1. Division of Applied Geology, Luleå University of Technology, 971 87 Luleå, Sweden, Department of Geology and Geochemistry, Stockholm University, 106 91 Stockholm, Sweden, Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, 104 05 Stockholm, Sweden, and Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK