1. Science Institute, University of Iceland, Reykjavik, Iceland
2. Geophysical Institute. University of Copenhagen, Haraldsgade 6, DK-2200 Copenhagen, Denmark
3. Laboratoire de Geochimie lsotopique, LODYC(UA CNRS 1206) CEA/IRDl/DESICP
4. Departement de Physico-Chimie, 91191 Gif sur Yvette, Cedex, France, and Geochemistry Department, Lamont-Doherty Geological Observatory, Columbia University, Palisades, New York 10964, USA