Polycrystalline samples of Mg2SiO4 forsterite and wadsleyite were synthesized and then dynamically loaded to pressures of 39-200 GPa. Differences in initial density and internal energy between these two phases lead to distinct Hugoniots, each characterized by multiple phase regimes. Transformation to the high-pressure phase assemblage MgO + MgSiO3 perovksite is complete by 100 GPa for forsterite starting material but incomplete for wadsleyite. The datum for wadsleyite shocked to 136 GPa, however, is consistent with the assemblage MgO + MgSiO3 post-perovksite. Marked increases in density along the Hugoniots of both phases between ∼130 and 150 GPa are inconsistent with any known solid-solid phase transformation in the Mg2SiO4 system but can be explained by melting. Density increases upon melting are consistent with a similar density increase observed in the MgSiO3 system. This implies that melts with compositions over the entire Mg/Si range likely for the mantle would be negatively or neutrally buoyant at conditions close to the core-mantle boundary, supporting the partial melt hypothesis to explain the occurrence of ultra-low velocity zones at the base of the mantle. From the energetic difference between the high-pressure segments of the two Hugoniots, we estimate a Grüneisen parameter (γ) of 2.6 ± 0.35 for Mg2SiO4-liquid between 150 and 200 GPa. Comparison to low-pressure data and fitting of the absolute pressures along the melt Hugoniots both require that γ for the melt increases with increasing density. Similar behavior was recently predicted in MgSiO3 liquid via molecular dynamics simulations. This result changes estimates of the temperature profile, and hence the dynamics, of a deep terrestrial magma ocean.