The data obtained as a result of experimental investigation of the distribution of Pb and U isotopes between zircon and different composition solutions are essential contribution to the conception of mechanism of discordance of age determinations calculated by three isotopic relationships: 206Pb/238U, 207Pb/235U,and 207Pb/206Pb. Cold-seal, hydrothermal pressure vessels were used in all experiments. Runs were carried out at 200-800°C and 1000 bars. Equilibrium concentrations of Pb and U in solutions at 400°C were achieved for metamict zircon during 7 days for crystalline one - 14 days. The duration of runs at lower temperatures was 30 days. As solutions H2O, 1mNaCl+0.5mHCl, 1mNaCl, 1mNaOH, 1mNaHCO3, 0.1mNa2CO3, 1m Na2CO3, 2m Na2CO3 were used.Isotopes of Pb and U were analysed by mass - spectrometer MAT-261.
The partition coefficients (KD = Csol./Czirc.) of Pb and U usotopes are due to acidic-alkaline properties of the solutions in the main. In the acidic medium Pb is preferably concentrated in the solution, lgKD(1mNaCl+0.5HCl) = 0.3, in comparison with U (lgKD=-3.58). In contrast, U enter the solution in the most alkaline ones, lgKD(1mNaOH)=-2.14, more of Pb, lgKD=-2.27. The extent of metamictness influences the distribution of Pb and U isotopes as well. Partition coefficients of U between metamict zircon and acidic solutions are approximately one order as large as crystalline whereas those of Pb are several order as large. For metamict zircon as well as for crystalline one partition coefficients of Pb and U in alkaline solutioms line up but for metamict zircon they are in excess of more then one order.
On concordia diagram the experimental data depending on composition of solution are settled in different sides from starting composition and different distances.