The incorporation of Pb into zircon

The incorporation of Pb into zircons grown from Pb-rich solutions was evaluated using three different approaches: (1) high-temperature growth of large crystals from Pb-silicate melts; (2) hydrothermal overplating of thin epitaxial layers on substrates of natural zircon; and (3) growth of small, homogeneously nucleated crystals from aqueous fluids. The melt-grown zircons (50–400 μm) were crystallized from PbOSiO2ZrO2 (±P2O5) liquid at atmospheric pressure by cooling from 1430° to 1350°C. In the P2O5 free system, despite 66 wt% PbO in the melt, these zircons contain < 1 ppm Pb, yielding an apparent crystal/melt partition coefficient (DPb) for Pb2+ of 7 × 10−7. Addition of ∼ 5 wt% P2O5 to the melt results in uptake not only of P ( ∼ 3400 ppm) in the zircons but also Pb (∼ 1500 ppm), increasing the apparent DPb to about 10−3. Hydrothermal overplating of ZrSiO4 was carried out at 1.5 GPa in a piston-cylinder apparatus by slow cooling from 500°C or 550°C to 140°C of polished slabs of natural zircon immersed in zircon-saturated aqueous solutions containing either PbO2 or PbO + P2O5. In both cases, the resulting epitaxial layers of ZrSiO4 (∼ 60 nm thick) contain > 3 atom% Pb, with apparent zircon/fluid partition coefficients of 4.2 and 2.6, respectively, for Pb4+ and Pb2+. In contrast to the case of melt-grown zircons, available P is excluded from the aqueous epitaxial zircon, suggesting that charge balance is accomplished by H+ instead. Small (2–5 μm) zircons grown by cooling aqueous solutions (PbO + SiO2 + ZrO2 ± P2O5) from 800°C or 900°C contain ∼ 0.25–0.5 atom% Pb (∼ 2–4 wt% PbO), yielding apparent DPb values of ∼ 0.2–0.3. Available P5+ is incorporated in a 2:1 ratio with Pb2+, suggesting a specific charge-balance mechanism: [2P5+ + Pb2+] = [2Si4+ + Zr4+]. However, Pb enters the zircon even when P is unavailable, so H+ may again play a charge-balancing role. e of the rapid, polythermal modes of zircon growth and the high Pb content of the experimental systems, the apparent partition coefficients should not be viewed as equilibrium values, but as qualitative indicators of Pb compatibility under various growth circumstances. The overall results are consistent with the low but variable levels of non-radiogenic (common) Pb in natural zircons. The increased compatibility of Pb in fluid-grown, low-temperature zircons suggests a possible fingerprint for zircons from hydrothermal and wet-metamorphic rocks, i.e., high concentrations of common Pb.