Delamination lower crust

An additional, unquantified contribution is also made by the delamination of the lower crust and the subcontinental lithosphere (Chapter 4, Section 4.5.2.1). 

In (i)-(iii), delamination would be unnecessary if primitive cumulates or residues of lower crustal melting were ultramafic, and remained present below the seismic Moho. However, the absence of pyroxenite layers —10 km in thickness in the Talkeetna and Kohistan arc sections suggests that large proportions of ultramafic, igneous rocks are not present at the base of arc crust. 

If delamination of dense lower crustal rocks has been essential to continental genesis, and the delaminated rocks constitute —20-40% of the mass of the continents, one might expect to see evidence for this component in magmas derived from the convecting mantle. However, continental crust comprises only —0.5% of the sihcate Earth, so that—even if it represents 40% of the original crustal mass— recycled lower cmst might comprise a very small fraction of the convecting mantle. 

Similar observations have been made in the younger crust of the Sierra Nevada batholith in the USA, where Ducea and Saleeby (1998) proposed that the very thin granite crust (30-40 km), underlain by peridotitic upper mantle, can be explained by the delamination of a thick eclogitic root. Boyd et al. (2004) support this view with evidence from a seismic tomographic study which shows the descent of a two layer slab comprising an eclogitic upper part and a peridotitic lower part, into the mantle. This may be the only place on Earth where dense material is currently being removed from the continental crust into the mantle. 

It is clear, from the examples cited above, that lower crustal delamination takes place. What is less certain is the relative importance of this process, for the number of convincing examples of lower crustal delamination is small. Kramers and Tolstikhin (1997) argued on the basis of their Pb-isotope forward transport model that lower crustal delamination is a minor process and not important in modifying crustal compositions. In contrast, Plank (2005) argued that between 40 and 50% lower crustal loss is required over geological time, to achieve the present-day Th/La ratio of the bulk crust. Whilst this volume equates to only 0.3% of the mantle mass, and less than 10% of the mass of subducted slabs over geological... 

---