Helium Isotopic Evidence for a Lower Mantle Component in Depleted Archean Komatiite

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Science  05 Jul 1996:
Vol. 273, Issue 5271, pp. 93-95
DOI: 10.1126/science.273.5271.93


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.