1. Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, U.S.A.
2. † Present address: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
3. ‡ Present address: Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, U.S.A.
4. GeoSoilEnviroCARS, University of Chicago, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A.
5. § Present address: Institut für Geologie und Mineralogie, Universität zu Köln, 50674 Köln, Germany.