Global ³He/⁴He isotopic systematics in oceanic basalts and implications of their origin for Hawaii

Anders Meibom

Laboratoire d'Etude de la Matiere Extraterrestre, USM 205 (LEME), Case Postale 52, Museum National d'Histoire Naturelle, 61 rue Buffon, 75005 Paris

Summary

1. A comparison of He isotopes and the major lithophile element isotope systems (i.e. Pb, Sr, and Nd) in oceanic basalts world wide: There are no clear correlations between He isotopes and any of the other isotope systems on oceanic basalts world wide. He isotopes seem largely decoupled from Pb, Sr, and Nd (although Eiler et al. have shown He-Pb isotope correlations for subsets of the Hawaiian basalts).
2. Local examples, e.g. Iceland, of decoupling between He and the conventional "plume" indicators, such as Sr, Pb, and La/Sm: On the Reykjanes Ridge He isotopes are doing the exact opposite of what would be expected if He with high ³He/⁴He ratios was spreading out from underneath Iceland, where a plume is envisaged to be situated. Galapagos is another local example of decoupling between the classical plume indicators and He isotope systematics.
3. Large temporal variations occur, e.g. Samoa, Kauai, sometimes on timescales as short as 100 years, which do not appear to be consistent with the conventional model of a primordial, undegassed lower mantle He reservoir.
4. As an alternative to this classical model I discussed the "olivine as He time capsules" model in which olivine crystals overgrow CO₂-rich bubbles (which contain a lot of He), and thereby isolate He from U and Th, especially once olivine-rich lithologies are formed in the upper mantle. Remelting of these olivine crystals at some later time and release of unradiogenic He is one way to account better for the lack of correlation with Pb, Sr, and Nd isotopes, the large local He isotope variations (on small length scale) and fast temporal variations in He isotope composition in single volcanoes.

Discussion

The question of how high the ³He/⁴He ratio in the atmosphere was back in time was raised. The answer to this seems to be that originally the ³He/⁴He of the atmosphere was ~ 120-300 times the present-day atmospheric ratio (Ra), judging from the current composition of Jupiter's atmosphere (which hasn't changed much) and data from meteorites. How old does recycled olivine have to be, then, to give the high ³He/⁴He values of 25-40 Ra, as measured in Hawaii and Iceland? (These compare with typical values of 8 Ra for MORB.) A few hundred million years, perhaps. There was speculation concerning how this could be tested. It was suggested that the ³He/⁴He in olivine from dunite cumulates in ophiolites might be measured, if sufficiently unweathered samples could be obtained.