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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,à reactions on 24Mg and 25Mg, which produce 21Ne and 22Ne, respectively. The à particles are derived from U-series decay chains, while the neutrons are mostly produced by secondary reactions from à 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.

Source of text: This review was assembled by Eric Caldwell, primarily from Dicken (1995).

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 Planet. Sci. Lett., 31: 369-385.
Dicken, A.P. (1995). Radiogenic Isotope Geology. Cambridge University Press, New York, 452 p.
Farley, K.A. and Poreda, R.J. (1993). "Mantle neon and atmospheric contamination." Earth Planet. Sci. Lett., : 325-339.
Honda, M., Mc Dougall, I. and Patterson, D.B. (1993). "Solar noble gases in the Earth: The systematics of helium-neon isotopes in mantle-derived samples." Lithos, 30: 257-265.
Honda, M., Reynolds, J.H., Roedder, E. and Epstein, S. (1987). "Noble gases in diamonds: occurrences of solar-like helium and neon." J. Geophys. Res., 92: 12507-12521.
Kennedy, B.M., Hiyagon, H. and Reynolds, J.H. (1990). "Crustal neon: a striking uniformity." Earth Planet. Sci. Lett., 98: 277-286.
Marty, B. (1989). "Neon and Xenon isotopes in MORB: implications for the Earth-atmosphere evolution." Earth Planet. Sci. Lett., 94: 45-56.
Marty, B. and Craig, H. (1987). "Cosmic-ray-produced neon and helium in the summit lavas of Maui." Nature, 325: 335-337.
Matsuda, J., Murota, M. and Nagao, K. (1990). "He and Ne isotopic studies on the extraterrestrial material in deep-sea sediments." J. Geophys. Res., 95: 7111-7117.
Ozima, M. and Zashu, S. (1991). "Noble gas state of the ancient mantle as deduced from noble gases in coated diamonds." Earth, Planet. Sci. Lett., 105: 13-27.
Ozima, M. and Zashu, S. (1988). "Solar-type Ne in Zaire cubic diamonds." Geochim. et Cosmochim. Acta, 52: 19-25.
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