Helium in the Archean komatiites revisited: significantly high 3He/4He ratios revealed by fractional crushing gas extraction

In order to provide constraints on 3He/4He ratios in the Archean mantle source, we have analyzed helium isotopic compositions in 2.7 Ga old Archean komatiites from the Abitibi green stone belt, Ontario, Canada. Two spinifex-textured komatiites yielded significantly high 3He/4He ratios of about 30 Ra (where Ra denotes the atmospheric 3He/4He ratio) in fractions released by sequential crushing. These results are the first confirmation of the occurrence of high 3He/4He ratios in Archean komatiites after the intriguing finding by Richard et al. [Science 273 (1996) 93–95] in komatiites from a nearby locality, Alexo. We also found that the crystal structure of the komatiites was significantly enriched in a radiogenic component (4He) and that this 4He was actually degassed by crushing gas extraction, indicating that the nominal 3He/4He ratios measured by crushing are lower limits for the 3He/4He ratio of the intrinsic component. By constraining the release behavior of radiogenic 4He by crushing, we have estimated the initial 3He/4He ratio of the inclusion-trapped component to be 73.0+7.8−5.5 Ra. A mantle source with such a high 3He/4He ratio at 2.7 Ga, if evolved in a closed system, would have a present-day 3He/4He ratio of 46–60 Ra, indicating that the komatiites from Munro have trapped their helium from a mantle reservoir with a very high 3He/4He ratio in the context of the present-day value. However, whether or not such a source can be considered as equivalent to the primitive mantle source (such that sampled at hotspots) is highly model-dependent. If a closed system evolution model is assumed, helium in the Munro komatiites is not likely to be derived from the mid-ocean ridge basalt (MORB) source-like reservoir. However, the notion that the komatiites may be derived from a depleted reservoir in terms of trace elemental and isotopic geochemistry might require an alternative view for the 3He/4He evolution in ancient mantle reservoirs, as has been demonstrated by a recent model calculation by Seta et al. [Earth Planet. Sci. Lett. 188 (2001) 211–219] in which the 3He/4He ratios in the MORB mantle source could have been as high as those in the primitive (less degassed) mantle source in the Archean.