[1] Tufts Univ, Dept Earth & Ocean Sci, Medford, MA 02155 USA
[2] Berkeley Geochronol Ctr, Berkeley, CA 94709 USA
[3] Continental Resources, Oklahoma City, OK 73126 USA
[4] Rutgers State Univ, Dept Earth & Planetary Sci, Piscataway, NJ 08854 USA
[5] Univ Bristol, Sch Earth Sci, Bristol BS8 1RJ, Avon, England
[6] Univ Minnesota, Large Lakes Observ, Duluth, MN 55812 USA
[7] Univ Minnesota, Dept Geol Sci, Duluth, MN 55812 USA
[8] Univ Massachusetts, Dept Geosci, Amherst, MA 01003 USA