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 growt h of large crystals from Pb-silicate melts; (2) hydrothermal overplati ng of thin epitaxial layers on substrates of natural zircon; and (3) g rowth of small, homogeneously nucleated crystals from aqueous fluids. The melt-grown zircons (50-400 mu m) were crystallized from PbO-SiO2-Z rO2 (+/- P2O5) liquid at atmospheric pressure by cooling from 1430 deg rees to 1350 degrees C. In the P2O5-free system, despite 66 wt% PbO in the melt, these zircons contain < 1 ppm Pb, yielding an apparent crys tal/melt partition coefficient (D-Pb) for Pb2+ of 7 x 10(-7). Addition of similar to 5 wt% P2O5 to the melt results in uptake not only of P (similar to 3400 ppm) in the zircons but also Pb (similar to 1500 ppm) , increasing the apparent D-Pb to about 10(-3). Hydrothermal overplati ng of ZrSiO4 was carried out at 1.5 GPa in a piston-cylinder apparatus by slow cooling from 500 degrees C or 550 degrees C to 140 degrees C of polished slabs of natural zircon immersed in zircon-saturated aqueo us solutions containing either PbO2 or PbO + P2O5. In both cases, the resulting epitaxial layers of ZrSiO4 (similar to 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 milt-grown zircons, available P is excluded from the aqueous epitaxial zircon, suggesting that charge balance is accomplished by H+ instead. Small (2-5 mu m) zircons grown by cooling aqueous solutions (PbO + Si O2 + ZrO2 +/- P2O5) from 800 degrees C or 900 degrees C contain simila r to 0.25-0.5 atom% Pb (similar to 2-4 wt% PbO), yielding apparent D-P b values of similar to 0.2-0.3. Available P5+ is incorporated in a 2:1 ratio with Pb2+, suggesting a specific charge-balance mechanism: [2P( 5+) + Pb2+] = [2Si(4+) + Zr4+]. However, Pb enters the zircon even whe n P is unavailable, so H+ may again play a charge-balancing role. Beca use of the rapid, polythermal modes of zircon growth and the high Pb c ontent of the experimental systems, the apparent partition coefficient s should not be viewed as equilibrium values, but as qualitative indic ators of Pb compatibility under various growth circumstances. The over all results are consistent with the low but variable levels of non-rad iogenic (common) Pb in natural zircons. The increased compatibility of Pb in fluid-grown, low-temperature zircons suggests a possible finger print for zircons from hydrothermal and wet-metamorphic rocks, i.e., h igh concentrations of common Pb. (C) 1997 Elsevier Science B.V.