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 Pb-207/P b-206 apparent ages obtained by single zircon Pb evaporation/ionization met hod, in rock samples previously dated by conventional U-Pb zircon dissoluti on procedures. The technique applied here uses a single rhenium center fila ment to evaporate, ionize and measure the Pb isotopic ratios, in a thermal ionization mass spectrometer, by direct evaporation/ionization of a single zircon grain. The single zircon Pb evaporation/ionization method was applied in igneous a nd metamorphic rocks of Archean to Neoproterozoic age. The Pb-207/Pb-206 Ob tained 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, unce rtainties on the Pb-207/Pb-206 ratios measured on zircons may constitute a limitation of the Pb evaporation/ionization method, particularly for the pr ecise determination of the age of crystallization of Neoproterozoic rocks o r younger. For granulite facies rocks, where a Pb loss discordancy has been imprinted upon the zircons, the Pb-207/Pb-206 ages obtained by Pb evaporat ion/ionization method must be considered as minimum ages. The results obtained here along with the relative simplicity of the analyti cal procedures used in the single zircon Pb evaporation/ionization method s how great potential for the application of this technique in the reconnaiss ance evaluation of the ages of rocks units of Precambrian crust where a geo chronological framework is not well established. (C) 1998 Elsevier Science Ltd. All rights reserved.