1. School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK.
2. Instituto de Geociências, Universidade de Brasília, CEP 70910-900 Brasília, DF, Brazil.
3. Department of Terrestrial Magnetism, Carnegie Institution, Washington, DC 20015, USA.
4. Geophysical Laboratory, Carnegie Institution, Washington, DC 20015, USA.