FATE OF PB IN AN ARCHEAN ZIRCON

We report, for the first time, a direct, atomic-scale characterization of Pb in zircon (3.3-4.4 Ga) from the early Archean Yilgarn craton in Australia using high-resolution HAADF-STEM. The form of Pb (valence and coordination geometry) in zircon has a direct effect on the Pb diffusion rate, thus affecting the U-Th-Pb age date. We found that the Pb atoms are incorporated into zircon without distortion by direct substitution for Zr⁴⁺. Pb was also found concentrated within the amorphous domains created by fission fragment damage. The first result suggests that the Pb is tetravalent, and the latter observation demonstrates that Pb diffusion may occur through amorphous regions created by radiation damage. At the first percolation point, that is when the radiation-damaged domains become fully interconnected, one expects an increase in the loss of Pb due to the more rapid diffusion through the interconnected, amorphous domains.