1. Hanse Institute of Advanced Study; D-27753 Delmenhorst Germany
2. Department of Marine Chemistry and Geochemistry; Woods Hole Oceanographic Institution; Woods Hole Massachusetts 02543 USA
3. Department of Geology and Geophysics; University of Oklahoma; Norman Oklahoma 73019 USA
4. Australian Geological Survey Organisation; GPO Box 378, Canberra ACT 2601 Australia