Comparison of single filament Pb evaporation/ionization zircon ages with conventional U-Pb results: examples from the Precambrian of Brazil

This paper documents a systematic and detailed study comparing the 207Pb/206Pb apparent ages obtained by single zircon Pb evaporation/ionization method, in rock samples previously dated by conventional U-Pb zircon dissolution procedures. The technique applied here uses a single rhenium center filament to evaporate, ionize and measure the Pb isotopic ratios, in a thermal ionization mass spectrometer, by direct evaporation/ionization of a single zircon grain. ngle zircon Pb evaporation/ionization method was applied in igneous and metamorphic rocks of Archean to Neoproterozoic age. The 207Pb/206Pb obtained for the igneous rocks with little or no evidence of deformation are, within the error, similar to the published U-Pb ages and may be considered as good indicator of the crystallization age of these rocks. However, uncertainties on the 207Pb/206Pb ratios measured on zircons may constitute a limitation of the Pb evaporation/ionization method, particularly for the precise determination of the age of crystallization of Neoproterozoic rocks or younger. For granulite facies rocks, where a Pb loss discordancy has been imprinted upon the zircons, the 207Pb/206Pb ages obtained by Pb evaporation/ionization method must be considered as minimum ages. sults obtained here along with the relative simplicity of the analytical procedures used in the single zircon Pb evaporation/ionization method show great potential for the application of this technique in the reconnaissance evaluation of the ages of rocks units of Precambrian crust where a geochronological framework is not well established.