Cosmogenic helium and neon in 11 Myr old ultramafic xenoliths: Consequences for mantle signatures in old samples
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 ccS...
Published in: | Geochemistry, Geophysics, Geosystems |
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Main Authors: | , |
Other Authors: | , |
Format: | Article in Journal/Newspaper |
Language: | English |
Published: |
HAL CCSD
2005
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Subjects: | |
Online Access: | https://hal-insu.archives-ouvertes.fr/insu-03601115 https://hal-insu.archives-ouvertes.fr/insu-03601115/document https://hal-insu.archives-ouvertes.fr/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 |
Summary: | 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. |
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