The role of plumes in mantle helium fluxes

We present a simple model of ³He and ⁴He transport in the mantle using the appropriate rates of mass and species transfer and ⁴He production. Previous workers have shown the presence of excess ³He in hotspots such as Hawaii and Iceland and inferred that these hotspots tap a source with a higher ³He ⁴He ratio than the source region of mid-ocean ridge basalts (MORB). Hotspot ocean islands probably originate over upwelling plumes which carry material from the lower mantle to the upper mantle. Melting at hotspots and at mid-ocean ridges degasses the mantle of volatiles such as helium. The upper mantle is outgassed largely of helium due to melting at mid-ocean ridges and hotspots. We postulate that the excess ³He seen in MORB originates in material that was carried from the lower mantle in plumes but not completely outgassed at hotspots. This helium is incorporated into the depleted upper mantle. Assuming that the upper mantle is in a quasi-steady-state with respect to helium, a simple model balancing ³He and ⁴He fluxes in the upper mantle indicates that the hotspots significantly outgas the lower mantle of ³He. The concentration of ⁴He in the plume source reservoir is 2-3 orders of magnitude lower than the concentration in carbonaceous chondrites. The residence time of helium in the upper mantle depends on the outgassing efficiency at hotspots, since the hotspots may outgas some upper mantle material which has been entrained in the plumes. The residence time of He in the upper mantle is about 1.4 × 10 ⁹ yr. We conclude that the efficiency of outgassing of He from plumes is high and that the plumes dominate the present ³He loss to the atmosphere. The ⁴He in the less depleted layer of the mantle is not trapped "primordial" but is predominantly from in situ decay of U and Th in the depleted layer over ∼ 1.4 × 10 ⁹ yr. The ⁴He in the lower mantle is dominantly from in situ decay of U and Th over 4.4 × 10 ⁹ yr. This model appears to explain the total Ar in the atmosphere as there is about the same amount of K in the depleted mantle as is in the continents.