mf • P , 9 , u , P , 9 , 9 x ' ' Us ' Usb ' • Pm ; s GPa

Shock compression data are reported for hot-pressed Bamble bronzite (En86) loaded to pressures between 104 and 161 GPa. When compared to earlier shock wave data on En90 at lower pressures and to static compression data, our data require the presence of a phase change. In space the data yield two distinct trajectories, which cannot be explained by experimental error. The higher-density data, corrected for porosity and a small amount of metallic iron impurity, agree with a theoretical En86 high-pressure phase Hugoniot calculated from static compression equation of state data for perovskite (pv) structure silicates when experimental errors and uncertainties in the equation of state parameters are considered. All the En86 data can be described by a calculated Hugoniot if the first pressure derivative of the MgSiO3(PV) bulk modulus is taken as 4.5+1.0. Combining the present preferred data with recent shock wave data for single-crystal forsterite, we find that En86 is slightly more dense than Fo86 at pressures above 110 GPa. Comparison of the forsterite and enstatite data with the Preliminary Reference Earth Model (PREM) lower mantle densities, with corrections applied for the higher shock temperatures relative to lower mantle temperatures, shows that PREM densities are satisfied by olivine or pyroxene stoichiometries with Mg mole fractions from 0.82 to 0.90. These values are lower than estimates of 0.90 to 0.95 developed from extrapolating static compression data to lower mantle conditions.