[1] The phase relations and density of a natural mid-ocean ridge basalt (MORB) were investigated from 28 to 89 GPa and 1600 to 2700 K by in situ X-ray diffraction measurements and chemical analysis of the quenched samples using transmission electron microscopy (TEM). We observed an assemblage of five phases up to 50 GPa, namely an aluminum-bearing magnesium perovskite phase, a calcium perovskite phase, a stishovite phase, the new aluminum-rich (NAL) phase, and a calcium ferrite-type phase. The NAL phase was no longer observed above 50 GPa. The phase proportions were obtained by Rietveld refinement of the in situ X-ray diffraction patterns. After the disappearance of the NAL phase beyond 50 GPa, the proportion of each phase remains constant up to 89 GPa. The density of MORB was calculated using the measured volumes, phase proportions, and chemical compositions of the coexisting phases. The thermoelastic parameters of the MORB sample were estimated from the fit of the measured densities at various pressure and temperature conditions. Resulting MORB density profiles were calculated for different subducting slab temperature profiles. MORB densities are 0.5% to 2% greater than those of the surrounding mantle over the entire lower mantle range, suggesting MORB likely subducts to the core-mantle boundary.

The high pressure behavior of bundled 1.35Ϯ 0.1 nm diameter single wall carbon nanotubes ͑SWNT͒ filled with C 70 fullerenes ͑usually called peapods͒ has been investigated by Raman spectroscopy and compared with the corresponding behavior of the nonfilled SWNT. We show experimentally that two reversible pressureinduced transitions take place in the compressed bundle SWNT. The first transition, in the 2-2.5 GPa range, is in good correspondence with predictions of the thermodynamic instability of the nanotube circular cross section for the studied tube diameter. An interaction between the fullerenes and the tube walls is then observed at about 3.5 GPa, which evidences a progressive deformation of the tube cross section. The second transition takes place at pressures between 10 and 30 GPa, and is evidenced by two effects by a strong frequency downshift of the Raman transverse modes and the concomitant disappearance of the fullerenes Raman modes in peapods. The pressure at which the second transition takes place is strongly dependent on the nature of the pressure transmitting medium. We also report irreversible effects at high pressure as the shortening of the tubes, the formation of nanostructures and the disappearance of the C 70 Raman signal in some cases. Transmission electron microscopy studies are also reported supporting these transformations.