Preserving noble gases in a convecting mantle

Noble gases preserved in the 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.