1. Department of Geological Sciences, Stanford University, Stanford, California 94305, USA
2. Department of Geological Sciences, Stockholm University, SE-106 91, Stockholm, Sweden
3. U.S. Geological Survey, Menlo Park, California 94025, USA
4. School of Earth and Environmental Sciences, Washington State University, Pullman, Washington 99164, USA
5. Department of Earth and Planetary Sciences, University of California, Santa Cruz, California 95064, USA