1. Department of Geology and Geophysics, University of California, Berkeley, CA 94720. O. A
2. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109
3. Research School of Earth Sciences, Australian National University, Canberra ACT 2601, Australia
4. Department of Integrative Biology, University of California, Berkeley, CA 94720
5. Department of Soil and Water Science, University of Arizona, Tucson, AZ 85721
6. BHP-Utah International Metals, 550 California Street, San Francisco, CA 94104