Diamond is an evidence for carbon existing in the deep Earth. Some diamonds are considered to have originated at various depth ranges from the mantle transition zone to the lower mantle. These diamonds are expected to carry significant information about the deep Earth. Here, we determined the phase relations in the MgCO3-SiO2 system up to 152 GPa and 3,100 K using a double sided laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction. MgCO3 transforms from magnesite to the high-pressure polymorph of MgCO3, phase II, above 80 GPa. A reaction between MgCO3 phase II and SiO2 (CaCl2-type SiO2 or seifertite) to form diamond and MgSiO3 (bridgmanite or post-perovsktite) was identified in the deep lower mantle conditions. These observations suggested that the reaction of the MgCO3 phase II with SiO2 causes formation of super-deep diamond in cold slabs descending into the deep lower mantle.

Hydrogen transport from the surface to the deep interior and distribution in the mantle are important in the evolution and dynamics of the Earth. An aluminum oxy-hydroxide, δ-AlOOH, might influence hydrogen transport in the deep mantle because of its high stability extending to lower mantle conditions. The compressional behavior and spin states of δ-(Al,Fe3+)OOH phases were investigated with synchrotron X-ray diffraction and Mössbauer spectroscopy under high pressure and room temperature. Pressure-volume (P-V) profiles of the δ-(Al0.908(9)57Fe0.045(1))OOH1.14(3) [Fe/(Al+Fe) = 0.047(10), δ-Fe5] and the δ-(Al0.832(5)57Fe0.117(1))OOH1.15(3) [Fe/(Al+Fe) = 0.123(2), δ-Fe12] show that these hydrous phases undergo two distinct structural transitions involving changes in hydrogen bonding environments and a high- to low-spin crossover in Fe3+. A change of axial compressibility accompanied by a transition from an ordered (P21nm) to disordered hydrogen bond (Pnnm) occurs near 10 GPa for both δ-Fe5 and δ-Fe12 samples. Through this transition, the crystallographic a and b axes become stiffer, whereas the c axis does not show such a change, as observed in pure δ-AlOOH. A volume collapse due to a transition from high- to low-spin states in the Fe3+ ions is complete below 32–40 GPa in δ-Fe5 and δ-Fe12, which i ~10 GPa lower than that reported for pure ε-FeOOH. Evaluation of the Mössbauer spectra of δ-(Al0.824(10)57Fe0.126(4))OOH1.15(4) [Fe/(Al+Fe) = 0.133(3), δ-Fe13] also indicate a spin transition between 32–45 GPa. Phases in the δ-(Al,Fe)OOH solid solution with similar iron concentrations as those studied here could cause an anomalously high ρ/νΦ ratio (bulk sound velocity, defined as K/ρ at depths corresponding to the spin crossover region (~900 to ~1000 km depth), whereas outside the spin crossover region a low ρ/νΦ anomaly would be expected. These results suggest that the δ-(Al,Fe)OOH solid solution may play an important role in understanding the heterogeneous structure of the deep Earth.

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[1] We determined the compressional velocity of hcp-Fe using high-resolution inelastic X-ray scattering combined with in situ X-ray powder diffraction: Our measurements extend up to 174 GPa at room temperature, to 88 GPa at 700 K, and to 61.5 GPa at 1000 K. Our data, including those obtained at high temperature, are well described by a linear relation to density, extending the range of verification of Birch's law and suggesting only small temperature dependence up to 1000 K. This result, once compared to the preliminary reference Earth model seismologically based model, indicates that there is either a strong temperature effect on Birch's law above 1000 K or the composition of the core is rather different than expected, containing, e.g., heavy impurities. Noting that both recent theoretical calculations and shock wave velocity measurements are consistent with modification of Birch's law at high temperature, we favor the former explanation.