1Department of Earth and Planetary Sciences, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854-8066, USA
Correspondence and requests for materials should be addressed to C.H. ([email protected]).
Received 04 Spetember 2008; accepted 28 January 2009
Acknowledgements We are grateful to N. Sleep and A. Kerr for reviews, and to C. Class, M. Hirschmann, P. Asimow, M. Humayun and K. Hoernle for discussions.
