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It is well established that the present-day mantle exhibits a layered structure, with an upper region depleted in incompatible elements, but considerable uncertainty surrounds the state of the mantle in the past. In this context, the isotopic composition of helium is an excellent diagnostic tracer for the origin of mantle-derived magmas. The presence of variable but generally high 3He/4He ratios in magmas associated with modern mantle plumes is firm evidence for the presence of a deep mantle component
rich in primitive 3He (3He/4He up to 32 Ra, where Ra is the atmospheric ratio of 1.4 x 10-6) whereas the lower and near-constant 3He/4He ratios of mid-ocean ridges basalts (8 Ra) are thought to represent the more degassed and well homogenised upper mantle. So far almost all studies have focused on present-day volcanism. Exceptions are plume-type 3He/4He ratios up to 20 Ra in 30 Ma-old Ethiopian flood basalts, up to 22 Ra in 60 Ma-old East Greenland flood basalts (both measured in our laboratory), and ratios up to 14 Ra observed in 68.5 Ma-old volcanics associated with the Deccan flood basalts (Basu et al., 1994) and up to 12.7 Ra in 253 Ma-old volcanics associated with the Siberian flood basalts (Basu et al., 1995). 3He/4He around 0.2 Ra have been measured in amphiboles from 2.8 Ga-old charnockite series from the Kola Peninsula, presumably reflecting trapping of ancient mantle He (Kamiensky et al., 1990) but such relatively low value indicates secondary addition of radiogenic helium and does not allow to precise the original mantle source region.
Komatiites are highly magnesian volcanic rocks. They erupted at very high temperature (up to 1600°C), and with the exception of one Cretaceous occurrence in Colombia, are restricted to the Archean and Proterozoic greenstone belts (Arndt, 1994). Their unique petrology is attributed to melting at extreme depths (400 km or more) in unusually hot mantle sources. When filtered for crustal assimilation, their trace element patterns and radiogenic isotopes suggest they are derived from a depleted source, either MORB-type (Anderson, 1994) or part of a mantle plume (McDonough and Ireland, 1993; Kerr et al., 1995).
We separated fresh olivine crystals from three komatiites (Ottawa Islands in the 1.9 Ga Circum-Superior belt of northern Quebec; Alexo in the 2.7 Ga Abitibi belt of Ontario (Arndt, 1986); and the Zwishevane region of 2.7 Ga Belingwe greenstone belt of Zimbabwe) and analysed helium isotopic ratios following stepwise crushing. 3He/4He ratios vary over 4 orders of magnitude, from a very low value of 0.046±0.020 Ra (Belingwe), to an extremely high ratio of 39.1 ± 0.6 Ra in the first crushed fraction of Alexo sample M712. In order to estimate the possible contributions of radiogenic and nucleogenic He, the U, Th and Li contents of the magma were estimated following ion probe determination of lithium and potassium in a glass inclusion of the Alexo sample. For this sample, both the Li content (0.48 ppm) and the U content (18 ppb, as computed from a measured K content of 226 ppm) are low, in agreement with the depleted character of this komatiite (e.g. Arndt, 1994), and cannot account for significant enhancement of 3He during in-situ nucleogenic production. The possibility that the extreme 3He enrichment in sample M712 from Alexo resulted from incorporation of cosmogenic helium-3 is negated using two different lines of evidence. First the host komatiite was probably never exposed for a long period at the surface. The flow became part of the crust during continent accretion 2.7 Ga ago and remained buried until exhumation following the last glaciation ~ 9000 years ago. Second, stepwise crushing reveals that helium with extremely high 3He/4He ratio was extracted during the very first crushing step and that subsequent extraction released helium increasingly diluted by radiogenic He. Such decrease in the 3He/4He ratio is not only due to the increase in the radiogenic 4He content but also to the concomittant decrease in the 3He content.
In the Alexo sample, the He isotopes provide firm evidence of a plume origin. The value of 39 Ra is the highest He isotopic ratio ever measured in terrestrial lavas (with the exception of diamonds for which the origin of elevated He isotopic ratios is unclear). Our modelling of He isotope evolution through time for different mantle reservoirs in closed and open conditions shows that such high values cannot represent an ancient MORB-type reservoir. When combined with the published trace-element and Nd isotope data (Arndt, 1986), the results show that the portion of the plume source from which the komatiite was derived was geochemically and isotopically depleted, unlike that of the dominant component of most modern oceanic island basalts, but similar to that of certain picrites and the Cretaceous Gorgona komatiites.
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