Are He isotopes decoupled from other geochemical tracers in the deep Earth?

Lead Research Organisation: Scottish Universities Environmental Research Centre (SUERC)
Department Name: SUERC

Abstract

Basalts erupted at the mid-ocean ridges, where new ocean crust is forming, have a very uniform isotopic composition of the gas 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 released during radioactive decay of U and Th in 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 of all the noble gases (including Ar and Xe) 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. Recent results from basalts from the Canadian Arctic and Greenland suggest that He isotopes may not trace the mantle source at all, and that the Earth's mantle may indeed be nearly completely degassed. Using a variety of isotope techniques, many of which have only been developed in the last few years, and some spectacular basalt samples collected inaccessible parts of northern Ethiopia we are able to propose a programme of isotopic analyses that will test this suggestion. Our results will significantly influence understanding of the deep Earth, the generation of magmas and the evolution of the planet.

Publications

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Description That the earlieat basalts erupted by the Ethiopia mantle plume are chemically distinct from the Iceland plume and do no appear to originate in deep mantle.
Exploitation Route Publication of results
Sectors Environment