1. Earth and Planetary Sciences Department, McGill University, 3450 University Street, Montreal, Quebec, H3A 2A7, Canada.
2. Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015, USA.
3. GEOTOP (Centre de recherche en géochemie et géodynamique), Université du Québec à Montréal, Post Office Box 8888, Succursale Centre-ville, 210, Président-Kennedy Avenue, Montreal, Quebec H3C 3P8, Canada.