1. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139–4307, USA.
2. Department of Geosciences, Princeton University, Princeton, NJ 08544, USA.
3. Department of Geophysics, Stanford University, Stanford, CA 94305–2004, USA.
4. Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA.