1. 1 Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, U.S.A.
2. 2 Department of Earth Science, University of Bergen, Allegaten 41, 5007 Bergen, Norway
3. 3 CNR–Istituto di Geoscienze e Georisorse, Consiglio Nazionale delle Ricerche Area della Ricerca di Pisa, 56127 Pisa, Italy