Acasta Gneiss

Meteorites present an opportunity to look at geological time or the time told by radionucleotides within rocks. The oldest rocks found on Earth are not as old as the age of the Earth due to continual reprocessing of the Earth s surface. The oldest discovered rocks so far are the Acasta gneisses from Northwestern Canada, which are 4.03 Gyr, but these are young compared with the CAIs found in the Allende meteorite, which are 4.566 0.002 Gyr or 4.556 billion years. The ages of these species are derived from the relative abundances of radioisotopes and their daughter species, as seen in Table 6.3. 

All in all, the complexity of the isotopic data and their interpretation in the Acasta Gneiss Complex illustrate the difficulties inherent in study of planetary evolution using polymeta-morphic gneisses. This is the ultimate cause for the controversies over relaibility of radiogenic isotopic parameters derived from these rocks (e.g., Vervoort et al., 1996 Gmau et al., 1996 Moorbath et al., 1997). 

Bowring S. A., Housh T. B., and Isachsen C. (1990) The Acasta gneisses remnant of Earth s early crust. In Origin of the Earth (eds. H. E. Newsom and J. H. Jones). Oxford University Press, Oxford, pp. 319-343. 

Stern R. and Bleeker W. (1998) Age of the world s oldest rocks refined using Ganada s SHRIMP the Acasta Gneiss Gomplex, Northwest Territories, Ganada. Geosci. Can. 25, 27-31. 

Oldest preserved terrestrial rocks Acasta Gneiss Complex, metatonalites and gabbros, no supracrustal rocks... 

The Acasta Gneisses, northwestern Canada - the most ancient rocks... 

Whilst the great age of the Acasta Gneisses is not in doubt, they have been the subject of some controversy. Bowring and Housh (1995) calculated initial eNd values for a range of samples from this area and found that they are extremely heterogeneous. This observation, coupled with the ancient zircon ages for these samples, has profound implications for the... 

Iizuka, T., Horie, K., Komiya, T., Maruyama, S., Hirata, T., Hidaka, H., and Windley, B., 2006. 4.2 Ga zircon xenocryst in an Acasta gneiss from northwestern Canada evidence for early continental crust. Geology, 34, 245-8. 

Sano Y, Terada K, Hidaka H, Yokoyama K, Nutman AP (1999b) Palaeoproterozoic thermal events recorded in the 4.0-Ga Acasta gneiss, Canada Evidence from SHRIMP U-Pb dating of apatite and zircon. Geochim Cosmochim Acta 63 899-905... 

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