After HCl dissolution, the zircon solution is aliquoted and spiked with 208Pb/235U tracer. The Pb/U ratio of this spike has been calibrated to +/- 0.4% by comparison with NBS trtace-element glasses. Chemical isolation follows the procedure outlined by Krogh (1973), except that 200-to 400-mesh AG 1X8 ion-exchange resin is used for better retention. The samples are loaded with H3PO4 and silica gel (for Pb and most U) or graphite (for some U) onto degassed Re filaments. U was run with graphite as U metal initially, and later with silica gel as UO2. UO2 is run together with the spiked Pb aliquit on the same filament, with PB iionizing at a slightly lower temperature than that for U. The mass spectrometer utilized is a VG-354 equipped with [...]