The mantle sources of xenolith He

There appears to be a general consensus that noble gases found in xenoliths have been largely introduced relatively recently and have not been trapped and stored since the stabilization of that region of the lithosphere. The similarity between the isotopic compositions of He found in continental xenoliths and that found in MORB indicates that a dominant fraction of the xenolith He originated within the convecting upper mantle. However, the shifts to somewhat lower values at some locations indicate that another component is present. With reference now to the detailed regional studies, there are several possible sources that have been discussed for the source of the mantle He trapped in the xenoliths  

MORB mantle. The convecting upper mantle sampled by MORB has He/ He = (8 1) Ra away from hotspots (see Graham 2002, this volume). In some areas, such as southeast Australia, xenolith He appears to have MORB He isotope compositions. This is not surprising, considering that much of the mantle underlying the continents must have this composition. It is also not incompatible with the involvement of diapiric mantle hotspots in the local volcanism. The He isotope composition of many hotspots have not been clearly characterized, and while hotspots are often assumed to have very high He/ He ratios as seen in Iceland and Hawaii, it has not been established that other hotspots, with very different trace element characteristics, all have such ratios. Further, the presence of such material does not preclude the involvement of MORB mantle as well within a particular lithospheric region. 

While the effects of magma contamination and radiogenic production cannot be discounted completely from some of these samples, the general pattern seems consistent with a mantle region of lower He/ He ratios. As discussed above, it appears that these mantle domains directly provide the He seen in the continental lithosphere. In western Europe, the Cameroon Line, Meidob Hills and Kivu, data from other isotopic tracers in the host basalts are consistent with such a source, which also then provides a reasonable explanation for the xenolith He. However, such domains are sampled at only a small proportion of oceanic vents (see Graham 2002, this volume), and so it is unlikely that such a source has contributed to many of the other continental locations. 

Lithosphere metasomatism. It has been suggested that metasomatic fluids can transport into magma source regions trace elements that have been highly fractionated and so can generate the unique isotopic signatures seen in HIMU basalts (e.g., Halliday et al. 1995). While this process has been suggested for the oceanic asthenosphere, low 

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