Virginia Tech, Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall (0420), Blacksburg, VA 24061,
University of Arizona, Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721-0077,
Carnegie Institution of Washington, Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd., Washington, DC 20015,