1. Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road NW, Washington, D.C. 20015, USA
2. Department of Earth, Environmental and Planetary Sciences, Brown University, 324 Brook Street, Providence, Rhode Island 02912, USA
3. Department of Geology, University of Georgia, 210 Field Street, Athens, Georgia 30602-2501, USA
4. Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964, USA
5. Department of Earth Sciences, ETH Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland