1. Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, Virginia 24060, USA
2. Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221, USA
3. Kentucky Geological Survey, 228 Mining and Mineral Resources Building, University of Kentucky, Lexington, Kentucky 40506, USA
4. Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA
5. Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA