Cosmogenic neon produced from sodium in meteoritic minerals

Abstract

Cosmogenic neon in sodium-rich oligoclase feldspar from the ordinary chondrites St. Severin and Guareña is characterized by an unusually high ^(22)Ne/^(21_Ne = 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 of ^(21)Ne per gram sodium is one quarter the production rate per gram magnesium. The striking enrichment of ^(22)Ne relative to ^(21)Ne in sodium arises from enhanced indirect production from ^(23)Na via ^(22)Na. The unusual composition of cosmogenic neon in sodium and sodium-rich minerals explains the high ^(22)Ne/^(21)Ne 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 this ^(22)Ne-enriched cosmogenic neon from meteoritic feldspar can result in a 2–3% drop in the measured cosmogenic ^(22)Ne/^(21)Ne 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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