1. 1Department of Geological Sciences, Stanford University, Stanford, California 94305, USA
2. 2Department of Earth and Planetary Science, University of California−Berkeley, Berkeley, California 94720, USA
3. 3Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA
4. 4Department of Earth and Planetary Sciences, University of California−Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA
5. 5U.S. Geological Survey, Alaska Science Center, 4210 University Drive, Anchorage, Alaska 99508, USA