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Science 26 May 2006:
Vol. 312. no. 5777, p. 1139
DOI: 10.1126/science.1125408
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Technical Comments

Response to Comment on "Heterogeneous Hadean Hafnium: Evidence of Continental Crust at 4.4 to 4.5 Ga"

T. M. Harrison,1,2* J. Blichert-Toft,3 W. Müller,4 F. Albarede,3 P. Holden,2 S. J. Mojzsis5

Valley et al. review the lines of evidence on which we drew to conclude that continental crust formed much earlier than previously thought. Their comment contains some misrepresentations that we correct, but new information they provide appears to bolster our hypothesis. Nothing in their comment refutes the presence of continental crust or plate boundary processes prior to 4 billion years ago.

1 Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095, USA.
2 Research School of Earth Sciences, Australian National University, Canberra, A.C.T. 0200 Australia.
3 Ecole Normale Superieure, CNRS UMR 5570, 69364 Lyon Cedex 7, France.
4 Department of Geology, Royal Holloway University of London, Egham, TW20 0EX, UK.
5 Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA.

* To whom correspondence should be addressed. E-mail: mark.harrison{at}anu.edu.au

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Science. ISSN 0036-8075 (print), 1095-9203 (online)