[1] British Geol Survey, NERC Isotope Geosci Lab, Keyworth NG12 5GG, Notts, England
[2] Univ Milan, Dipartimento Sci Terra A Desio, I-20133 Milan, Italy
[3] Inst Geol Econ CSIC UCM, Fac Ciencias Geol, Dept Paleontol, Madrid 28040, Spain
[4] British Geol Survey, Edinburgh EH9 3LA, Midlothian, Scotland