1. School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK
2. Aspinall and Associates, Tisbury SP3 6HF, UK
3. School of Mathematics, University of Bristol, Bristol BS8 1QU, UK
4. Institute of Applied Mathematics, Delft University of Technology, Building 28, Mourik Broekmanweg 6, 2628 XE Delft, The Netherlands
5. Resources for the Future, 1616 P Street NE, Washington, DC, USA
6. Departamento de Mineralogía y Petrología, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain