1. Department of Earth Sciences, Durham University, Durham DH1 3LE, UK
2. Department of Earth Sciences, Memorial University of Newfoundland, St John's, NL, A1C 5S7, Canada
3. Geological Survey of Norway, Leiv Erikssons vei 39, Trondheim, Norway
4. Department of Geoscience, Aarhus University, 8000 Aarhus, Denmark