1. Division of Geological Sciences, California Institute of Technology, Pasadena 91109; Mineralogical Institute, Swiss Federal Institute of Technology, Zurich, Switzerland.
2. Division of Geological Sciences, California Institute of Technology, Pasadena 91109; Isotape Geology Branch, U.S. Geological Survay, Denver Colorado.
3. Division of Geological Sciences, California Institute of Technology, Pasadena 91109