HELIUM ISOTOPIC EVIDENCE FOR A LOWER MANTLE COMPONENT IN DEPLETED ARCHEAN KOMATIITE

Archean magnesium-rich komatiites require hot and presumably deep mant le sources, but their trace-element composition and radiogenic isotope composition are similar to those of modern mid-ocean ridge basalts, w hich 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 o f magnitude, between a radiogenic end-member rich in helium-4 and a co mponent rich in helium-3, Such helium-3 enrichment suggests the presen ce of a lower mantle component in Archean komatiites.