HR: 0800h
AN: U21B-0413    [Abstracts]
TI: Chemical Geodynamics: a perspective from the Continental Crust
AU: * Hawkesworth, C J
EM: c.j.hawkesworth@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol, Queens Road, Bristol, BS8 1RJ, United Kingdom
AU: Kemp, T
EM: tony.kemp@jcu.edu.au
AF: School of Earth Sciences, James Cook University, Townsville, QLD 4811, Australia
AB: Chemical geodynamics has been applied both to the differentiation and evolution of the continental crust, and to its recycling back into the Earth's mantle. Estimates of the residence time of material in different reservoirs within the continental crust constrain models for the evolution of the crust and the volumes of material recycled. The present data indicate that the generation of new crust in the Archaean and Early Proterozoic may have occurred in restricted periods of rapid crust generation. If correct, the rates at which new crust is reworked into the sedimentary reservoir are much slower than had been previously envisaged. More widely, if the average residence times in the upper crust are typically of the order of 2 Ga, those in the lower crust are ~0.4-0.6 Ga. The relative volumes of recycled upper and lower crustal material are therefore better constrained, and the effects of the recycling of both upper and lower crust compositions on the trace element and radiogenic isotope ratios of mantle sampled by oceanic basalts can be further explored. For example, the differentiation of basaltic magma to the composition of the bulk continental crust yields large volumes of residue with distinctive Rb/Sr, Sr/Nd, Sm/Nd, Lu/Hf and U/Pb ratios delaminated into the mantle after relatively short periods in the crust. Current issues that can now be investigated in more detail include the extent that the pulses of crust generation are global, rather than of slightly different ages in different continental blocks, the Lu/Hf ratio, and hence by inference the composition of early crust from in situ Pb and Hf isotope data on old zircons, and the separate evolution of the sedimentary and igneous portions of the continental crust.
DE: 8125 Evolution of the Earth (0325)
SC: Union [U]
MN: 2007 Fall Meeting