Part of this project was funded by a grant from the Center of Advanced Studies of the Norwegian Academy of Sciences and by the EU-Network "Precip-Dissolution" contract number HPRN-CT-2000-00058. JA acknowledges funding by the DFG-Graduiertenkolleg "Composition and Evolution of Crust and Mantle". Discussions with Francois Renard, Renault Toussaint, Jens Feder and Dag Dysthe are gratefully acknowledged.