1. Structural Transitions in Amorphous Mantle Phases Dan Shim Massachusetts Institute of Technology Thanks to: K. Catalli, T. Grove, NSF CIDER 2004

2. Melting in Earth Core formation Magma Ocean Top of the transition zone Bercovici and Karato (2003) Nature 425, 39+ Revenaugh and Sipkin (1994) Nature 369, 474+ Core-Mantle Boundary Revenaugh and Meyer (1997) Science 277, 670+ Williams et al (1998) Science 280, 546+ Stevenson (2008) Nature 451, 261+

3. Coordination Number increase in Si--O Stolper and Ahrens (1987) GRL 14, 1231+

4. Raman Spectra of Silicate Glasses Shim and Catalli (2009) EPSL, in press Monomer (Q 0 ) Chain (Q 2 )

5. Raman Spectra Shim and Catalli (2009) EPSL, in press

6. Differentiation in Magma Ocean Boyet and Carlson (2005) Science 309, 576+

7. Magma Ocean Differentiation in Earth and Mars

8. Direct Measurements on Melts Slotznick and Shim (2008) AM

9. Summary  In the present-day mantle, melting will further enhance the degree of differentiation atop the transition zone and the core-mantle boundary.  Si-rich melts undergo the structural transition at systematically lower pressure than Si-poor melts.  Mg-silicate glasses undergo a series of structural transitions at high pressure: increases in the Si—O coordination number and changes in the Si—O polyhedral connectivity.  The compositional sensitivity of the structural transition would result in Si-enrichment of the deeper part of the magma ocean. Therefore, the magma ocean would be compositionally stratified.