1. Boone Pickens School of Geology, Oklahoma State University, 105 Noble Research Center, Stillwater, OK 74078-3031, USA
2. Department of Geosciences, Virginia Tech, Blacksburg, VA, USA.
3. Lamont-Doherty Earth Observatory, Columbia University, 61 Rte 9W, Palisades, NY 10964, USA
4. BP America, 501 West Lake Blvd., Houston, TX 77079, USA
5. Geological Survey Department, P.O. Box 27, Zomba, Malawi
6. Department of Earth and Planetary Sciences, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA