1. Lunar and Planetary Laboratory, The University of Arizona, Tucson, Arizona 85721, U.S.A.
2. Materials Science and Engineering, The University of Arizona, Tucson, Arizona 85721, U.S.A.
3. † Present address: Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, U.S.A.
4. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A