Manuel Moreira

The first isotopic data and concentrations of helium are reported for the Pacific‐Antarctic ridge between 52.5°S and 41.5°S. The ⁴He/³He ratio is extremely homogeneous over more than 1200 km, with a mean ratio of 99,275 (R/Ra = 7.29) and a standard deviation of 2719 (0.19), which is the lowest dispersion observed for the global mid oceanic ridge system. Moreover, the Menard T.F. is a frontier between two mantles with slightly different helium isotopic ratios (96,595 ± 1520 and 100,347 ± 2330). No difference in the helium concentration between the two ridge segments defined by the Menard T.F. can be observed, as well as no significant difference in the U and Th contents suggesting that the difference in helium isotopic ratio is old (>500 My) and may represent a slight difference in degassing or/and trace element depletion history.

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.

New experimental data on the isotopic variations of neon, argon, and xenon in a popping rock imply that the ⁴⁰ Ar/ ³⁶ Ar ratio of the upper mantle is less than 44,000 and that the ¹²⁹ Xe/ ¹³⁰ Xe ratio is less than 8.2. The elemental abundance pattern of rare gases is chondritic-like and is quite distinct from the solar pattern. These data imply that Earth accreted from planetesimals that probably underwent a transformation of their rare gas budget from solar- to chondritic-like, leaving the isotopic composition unchanged from the solar pattern.