A three-dimensional method for calculating independent chemical U/Pb- and Th/Pb-ages of accessory minerals

Previous theoretical considerations on the chemical UTh-total Pb dating method failed to distinguish between thorogenic and uranogenic lead. However, it can be shown that the data points are located on a plane in the three-dimensional ThO2, PbO, UO2 space. The calculation of the best-fit plane yields a slope in the ThO2PbO and UO2PbO coordinate projections, and an initial PbO value. From the two slopes, Th/Pb- and U/Pb-ages can be calculated independently. thod described in this paper is applied to monazites from the Hercynian G4-granite of the Fichtelgebirge (Germany). Th/Pb- and U/Pb-ages were calculated at 323 ± 20 Ma and 304 ± 15 Ma, respectively. The intercept value close to zero indicates that no significant amounts of common lead are present in the monazites studied.