1. Department of Terrestrial Magnetism; Carnegie Institution of Washington; Washington, D. C. 20015 USA
2. Division of Earth and Ocean Sciences, Nicholas School of the Environment; Duke University; Durham North Carolina 27708 USA
3. Department of Geography and Earth Sciences; University of North Carolina; Charlotte North Carolina 28223 USA
4. Lamont-Doherty Geological Observatory; Columbia University; Palisades New York 10964 USA
5. School of Ocean and Earth Sciences and Technology; University of Hawaii; Honolulu Hawaii 96822 USA
6. Department of Geological Sciences; Cornell University; Ithaca New York 14853-1504 USA