Cosmogenic helium and neon extracted by crushing: A technique for discriminating between mantle and cosmogenic helium

Abstract

The helium and neon isotopic compositions of olivines coming from a 11Ma old xenolith sampled at Mt. Hampton (West Antarctica) were analyzed by crushing and heating. The 4He/3He isotopic ratio obtained by crushing varies between 1340 and 6300 (R/Ra between 115 and 539) with 4He content around 3-5 10-10 ccSTP/g confirming that cosmogenic helium can be extracted by crushing [Scarsi, 2000; Yocochi et al., 2004]. The neon also shows a clear cosmogenic origin (20Ne/22Ne down to 7.7 and 21Ne/22Ne>0.32) indicating that some cosmogenic neon can also be extracted by crushing out of the olivines. This result indicates that for samples that had been exposed for a long time (e.g. few Ma to Ga), a step crushing procedure may not give the mantle ratios without ambiguity and that measurement of neon can discriminate between cosmogenic and mantle origin of the 3He. Melting of the powder left after the crushing experiment gives 4He/3He ratio as low as 51±5 (R/Ra=14 230) and 21Ne/22Ne as high as 0.78, close to the cosmogenic end-member. Our results show that ~0.4% of the cosmogenic helium and ~0.3% of the cosmogenic neon can be extracted out of olivines by crushing