1. Lamont-Doherty Earth Observatory and Department of Earth and Environmental Sciences; Columbia University; Palisades New York 10968 USA
2. Oberlin College; Oberlin Ohio 44074 USA
3. Lamont-Doherty Earth Observatory; Columbia University; Palisades New York 10968 USA
4. CLRC; Daresbury Laboratory; Warrington WA4 4AD England, UK
5. Institut fuer Mineralogie; J. W. Goethe-Universitaet; Senckenberganlage 28, 60054 Frankfurt a.M. Germany