[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 global geochemical carbon cycle involves exchanges between the Earth's interior and the surface. Carbon is recycled into the mantle via subduction mainly as carbonates and is released to the atmosphere via volcanism mostly as CO 2 . The stability of carbonates versus decarbonation and melting is therefore of great interest for understanding the global carbon cycle. For all these reasons, the thermodynamic properties and phase diagrams of these minerals are needed up to core mantle boundary conditions. However, the nature of C-bearing minerals at these conditions remains unclear. Here we show the existence of a new Mg-Fe carbon-bearing compound at depths greater than 1,800 km. Its structure, based on three-membered rings of corner-sharing ðCO 4 Þ 4− tetrahedra, is in close agreement with predictions by first principles quantum calculations [Oganov AR, et al. (2008)