[1] Univ Paris 07, Lab Geochim & Cosmochimie, Inst Phys Globe, F-75252 Paris 05, France
[2] ETH, Inst Isotope Geochem & Mineral Resources, CH-8092 Zurich, Switzerland
[3] Univ Oxford, Dept Earth Sci, Oxford OX1 3PR, England
[4] Ecole Normale Super Lyon, Lab Sci Terre, F-69364 Lyon 7, France