1. a Department of Geology , University of the West Indies , Kingston 7, Jamaica
2. b SLOWPOKE Reactor Facility , University of Alberta , Alberta T8G 2N0, Edmonton, Canada
3. c Camborne School of Mines , University of Exeter, Redruth , TR15 3SE, Cornwall, England
4. d Petroleum Office, Ministry of Natural Resources , Belmopan, Belize
5. e Department of Chemistry , University of Hull , Hull HU6 7RX, England