1. Department of Geosciences, Smith College, Northampton, Massachusetts 01063, USA
2. Department of Geology, Amherst College, Amherst, Massachusetts 01002, USA
3. Department of Earth & Planetary Sciences, Washington University in St. Louis, St. Louis, Missouri 63130, USA
4. Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK
5. The School of Earth Sciences, Leeds University, Leeds, West Yorkshire LS2 9JT, UK