Neon

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Periodic Table--Neon

Neon has three isotopes: 20Ne (90.48%), 21Ne (0.27%) and 22Ne (9.25%). 21Ne and 22Ne are nucleogenic and their variations are well understood. In contrast, 20Ne is not known to be nucleogenic, and the causes of its variation in the Earth have been hotly debated. The principal nuclear reactions which generate neon isotopes are n,Ðo reactions on 24Mg and 25Mg, which produce 21Ne and 22Ne, respectively. The Ðo particles are derived from U-series decay chains, while the neutrons are mostly produced by secondary reactions from Ðo particles. The net result yields a trend towards lower 20Ne/22Ne and higher 21Ne/22Ne ratios observed in uranium-rich rocks such as granites. Isotopic analysis of exposed terrestrial rocks has demonstrated the cosmogenic production of 21Ne (Marty and Craig, 1987). This isotope is generated by spallation reactions on Mg, Na, Si and Al. By analyzing all three isotopes, the cosmogenic component can be resolved from magmatic neon and nucleogenic neon. This suggests that neon will be a useful tool in determining cosmic exposure ages of surficial rocks.

Similar to xenon, neon (Craig and Lupton, 1976) contents observed in samples of MORB and volcanic gases are enriched in 20Ne, as well as nucleogenic 21Ne, relative to 22Ne contents. The neon isotopic contents of these mantle-derived samples represent a non-atmospheric source of neon. The 20Ne-enriched components were attributed to exotic primordial rare gas components in the Earth, possibly representing solar neon. Elevated 20Ne abundances were also found in diamonds (Honda et al., 1987; Ozima and Zashu, 1988, 1991), further suggesting a solar neon reservoir in the Earth.

References

* Allegre, C.J., Sarda, P. and Staudacher, T. (1993). "Speculations about the cosmic origin of He and Ne in the interior of the Earth." Earth Planet. Sci. Lett., 117: 229-233.

* Black, D.C. (1972)." On the origins of trapped helium, neon and argon isotopic variations in meteorites, II. Carbonaceous chondrites." Geochim. et Cosmochim. Acta, 36: 377-394.

* Craig, H. and Lupton, J.E. (1976). "Primordial neon, helium, and hydrogen in oceanic basalts." Earth