Continental crust intracrustal fractionation

However, fractionation of primitive andesite is clearly just one of many possibilities for the genesis of continental crust with the composition of high Mg andesite. An alternative view is that intracrustal differentiation, perhaps involving residual garnet and/or magnetite, has converted... 

Fig. 2. Reservoirs and fluxes used in previous and present modelling work (not to scale). Rectangles used for reservoirs ellipses for loci of fractionation by melt processes, m-f, depleted mantle melting, oceanic crust (MORB) formation s-f, subduction zone melting leading to continental crust formation c-f, intracrustal fractionation leading to upper and lower crust formation. Bold arrows, fluxes involving trace element fractionation line arrows flirxes without trace element fractionation stippled arrows, fluxes operating only during accretion and core formation.

It has already been shown that the composition of the Earth s continental crust and the Earth s mantle have evolved chemically over time (see Section 4.3 and Chapter 3, Section 3.2.3). Hence, as the continental crust has grown, so has its composition changed, as is apparent from the differences in the REE content and Rb/Sr ratio of granitoids and the Th/La ratio of sediments (Section 4.3.2). These chemical differences could indicate that the mechanism of crust formation has also changed with time. Further support for this hypothesis comes from Plank s 2005 study of crustal Th/La ratios, discussed above. Plank argued that the present-day high Th/La ratio (0.28-0.31) of the continental crust is the product of internal crustal fractionation. However, Archaean continental crust has a much lower Th/La ratio (0.18) than modern continental crust, and does not require intracrustal differentiation, and so may have formed in a different manner. 

Figure 9 (a) Chondrite-normalized REE patterns for the upper, bulk, and lower continental crust reservoirs, (b) Upper and lower continental crust normalized to the bulk continental crust, highlighting the fractionation of europium and the more incompatible FREE during intracrustal differentiation... 

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