Mantle crust-forming reservoirs

The formation of basalts by partial melting of the upper mantle at mid-oceanic ridges and hot spots provides the opportunity to determine mantle composition. Early studies of radiogenic isotopes in oceanic basalts (e.g., Faure and Hurley, 1963 Hart et al., 1973 Schilling, 1973) showed fundamental chemical differences between OIBs and MORBs (see Chapter 2.03). This led to the development of the layered mantle model, which consists essentially of three different reservoirs the lower mantle, upper mantle, and continental crust. The lower mantle is assumed primitive and identical to the bulk silicate earth (BSE), which is the bulk earth composition minus the core (see also Chapters 2.01 and 2.03). The continental crust is formed by extraction of melt from the primitive upper mantle, which leaves the depleted upper mantle as third reservoir. In this model, MORB is derived from the depleted upper mantle, whereas OIB is formed from reservoirs derived by mixing of the MORB source with primitive mantle (e.g., DePaolo and Wasserburg, 1976 O Nions et al., 1979 Allegre et al., 1919). 

A potentially significant reservoir, and one which a number of authors have suggested is important in the context of continent formation, is the subcontinental lithospheric mantle (SCLM). Kramers (1987, 1988), suggested that the TTG magmas of the Archaean crust formed in an open-system magma layer in the early Earth, the cumulates from which are now preserved as the SCLM. More recently Abbott et al. (2000) proposed a model of continental growth founded upon the premise that the continental crust was extracted from the SCLM. 

Table 4.3 below summarizes the different reservoirs thought to be present in the modern silicate Earth. However, of these, only the depleted mantle and buried eclogitic slabs have compositions which make them directly complementary to the composition of the continental crust. It is suggested here that Archaean SCLM is not closely related in a geochemical sense to Archaean felsic crust but is the product of basalt extraction, basalt which now is emplaced within the Archaean crust. In contrast, Phanerozoic SCLM may be the restite-complement of the basaltic precursor to modern crust. The proposal that there was an early-formed (pre 3.7 Ga), enriched, basaltic crust (Section 4.5.1.2.2), if confirmed, has important implications for the balance between the major Earth reservoirs, not least because the primitive mantle and the bulk silicate Earth can no longer be regarded as com-positionally identical. 

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... 

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