1. Dept. of Physics and Geology, University of Perugia, Piazza Università, 06100 Perugia, Italy
2. Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, F-63000 Clermont-Ferrand, France
3. Dept. Earth and Environmental Sciences, Ludwig-Maximilian-University (LMU), Theresienstrasse 41/III, 80333 München, Germany