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).