1. Earth and Environmental Sciences Area, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
2. Rocky Mountain Biological Laboratory, Gothic, CO 81224, USA.
3. Geology, Geophysics, and Geochemistry Science Center, U.S. Geological Survey, Denver, CO 80225, USA.
4. Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, USA.