1. Institute for Rock Magnetism,; University of Minnesota; 310 Pillsbury Drive SE,; Minneapolis,; Minnesota; 55455,; USA
2. Center for Electron Nanoscopy; Technical University of Denmark; DK-2800; Kongens Lyngby; Denmark
3. Department of Earth Sciences,; University of Cambridge,; Downing Street,; Cambridge; CB2 3EQ,; UK
4. Berkeley Geochronology Center,; 2455 Ridge Road,; Berkeley,; California; 94709,; USA