1. Univ. Lyon, Univ. Lyon 1, ENSL, CNRS, LGL-TPE, Villeurbanne, 69100 France
2. Univ. Göttingen, GZG, Dept. of Mineralogy, Goldschmidtstr. 1, 37077 Göttingen, Germany
3. Dept. of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912, USA