On the valency state of radiogenic lead in zircon and its consequences
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In zircon U-Pb systematics, extreme robustness up to the temperatures of granulite facies and anatexis contrasts with apparently easy loss of radiogenic Pb at low temperatures, often without any metamorphic event being in evidence. Here we propose that this paradoxical behaviour can be understood with the hypothesis that radiogenic Pb in zircon is tetravalent. We review data and arguments in favour of this hypothesis. Diffusion profiles calculated for Pb2+ in a 25 µm radius zircon xenocryst in a melt at 1000 °C, combined with the incompatibility of Pb2+, or for a zircon core inside a younger zircon rim at this temperature, show age effects that should have been observed in SIMS dating. Further, in zircon evaporation as well as in leaching experiments, common Pb is generally released preferentially to radiogenic Pb. After removal of less radiogenic Pb, the evaporation record generally shows pure radiogenic Pb during the final evaporation steps. The distribution of residual Pb in a leached titanite, revealed by PIXE, is similar to that of Ti. Lastly, XANES spectra of a 1 Ga old titanite (predominantly radiogenic Pb) and an Alpine one (predominantly common Pb) are significantly different, although the former does not resemble that of PbO2. The arguments why radiogenic Pb should be tetravalent are based on analogies with studies relating to the tetravalent state of 234Th and the hexavalent state of 234U, which show that a-recoil in silicates generates a strongly oxidizing environment at the site where the recoiling nucleus comes to rest. Further, a zircon grain, being small, should remain highly oxidizing in its interior by the constant loss of ß-particles, maintaining the 4+ state of radiogenic Pb.
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 a-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) a-decays in the 235U chain.
|Publication status||Published - 2009|