This research used samples provided by the Ocean Drilling Program (ODP). The ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. Funding for this research was provided by the Deutsche Forschungsgemeinschaft grant UR74/2 to Michael Urbat. Discussions with Peter Hofmann improved the manuscript. Alexander Weingarten and Mathias Höcker carried out the magnetic measurements. Reviews by Gary Acton and an anonymous reviewer are very much appreciated.