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Cosmogenic neon produced from sodium in meteoritic minerals
Authors:SP Smith  JC Huneke
Institution:The Lunatic Asylum of the Charles Arms Laboratory of the Geological Sciences, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, Calif. USA
Abstract:Cosmogenic neon in sodium-rich oligoclase feldspar from the ordinary chondrites St. Severin and Guaren?a is characterized by an unusually high22Ne/21Ne = 1.50 ± 0.02. This high ratio is due to the cosmogenic22Ne/21Ne production ratio in sodium which is 2.9 ± 0.3, two to three times the production ratio in any other target element. The relative production rate of21Ne per gram sodium is one quarter the production rate per gram magnesium. The striking enrichment of22Ne relative to21Ne in sodium arises from enhanced indirect production from23Na via22Na.The unusual composition of cosmogenic neon in sodium and sodium-rich minerals explains the high22Ne/21Ne ratios observed in inclusions of the Allende carbonaceous chondrite, and observed during low-temperature extraction of neon from ordinary chondrites. The isotopic composition of cosmogenic neon released during the stepwise heating of a trapped gas-rich meteorite containing sodium-rich phases can be expected to vary, and use of a constant cosmogenic neon composition to derive the composition of the trapped gas may not be justified. Preferential loss of this22Ne-enriched cosmogenic neon from meteoritic feldspar can result in a 2–3% drop in the measured cosmogenic22Ne/21Ne ratio in a bulk meteorite sample. This apparent change in composition can lead to overestimation of the minimum pre-atmospheric mass of the meteorite by a factor of two.
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