1. Graduate student in geochemistry, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125.
2. Senior scientist and member of the professional staff, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125.
3. Professor of geology in the Department of Geological Sciences at the University of Southern California, Los Angeles, CA 90089.
4. John D. MacArthur Professor of Geology and Geophysics in the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125.