Age of the “least radiogenic” galenas at Isua, west Greenland

Five samples from the Isua locality B (65°07′N, 50°12′W) additional to those previously reported by P.W.U. Appel and coworkers have been assayed for Pb isotope ratios. Whilst giving close support to data on the two samples which were reported for this locality, the new information can be seen to define two very good, but short, regression lines with slopes 0.7261 ± 0.0300 on the 207Pb204Pb206Pb204Pb diagram, and 0.8403 ± 0.1239 on the 208Pb204Pb 206Pb204Pb diagram. Both lines fit the data to within experimental error. Treated as a secondary isochron, the first line dates a relatively late emplacement in the sheared quartz host at 2.8 Ga, whilst the second line imperfectly suggests a 232Th238U for the provider of the radiogenic contaminant (the Isua supracrustals?) at 3.55 ± 0.54 (2a), apparently less than the corresponding ratio for the protolith from which the initial Pb and the supracrustals were derived. Compared with the growth-curve “average”, this protolith is characterised by high 238U204Pb as well as high 232Th238U, suggesting not only the familiar tendency for U (and Ph) to precede Th into the partial melts, but also a complexity in the region's pre-3.8 Ga history which has yet to be fully explored.