Séminaire de Chris Hawkesworth (University of Bristol, UK)


 AHLeGall    20/05/2016 : 23:55

chris_hawkesworth.jpg

Le vendredi 20 mai 2016 à 10:30, en salle de conf de l'OSUR (campus de Beaulieu, bât.14B, RDC), Chris Hawkesworth propose un séminaire intitulé "Geological cycles and the generation of the continental crust"

Le vendredi 20 mai 2016 à 10:30, en salle de conf de l'OSUR (campus de Beaulieu, bât.14B, RDC), Chris Hawkesworth propose un séminaire intitulé "Geological cycles and the generation of the continental crust"


Cette présentation est proposée dans le cadre du cycle des séminaires de Géosciences Rennes

Présentation :

The continental crust is the archive of Earth history, and the apparently cyclical nature of geological evolution is a feature of the geological record. The advent of radiometric ages has highlighted that the continental crust is characterised by distinctive peaks and troughs in the distribution of ages of igneous crystallization, metamorphism, continental margins and mineralization. It is argued that the temporal distribution largely reflects the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, and the peaks of ages are linked to the timing of supercontinent assembly. In contrast there are other signals, such as the Sr isotope ratios of seawater, mantle temperatures, and redox conditions on the Earth, where the records are regarded as primary because they are not sensitive to the numbers of samples of different ages that have been analysed.
Less than 5% of the geological record consists of rocks older than 3 Ga, and there no known rocks older than 4 Ga. The sedimentary record is biased by preferential sampling of relatively young material in their source terrains. The implication is that there were greater volumes of continental crust in the Archaean than might be inferred from the compositions of detrital zircons and sediments. Recent studies based on the U-Pb, Hf and O isotope ratios of detrital zircons suggest that at least ~60-70% of the present volume of the continental crust had been generated by 3 Ga. The growth of continental crust was a continuous process, but there was a decrease in the rate of crustal growth at ~3 Ga. Before 3 Ga the rates of continental growth were high (~3.0 km3.yr-1), broadly similar to the rates at which new crust is generated and destroyed at the present time. Since 3 Ga the net growth rate was much lower (~0.8 km3.yr-1), perhaps because of higher rates of destruction of crustal materials. It is inferred that subduction-driven plate tectonics and discrete subduction zones have been dominant since ~3 Ga.
The poor preservation of rocks and minerals after billions of years of crustal evolution makes it difficult to establish the composition of new/juvenile continental crust and hence the conditions and the tectonic setting(s) in which it was generated. However, these can be addressed using time-integrated
Rb/Sr ratios for new crustal material; these indicate that new continental crust was principally mafic over the first 1.5 Ga of Earth's evolution, and that it became more evolved subsequently. It is concluded that significant volumes of pre-3 Ga crust may have been associated with intraplate magmatism. Since ~3 Ga there has been an increase in Rb/Sr, SiO2, and the inferred thickness of new crust, consistent with an increase of continental input into the oceans and the onset of plate tectonics. s of generation and destruction of pre- and post-3 Ga continental crust through time are explored

Semin Chris Hawkesworth 20mai2016


Contact :
Chris Hawkesworth (School of Earth Sciences, University of Bristol, UK) - Fellow Royal Society

Contact OSUR :
Philippe Steer (Géosciences Rennes)
Benjamin Guillaume (Géosciences Rennes)