1. Department of Earth Science & Engineering, Royal School of Mines, Imperial College, London SW7 2AZ, UK
2. Department of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE, UK
3. Géosciences Montpellier, Université de Montpellier, Centre national de la recherche scientifique (CNRS), 34095 Montpellier cedex 05, France