Helium Isotopic Evidence for a Lower Mantle Component in Depleted Archean Komatiite
Archean magnesium-rich komatiites require hot and presumably deep mantle sources, but their trace-element composition and radiogenic isotope composition are similar to those of modern mid-ocean ridge basalts, which originate in the upper mantle. The isotopic composition of helium extracted by sequential crushing of fresh olivines separated from two Archean and one mid-Proterozoic komatiites varies over three orders of magnitude, between a radiogenic end-member rich in helium-4 and a component rich in helium-3. Such helium-3 enrichment suggests the presence of a lower mantle component in Archean komatiites.
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D. Richard, B. Marty, M. Chaussidon, CNRS, Centre de Recherches Petrographiques et Geochimiques, Rue Notre-Dame des Pauvres, B.P. 20, 54501 Vandoeuvre Cedex, and Ecole Nationale Superieure de Geologie, 94 Avenue De Lattre de Tassigny, 54001 Nancy Cedex, France. N. Arndt, Geosciences, Unite Propre de Recherches 4661 CNRS, Universite de Rennes, 35042 Rennes Cedex, France.
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