[1] Univ Maryland, ESSIC, College Pk, MD 20742 USA
[2] Univ Maryland, Dept Geol, College Pk, MD 20742 USA
[3] Univ Calif Los Angeles, Dept Earth & Space Sci, Los Angeles, CA 90095 USA
[4] Carnegie Inst Washington, Geophys Lab, Washington, DC 20015 USA
[5] Geol Survey Canada, Mineral Resources Div, Ottawa, ON K1A 0E8, Canada