The lower boundary layer reservoir

The boundary layer at the core-mantle boundary (CMB) has been explored as a reservoir for high He/ He ratio He, in the context of whole mantle convection. It has been suggested that subducted oceanic crust could accumulate there and form a distinct chemical boundary layer, accounting for the properties of the D layer (Christensen and 

1) The bulk mantle has a constant volume (a free parameter depends on D and RDM volumes). The starting U concentration is that of the bulk silicate Earth and that of He is calculated for a closed system with a Loihi He/ He ratio. The main He outflow is to the atmosphere by degassing a constant volume with time (i.e., first order degassing). U is largely transferred to the continental crust at a set first-order removal rate. Smaller fluxes of He and U to D and RDM (via the altered ocean crust and depleted oceanic lithosphere, respectively) also occur. Small inputs occur from subduction of continental crust and from D and RDM. Present isotopic compositions are those seen in MORE. 

2) The model continental crust grows linearly over 4.5 Ga to the present volume. It only serves to deplete the bulk mantle of U. Return to the bulk mantle by subduction occurs at a rate set by those of continental growth and mantle outflow. 

3) The atmosphere grows progressively and serves only to deplete the bulk mantle of He. Since He is lost to space, the atmospheric He abundance is not a constraint. 

4) The D layer grows linearly, with the present volume taken as a free parameter. There is a constant mass inflow from subduction, with a U concentration related to the bulk mantle U concentration by a large enrichment factor operating during formation of ocean crust. There is a mass flux out (dependent upon the growth rate of the layer). Present isotopic compositions are those seen in HIMU basalts. 

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