Summary

The most important questions relating to the history of the continental crust are as follows. Which processes produced the continental crust and what material did these involve? What were the transport and fractionation mechanisms occurring within the crust? How was the crust destroyed? The answers to these questions are fundamental for building a (semi-)quantitative chemical transport crust-mantle model and applying it to achieve a better understanding of the Earth's evolution.

The crustal abundances of indicative trace elements, such as enhanced Pb/La, Ba/Th and La/Nb ratios, point to subduction-related magmatism as the major crustal feeding process (Chapter 25). Moreover, because these arc-related signatures are also typical of ancient crustal rocks (Fig. 26.3(a)), arc-like magmas most probably also fed the crust in the geological past of the planet. During the Hadean and Archaean eras, subduction and arc-melt generation were different from that in the late Proterozoic and Phanerozoic: this difference is reflected in the chemical compositions of the Archaean and post-Archaean crustal rocks. Although the underplating and intrusions of mafic magmas in mature crustal domains are clearly seen to have made some contribution to crustal growth, it appears that subduction magmatism generated by far the greatest crust-feeding flux throughout Earth's history. This simplified solution of the first problem is used in the chemical Earth model (Chapter 28).

Intracrustal magmatism leading to the redistribution of chemical elements between these reservoirs is one of the most complicated geological phenomena. Different sources of heat initiate the magmatism, and highly differing PT-t conditions, source and wall-rock compositions and variable fluid contributions strongly affect the melt generation, transport and emplacement processes. This results in a great variety of both extracted magmas and residual matter. Because of this complexity it is difficult to establish whether the lower and upper crust are truly complementary reservoirs.

The continental crust is growing mainly by the addition of arc-generated magmas, but it is also being destroyed. Weathering and erosion along with sediment transfer from the continents are obvious processes. The dissolved and suspended riverine as well as aeolian sediment fluxes into the ocean are important, as they affect the composition of the oceans and (via subduction) introduce specific elemental and isotopic signatures into the mantle. The composition of this flux is in most respects close to the mean composition of the upper crust.

A comparison of the crustal abundances of incompatible elements (alkali metals but also U, Th, Ba and others) and those in potential crust-feeding magmas shows that the latter are not sufficiently enriched to supply the crust (Fig. 25.1). This means that crust-feeding magmas must have evolved by fractional crystallization and that cumulates sank back into the mantle. The process is known as delamination.

Chapter 27 shows how these processes, through geological time, have left their mark in the heterogeneous isotope ratios of reservoirs.

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