[1] Berkeley Geochronol Ctr, Berkeley, CA 94709 USA
[2] Univ Calif Berkeley, Dept Earth & Planetary Sci, Berkeley, CA 94720 USA
[3] Univ Arizona, Lunar & Planetary Lab, Tucson, AZ 85721 USA
[4] CALTECH, Div Geol & Planetary Sci, Pasadena, CA 91125 USA
[5] Univ Witwatersrand, Sch Geosci, Impact Cratering Res Grp, ZA-205 Johannesburg, South Africa
[6] Univ Vienna, Inst Geochem, A-1090 Vienna, Austria
[7] NYU, Earth & Environm Sci Program, New York, NY 10003 USA
[8] Open Univ, Dept Earth Sci, Milton Keynes MK7 6AA, Bucks, England
[9] Russian Acad Sci, Inst Dynam Geospheres, Moscow 11797, Russia