Preserving noble gases in a convecting mantle

High 3He/4He ratios sampled at many ocean islands are traditionally attributed to an essentially undegassed lower mantle reservoir with high primordial 3He concentrations. A large and mostly undegassed mantle reservoir is also thought to be required to balance the Earth's 40Ar budget. On the other hand, geophysical and geochemical observations suggest slab subduction into the lower mantle, implying that most of the Earth's mantle should have been processed by partial melting over the history of the Earth, leaving noble gases in both upper and lower mantle extensively outgassed. Helge Gonnermann and Sujoy Mukhopadhyay propose a simple solution to this contradiction, which lies with the recycling and mixing of noble-gas-depleted slabs. Such mixing 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, over the 4.5 billion years of Earth's history, the lower mantle can preserve high 3He contents, high 3He/4He ratios and 40Ar concentrations high enough to satisfy the 40Ar mass balance of the Earth, even with a substantial mass flux from the upper to lower mantle. An essentially undegassed mantle reservoir is thought to be required to balance the Earth's 40Ar budget and an undegassed lower mantle reservoir is traditionally believed to be responsible for the high 3He/4He ratios sampled at many ocean islands. On the other hand, geophysical and geochemical observations suggest slab subduction into the lower mantle; the subsequent process of partial melting over the history of the Earth would leave noble gases in both upper and lower mantle extensively outgassed. A simple solution to this contradiction is now proposed, which lies in the recycling and mixing of noble-gas-depleted slabs. 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.