[1] NASA, Lyndon B Johnson Space Ctr, Houston, TX 77058 USA
[2] Univ Maryland, Dept Geol, Isotope Geochem Lab, College Pk, MD 20742 USA
[3] Univ Chicago, Dept Geophys Sci, Chicago, IL 60637 USA
[4] Southampton Oceanog Ctr, Sch Ocean & Earth Sci, Southampton SO14 3ZH, Hants, England