Episodic growth models for the continental crust; new tests from Hf and O isotopes in zircon

The continental crust is unique to the Earth, and recent developments in analytical techniques and in approach now make it possible to tackle fundamental questions over its origins and evolution. The questions include when did the rocks of the continents form? Were there periods when large volumes of igneous rocks were generated and other periods when fewer rocks formed, and why might that be? To what extent did these igneous rocks represent new continental crust, or were they derived by remelting older crustal rocks? In what tectonic setting was the continental crust generated, and did those settings change in different periods of Earth history, as, for example, between periods of rapid and slow crustal growth? How do the records of the evolution of the crust obtained from sediments compare with those obtained from igneous rocks, and how may the two be reconciled? The critical step is to obtain representative records of the continental crust, and these are available in the mineral zircon. What is new is the information from isotopes and trace elements that can now be obtained from these tiny minerals. Zircons occur in most rocks from the upper parts of the continental crust, they are very robust and so they survive through repeated episodes of erosion and sedimentation, and even remelting and the generation of new igneous rocks. They contain growth zones that can be dated precisely using U-Pb isotopes, and so they provide an unparalleled time series of changing magmatic conditions that can now be determining using hafnium and oxygen isotopes and trace element abundances. These analyses are done in situ using ion beams or lasers, and the coupling of hafnium and oxygen isotopes can uniquely reveal whether even the individual growth bands in a zircon crystallised from a juvenile magma during crustal generation, or from a magma derived by reworking of pre-existing sedimentary rocks. Igneous rocks are generated in provinces of magmatic rocks that are restricted in both space and time. Sediments, by contrast, tend to average the various rocks that are available in their source areas. Thus they preserve very different records of the evolution of the continental crust, and now, for the first time, it is possible separately to unpick the record preserved in sediments from those preserved in igneous rocks. This is done using oxygen isotope ratios because they are higher in sediments than they are in igneous rocks that have not involved melting of sediment. Oxygen isotopes can therefore be used to recognise zircons that crystallised from magmas that were derived from sediment, from those that were not. The preliminary data indicate that there were restricted periods when lots of crust was generated, and then it took almost one billion years for the new crust to dominate the composition of the overlying sediments. This project is concerned to test those predictions. The second aspect is the extent that trace element abundances in zircons can be used broadly to distinguish those that crystallized from magmas generated in within plate settings linked to deep seated plumes from those generated in subduction related settings. The key question is to assess how much new continental crust was generated in response to deep-seated thermal disturbances reflected in mantle plumes, or to shallow level tectonic processes manifest in subduction zones, and how that balance changed with time. This approach will be applied to detrital zircons deposited near the end of the inferred periods of more rapid crustal growth. This offers a robust test of the significance of the inferred peaks of rapid crustal growth, in the sense that it will demonstrate that they are not just artefacts of the rocks presently exposed today. The areas to be studied include some of the oldest known rocks on Earth, which outcrop in Australia, and an area where new crust was generated in response to mantle plumes, in west Africa.