Heterogeneities preserved within the convecting mantle

The generation of He isotope compositions is only quantitatively described in the model. However, several difficulties with the description of He can be identified  

1) The authors recognize that the generation of the observed uniform MORB He/ He from the residue of heterogeneous OIB sources is a potential problem. They propose that rapid mixing of the shallow asthenosphere may homogenize this shallow reservoir. Numerical modeling may provide a future test of this. 

The two-stage melting model has yet to be quantitatively developed with respect to the noble gases, making it difficult to fully assess the viability of this approach. 

2) The source of the domains must have high He/ He ratios at the time of depletion. The He/ He evolution of the MORB-source mantle has not been calculated for whole mantle convection, and so it is not clear when ratios as high as now observed in OIB were present. As a reference, it can be noted that in a layered mantle model, the upper mantle did not have ratios 30 Ra for the last 2 Ga (Seta et al. 2001). If high ratios have not been present since that time, then the domains with high He/ He ratios must have been preserved since then. 

4) There must be a mechanism for the preferential involvement of this previously melted material at OIB. 

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