High 3He/4He ratios sampled at many ocean islands are usually attributed to an essentially undegassed lower-mantle reservoir with high 3He concentrations1,2,3,4. A large and mostly undegassed mantle reservoir is also required to balance the Earth’s 40Ar budget, because only half of the 40Ar produced from the radioactive decay of 40K is accounted for by the atmosphere and upper mantle5. However, geophysical6,7 and geochemical observations8 suggest slab subduction into the lower mantle, implying that most or all of Earth’s mantle should have been processed by partial melting beneath mid-ocean ridges and hotspot volcanoes. This should have left noble gases in both the upper and the lower mantle extensively outgassed, contrary to expectations from 3He/4He ratios and the Earth’s 40Ar budget. Here we suggest a simple solution: recycling and mixing of noble-gas-depleted slabs dilutes the concentrations of noble gases in the mantle, thereby decreasing the rate of mantle degassing and leaving significant amounts of noble gases in the processed mantle. As a result, even when the mass flux across the 660-km seismic discontinuity is equivalent to approximately one lower-mantle mass over the Earth’s history, high 3He contents, high 3He/4He ratios and 40Ar concentrations high enough to satisfy the 40Ar mass balance of the Earth can be preserved in the lower mantle. The differences in 3He/4He ratios between mid-ocean-ridge basalts and ocean island basalts, as well as high concentrations of 3He and 40Ar in the mantle source of ocean island basalts4, can be explained within the framework of different processing rates for the upper and the lower mantle. Hence, to preserve primitive noble gas signatures, we find no need for hidden reservoirs or convective isolation of the lower mantle for any length of time.
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We thank R. O’Connell, A. Dziewonski, C. Langmuir, E. Hellebrand for discussions and D. Graham for a thorough review of the manuscript. H.M.G. was in part supported by the University of Hawaii, SOEST Young Investigator programme. S.M. was in part supported by US National Science Foundation grant EAR 0509721.
Author Contributions H.M.G. and S.M. together developed the ideas presented here. H.M.G. developed the geochemical reservoir model and performed the numerical modelling. H.M.G. and S.M. analysed the results and co-wrote the paper.
This file contains Supplementary Data, Supplementary Figures S1-S5 with Legends, Supplementary Table 1 and Supplementary References.