1. S. S. Russell and G. J. MacPherson are in the Department of Mineral Sciences, MRC NHB-119, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA.
2. G. Srinivasan, G. R. Huss, and G. J. Wasserburg are in the Lunatic Asylum, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.