Fig. 1. Results of a model illustrating possible trajectories in _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 _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 (106K JPEG file)]

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