Isotopic analyses were performed on 1- to 9-mg zircon fractions by using an extraction method similar to that described by Krogh (1973). A mixed 205Pb-235U spike was used and isotopic neasurements were done on a 35-cm-radius, 90 degree-sector, single-focusing, solid-source Avco mass spectrometer. The standard silica get-H3PO4 emitter technique was used for lead isotopec determination; the uranium isotopes were measured by using the graphite-H3PO4 emitter technique.