Cosmogenic helium and neon in 11 Myr old ultramafic xenoliths: Consequences for mantle signatures in old samples

Manuel Moreira (1) , Pedro Madureira (1)

1 IPGP - Institut de Physique du Globe de Paris

The helium and neon isotopic compositions of olivines coming from a 11 Myr old xenolith sampled at Mt. Hampton (West Antarctica) were analyzed by crushing. The helium isotopic ratio varies between 1340 and 6300 (R/Ra between 115 and 539) with ⁴He content around 3-8 10^-10 ccSTP/g, confirming that cosmogenic helium can be extracted by crushing. The neon also shows a clear cosmogenic origin (²⁰Ne/²²Ne down to 7.7 and ²¹Ne/²²Ne > 0.32), indicating that the cosmogenic neon can also be extracted by crushing out of the olivines. Melting of the powder left after the crushing experiment gives a ⁴He/³He ratio as low as 42 ± 8 (R/Ra = 17,300) and ²¹Ne/²²Ne as high as 0.78, close to the cosmogenic production end-member. This study shows that up to ∼0.5% of cosmogenic helium and neon can be extracted by crushing. In this way, for samples that had been exposed to cosmic rays for a long time (e.g., a few Myr), a crushing procedure may not give the mantle ratios without ambiguity, and measurement of neon can help to discriminate between cosmogenic and mantle origin of the ³He.

Keywords: cosmogenic; crushing; helium; mantle; neon

Domains: Sciences of the Universe [physics]

Manuel Moreira, Pedro Madureira. Cosmogenic helium and neon in 11 Myr old ultramafic xenoliths: Consequences for mantle signatures in old samples. Geochemistry, Geophysics, Geosystems, 2005, 6, pp. 337-338. ⟨10.1029/2005GC000939⟩. ⟨insu-03601115⟩