1. Department of Earth & Planetary Sciences, University of California Santa Cruz, California 95064, U.S.A.
2. * Present address: Department of Mineral Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801, U.S.A. E-mail: marcus.dot.silva@gmail.com
3. Physical and Life Sciences, Physics Division, Lawrence Livermore National Laboratory, Livermore, California 94550, U.S.A.