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Second, the full trend in melt evolution from silicate-rich to carbonate-rich melts, which is also observed in inclusions in diamonds, can be explained by melting of K-and CO 2-bearing, water-undersaturated MORB compositions. In cratonic environments low-degree silicate and immiscible silicate and carbonate melts will metasomatize the overlying mantle in different ways, producing, in the first instance, Si enrichment and crystallization of additional orthopyroxene, phlogopite, pyrope-rich garnet and consuming olivine, and, in the second case, carbonate metasomatism, with additional magnesite^dolomite, clinopyroxene and apatite. Both metasomatic styles have been described in natural peridotite xenoliths from the cratonic lithosphere.

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The equations of state for solid (with bcc, fcc, and hcp structures) and liquid phases of Fe were defined via simultaneous optimization of the heat capacity, bulk moduli, thermal expansion, and volume at room and higher temperatures. The calculated triple points at the phase diagram have the following parameters: bcc–fcc–hcp is located at 7.3 GPa and 820 K, bcc–fcc–liquid at 5.2 GPa and 1998 K, and fcc–hcp–liquid at 106.5 GPa and 3787 K. At conditions near the fcc–hcp–liquid triple point, the Clapeyron slope of the fcc–liquid curve is dT/dP = 12.8 K/GPa while the slope of the hcp–liquid curve is higher (dT/dP = 13.7 K/GPa). Therefore, the hcp–liquid curve overlaps the metastable fcc–liquid curve at pressures of about 160 GPa. At high-pressure conditions, the metastable bcc–hcp curve is located inside the fcc-Fe or liquid stability field. The density, adiabatic bulk modulus and P-wave velocity of liquid Fe calculated up to 328.9 GPa at adiabatic temperature conditions started from 5882 K (outer/inner core boundary) were compared to the PREM seismological model. We determined the density deficit of hcp-Fe at the inner core boundary (T = 5882 K and P = 328.9 GPa) to be 4.4%.

In situ X‐ray diffraction study of post‐spinel transformation in hydrous peridotite (2 wt.% H2O) indicates that the phase boundary is shifted to higher pressures by 0.6 GPa relative to anhydrous peridotite at 1473 K, whereas, it shows no obvious shift at high temperature around 1873 K. A linear equation for the boundary is P (GPa) = −0.002 T (K) + 26.3, which is applicable for temperatures below 1800 K. The present data shows that a significant part of the depressions seen in the 660‐km seismic discontinuity may be affected by existence of water. It is also well resolved that the olivine‐wadsleyite phase transformation corresponding to the 410‐km seismic discontinuity is shifted to lower pressures by 1–2 GPa by the addition of water. Thus, the topography of seismic discontinuities in the transition zone associated with slabs can be attributed not only to cold subduction but also wet subduction.

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The importance for the global carbon cycle, the P–T phase diagram of CaCO3 has been under extensive investigation since the invention of the high-pressure techniques. However, this study is far from being completed. In the present work, we show the existence of two new high-pressure polymorphs of CaCO3. The crystal structure prediction performed here reveals a new polymorph corresponding to distorted aragonite structure and named aragonite-II. In situ diamond anvil cell experiments confirm the presence of aragonite-II at 35 GPa and allow identification of another high-pressure polymorph at 50 GPa, named CaCO3-VII. CaCO3-VII is a structural analogue of CaCO3-P21/c-l, predicted theoretically earlier. The P–T phase diagram obtained based on a quasi-harmonic approximation shows the stability field of CaCO3-VII and aragonite-II at 30–50 GPa and 0–1200 K. Synthesized earlier in experiments on cold compression of calcite, CaCO3-VI was found to be metastable in the whole pressure–temperature range.