[1] Univ Cambridge, Dept Earth Sci, Cambridge CB2 3EQ, England
[2] Univ Milan, Dipartimento Sci Terra, I-20133 Milan, Italy
[3] Univ Perugia, Dipartimento Sci Terra, I-06100 Perugia, Italy
[4] Univ Manchester, Dept Earth Sci, Manchester M13 9PL, Lancs, England
[5] Rutherford Appleton Lab, ISIS, Didcot OX11 0QX, Oxon, England
[6] Univ Bristol, Dept Geol, Bristol BS8 1RJ, Avon, England