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

• Published in: Geochemistry, Geophysics, Geosystems
• Main Authors: Moreira, Manuel, Madureira, Pedro
• Other Authors: Institut de Physique du Globe de Paris (IPGP), Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-Université de La Réunion (UR)-Institut de Physique du Globe de Paris (IPG Paris)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2005
• Subjects: cosmogenic; crushing; helium; mantle; neon; [SDU]Sciences of the Universe [physics]; Hampton; West Antarctica; Antarc*; Antarctica
• Online Access: https://insu.hal.science/insu-03601115; https://insu.hal.science/insu-03601115/document; https://insu.hal.science/insu-03601115/file/Geochem%20Geophys%20Geosyst%20-%202005%20-%20Moreira%20-%20Cosmogenic%20helium%20and%20neon%20in%2011%20Myr%20old%20ultramafic%20xenoliths%20Consequences%20for.pdf; https://doi.org/10.1029/2005GC000939

International audience 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 4 He 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 ( 20 Ne/ 22 Ne down to 7.7 and 21 Ne/ 22 Ne > 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 4 He/ 3 He ratio as low as 42 ± 8 (R/Ra = 17,300) and 21 Ne/ 22 Ne 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 3 He.