotope may have a lower or higher vapour pressure than the lighter one. For neon isotopes, the vapour pressure is a linear function of the atomic weight. ... 

Figure 3. Column assembly used by Clusius et al. (9) for the separation of the neon isotopes...

Figure 4. Distribution of the 3 neon isotopes along the columns Tj to Tg (9)...

Fain. D.E., and W.K. Brown, 1974, U.S. Atomic Energy Commission Report "Neon isotope separation by gaseous diffusion transport in the transition flow regime with regular geometries."... 

Graphite also contains a Ne-rich component, not obviously associated with other gases and not the same as the Ne-rich G-component in silicon carbide, but which can be identified as the complimentary form neon-E(L) (Amari et al, 1995), renamed the R-component (Table 2). It appears to contain no detectable amounts of the other neon isotopes, and is generally thought to result from in situ decay of Na (half-life 2.6 yr). 

Dixon E. T., Honda M., McDougall 1., Campbell 1. H., and Sigurdsson 1. (2000) Preservation of near-solar neon isotopic ratios in Icelandic basalts. Earth Planet. Sci. Lett. 180, 309 - 324. 

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... 

Neon isotope data from Samoan lavas show elevated e/ Ne relative to atmosphere. The data are not consistent with a model of mixing between a degassed MORE mantle with high e/ Ne and a deeper, undegassed plume source with atmospheric ONe/ Ne. The similarity of neon isotopes between the Samoan plume-Uke source and MORE supports the idea that neon isotopes in the mantle as a whole more closely resemble the solar composition than that of the atmosphere (see Chapter 2.06). Plume-like neon isotopic signatures have been identified in an apatite from a southeastern Australian spinel Iherzolite xenolith (Matsumoto et al, 1997). 

In the solid Earth, production of nucleogenic Ne is coupled to that of radiogenic " He. This is because production of Ne is directly proportional to the a-particle production ratio from the uranium and thorium series. The Ne/ He production ratio is constant and has been estimated at a value of 4.5 X 10 (Yatsevich and Honda, 1997). In this way, if the Earth accreted with solar helium and neon and initial ratios were modified by production of Ne and " He in a fixed proportion then the present-day He/ He and Ne/ Ne ratios in the mantle should be correlated. Honda et al. (1993) noted a strong correlation between OIB helium and neon isotopes such that steeper trajectories in three-isotope neon space were characterized by samples with high He/ He ratios. Indeed, they showed that it was possible to estimate the He/ He ratio of a suite of OIBs based solely on measurements of the neon isotope composition. 

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