4th Swiss Geoscience Meeting, Bern 2006 Emerged Archaean oceanic crust and its role for regulating the composition of the early atmosphere.

The first geochronological date obtained one century ago by Rutherford via the measurement of accumulated helium (He) from radioactive decay of uranium (U), while spectacular and elegant, was compromised by the tendency of He to diffuse from the U-bearing mineral. The report by Rutherford nevertheless further ignited the already fiery debate about the age of the Earth and how to best determine it. In 1907, Boltwood concluded from a survey of U-rich minerals that the final decay product of U must be lead (Pb). Arthur Holmes thus investigated the possibility of dating rocks with the U-Pb method. Namely, after elapse of sufficient time the quantity of Pb that had accumulated from decay of U could be used to estimate the age of the mineral. This phenomenon made it possible for Holmes to extract the first U-Pb geochrono-logical date of 370 million years from a U-rich rock from Norway. During WW1, with the realisation that the final decay product from radioactive Th was also Pb, it became necessary not only to determine the concentrations of U, Th and Pb, but also the exact atomic weight of Pb. From this, an estimate of the contribution of the then known two Pb isotopes to the overall Pb extracted from the mineral wa