Pb and U were eluted sequentially into the same beaker followed by the addition of ~10 ul of 0.6N ultra pure phosphoric acid. Each sample was loaded onto a single zone refined Re filament using a phosphoric acid-silica gel emitter (SiCl4). Isotopic ratios were measured using a modified single collector VG-54R thermal ionization mass spectrometer equipped with an analogue Daly photomultiplier. Both U and Pb, respectively, were run at 1450�C, in peak-switching mode on the Daly detector. U fractionation was determined directly on individual runs using the 233-235U tracer, and Pb isotopic ratios were corrected for fractionation of 0.37%/amu, based on replicate analyses of the NBS-981 Pb standard and the values recommended by Thirlwall [...]