Note to users. If you're seeing this message, it means that your browser cannot find this page's style/presentation instructions -- or possibly that you are using a browser that does not support current Web standards. Find out more about why this message is appearing, and what you can do to make your experience of our site the best it can be.

Originally published in Science Express on 17 November 2005
Science 23 December 2005:
Vol. 310. no. 5756, pp. 1947 - 1950
DOI: 10.1126/science.1117926
Prev | Table of Contents | Next

Reports

Heterogeneous Hadean Hafnium: Evidence of Continental Crust at 4.4 to 4.5 Ga

T. M. Harrison1,2*, J. Blichert-Toft3, W. Müller1,4, F. Albarede3, P. Holden1 and S. J. Mojzsis5

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia.
2 Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California at Los Angeles, Los Angeles, CA 90095, USA.
3 Ecole Normale Supérieure, CNRS Unité Mixte de Recherche 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.


Fig. 1. Results of a model illustrating possible trajectories in {epsilon}Hf(T) versus age space ensuing from mixing zircon components of differing age, U concentration, and Hf isotope composition. The model assumes concordant ages of T1 (core) and T2 (rim), which are characterized by the Hf isotope composition of bulk Earth at T1 and T2, respectively. Mixtures of the two components are shown adjacent the relevant curve in % of the rim component for the case of a 4.4-Ga core and 3.7-Ga rim. Where the bulk age of a mixture is characterized by a U-Pb age in the core, but incorrectly associated with a 176Hf/177Hf ratio that is a mixture of the two end-members, the potential exists for incorrect {epsilon}Hf values to be calculated. More surprising is that in the case where U-Pb age and Hf isotopes are measured on the same spot overlapping these two zones, aberrant values result, as shown in the mixing curves. [View Larger Version of this Image (20K GIF file)]
 

Fig. 2. Plot of {epsilon}Hf(T) versus age for new MC-ICP-MS Hf isotope analyses together with results of Amelin and co-workers (14) recalculated using the "terrestrial" 176Lu decay constant (17). Solid symbols are concordant to <12% discordant, whereas open symbols are >12% discordant. The line marked Lu/Hf = 0 corresponds to 176Hf/177Hf ratios equivalent to the bulk silicate Earth value at 4.5 Ga. The stippled region indicates values not attainable (i.e., lower than solar system initial 176Hf/177Hf). Error bars indicate 2 SE. [View Larger Version of this Image (34K GIF file)]
 

To Advertise     Find Products

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