On the valency state of radiogenic lead in zircon and its consequences

From its effective ion radius, similar to that of Zr4+, and its charge, Pb4+ has to be compatible in the zircon lattice. Also, by analogy with U4+, Th4+ and Hf4+, its diffusivity should be several orders of magnitude lower than that measured for Pb2+ in zircon. These factors can account for the extreme retentivity of U-Pb clocks even at high temperatures. On the other hand, radiogenic Pb situated in α-recoil damaged sites could be leached out by any electrolyte solution that reduces it to the divalent state, making it both incompatible and soluble. Thus, discordia can be generated in weathering. The curious observation that discordant Archaean zircon suites generally define trends to lower intercepts at up to 800 Ma instead of zero, appears to reflect a c. 14 to 28% better average accessibility to leaching of radiogenic 207Pb compared to 206Pb, in accord with the somewhat higher recoil energy of the last (branched) α-decays in the 235U chain.