1. Contribution from the Physics Department, Brookhaven National Laboratory, Upton, New York 11973-5000, School of Chemical Sciences, University of Birmingham, Birmingham, B15 2TT, UK, Geosciences Department, State University of New York, Stony Brook, New York 11794-2100, and Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, I-20133 Milano, Italy