THE CORE AS A POSSIBLE SOURCE OF MANTLE HELIUM

Abstract

The core has often been suggested as a source for He presently found in the mantle. If the core served as the long-term storage reservoir for He in ocean island basalts with distinctively high 3He/4He ratios, then the difficulties of maintaining an isolated mantle reservoir separate from the source of mid-ocean ridge basalts are removed. However, the possibilities of trapping He into the core and releasing it into the overlying mantle have not yet been systematically evaluated. Here some of the factors to be considered are discussed. Appealing to the core as a source of rare gases necessarily evokes specific conditions of terrestrial rare gas acquisition and core formation, as well as core composition characteristics. It is shown that even if partition coefficients between silicates (solid or melt) and liquid Fe are low, there may have been sufficient gas present in the mantle during core segregation to supply a substantial quantity of He and Ne to the core. Transfer from the core to the mantle by either bulk entrainment of core material or chemical interaction at the core-mantle boundary may provide a reasonable mechanism for supplying relatively unfractionated rare gases to plumes. However, this remains speculative. While this process may have considerable impact on other trace elements, further limits on the rate of such transfer are only possible once further constraints are available on the concentration of other elements in the outer core. There are presently insufficient data available to establish whether or not the core is a plausible source of mantle He with high 3He/4He ratios. Therefore, further systematic investigation of this possibility should be conducted.