1Department of Geological and Environmental Sciences, 450 Serra Mall, Building 320, Stanford, California 94305, U.S.A.;
2Department of Geological and Environmental Sciences, 450 Serra Mall, Building 320, Stanford, California 94305, U.S.A.; present address: Chevron Energy Technology Co., 6001 Bollinger Canyon Road, San Ramon, California 94305 U.S.A.