Pb in naturally irradiated monazites and zircons

Pb-L_III edge XANES and μ-XAFS experiments (bulk and μ-focalised) were performed to evaluate the speciation of radiogenic Pb in naturally irradiated monazites (ideally LaPO₄) and zircons (ideally ZrSiO₄) minerals after ca. one billion years. XANES spectra show that Pb is divalent in all the samples investigated. But large variation in the speciation of Pb has been evidenced from monazites to zircons. Ab-initio calculations (FEFF8.4) were preliminary tested in a set of Pb-sulfide, oxide, carbonate, phosphate and silicate minerals and converged using Dirac-Hara exchange potentials. In monazite, XANES spectra calculations results are consistent with Pb substituting to La.