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.,
||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:
||Marty, B. and Craig, H. (1987). "Cosmic-ray-produced
neon and helium in the summit lavas of Maui." Nature,
||Matsuda, J., Murota, M. and Nagao, K. (1990). "He
and Ne isotopic studies on the extraterrestrial material
in deep-sea sediments." J. Geophys. Res.,
||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:
||Ozima, M. and Zashu, S. (1988). "Solar-type Ne
in Zaire cubic diamonds." Geochim. et Cosmochim.
Acta, 52: 19-25.