Constraining the origin of high 3He/4He mantle: hypothesis testing using a multi-isotope approach.

Basalts erupted at the mid ocean ridges, where new ocean crust is forming, have a very uniform isotopic composition of the element helium (He). By contrast, basalts erupted at oceanic islands (e.g. Hawaii and Iceland) have much more variable He isotope composition and are often enriched in 3He. The two isotopes of He have very different origins. 3He is mostly 'primordial' i.e. it was incorporated from outer space when the Earth formed. 4He is mostly radiogenic, the product of alpha-particles and electrons released during radioactive decay of U and Th within the Earth. Thus, high 3He/4He has been viewed as evidence for a contribution from primordial material in the ocean island basalts. A widely accepted model for the origin of ocean island volcanoes places them above rising plumes of hot rock that originate deep in the Earth. The association of high 3He/4He with these 'mantle plumes' encouraged the suggestion of a deep reservoir containing primordial He. Mass balance models suggest that 1/2 to 1/3 of the mantle is required to balance the continental crust and atmosphere. The upper mantle, above a prominent discontinuity at 650 km depth observed from earthquake seismic data, represents about 1/3 of the mantle. Thus a standard model emerged in which the upper mantle is depleted in crust-forming elements and degassed of He while the upper mantle is undepleted and undegassed with primordial characteristics. This standard model makes predictions that have not been supported by data collected over the past 10-15 years and the evidence for a layered Earth with a primordial lower mantle has increasingly been challenged. A variety of alternative sources of high 3He/4He have been suggested, including: 1. The Earth's outer core sampled by mantle plumes originating at the bottom of the mantle. 2. Ancient depleted mantle that has remained isolated from mantle convection for 1-2 billion years or more. 3. A very early Earth differentiation event recorded by the daughter products of a radioactive isotope that is now extinct. 4. A minor component of He-rich, possibly primordial, mantle that when tapped by magmas dominates the He isotope composition of mixtures but is only a minor component for the other isotopic tracers. Using a variety of isotope techniques, many of which have only been developed in the last few years, we are able to propose a programme of isotopic analyses that will test these various suggestions. These hypothesis-testing measurements will constrain the origin of high 3He/4He and thereby clarify the significance of one of the most enigmatic geochemical tracers. Our results will significantly influence understanding of the deep Earth, the generation of magmas and the evolution of the planet.