Vol 64, No 9 (2019)

The specific features of the mineralogy of SiO₂ inclusions in sublithospheric diamonds are described in this study. Such diamonds are characterized by a complex growth history with stages of growth and dissolution and postgrowth processes of deformation and crushing. The nitrogen content in all studied crystals does not exceed 71 ppm and nitrogen is detected only as B-defects. The carbon isotope composition of diamonds varies widely from -26.5 to -6.7 ‰ of δ¹³С. SiO₂ inclusions associate with omphacitic clinopyroxenes, majoritic garnets, CaSiO₃, jeffbenite and ferropericlase. All SiO₂ inclusions are coesite, which is often accompanied by micro-blocks of kyanite. These phases are suggested to represent the product of the retrograde transformation of the primary Al-stishovite. Significant internal stresses in the inclusions and deformations around them can be evidence of thise phase transformation. The heavier oxygen isotope composition of SiO₂ inclusions in sublithospheric diamonds (up to 12.9 δ¹⁸O) indicates the crustal origin of their protoliths. The observed anti-correlation of δ¹⁸O of SiO₂ inclusions and δ¹³C of their host diamonds reflects the processes of interaction of slab-derived melts with reduced mantle rocks at depths above 270 km.

Here we present results on synthesis of double K−Ca carbonates at atmospheric pressure in closed graphite capsules. The mixtures of K₂CO₃ and CaCO₃ corresponding to stoichiometry of K₂Ca(CO₃)₂ and K₂Ca₂(CO₃)₃ were used as starting materials. The low-temperature modification of K₂Ca(CO₃)₂ was synthesized by a solid-state reaction at 500°C during 96 h. The high-temperature modification of K₂Ca(CO₃)₂ as well as the K₂Ca₂(CO₃)₃ compound were synthesized both by a solid-state reaction at 600°C during 72 h and during cooling of the melt from 830 to 650°C for 30 min. The obtained carbonates were studied by Raman spectroscopy. The Raman spectrum of bütschliite is characterized by the presence of an intense band at 1093 cm⁻¹ and several bands at 1402, 883, 826, 640, 694, 225, 167 and 68 сm⁻¹. The Raman spectrum of fairchildite has characteristic intense bands at 1077 and 1063 cm⁻¹, and several bands at 1760, 1739, 719, 704, 167, 100 сm⁻¹. In the Raman spectrum of K₂Ca₂(СO₃)₃ intense bands at 1078 and 1076 cm⁻¹ and several bands at 1765, 1763, 1487, 1470, 1455, 1435, 1402, 711, 705, 234, 221, 167, 125 and 101 сm⁻¹ were found. The collected Raman spectra can be used to identify carbonate phases entrapped as microinclusions in phenocrysts and xenoliths from kimberlites and other alkaline rocks.

The peritectic reaction of ringwoodite (Mg,Fe)₂SiO₄ and silicate-carbonate melt with formation of magnesiowustite (Fe,Mg)O, stishovite SiO₂ and Mg, Na, Ca, K-carbonates is revealed by experimental study at 20 GPa of melting relations of the multicomponent MgO−FeO−SiO₂−Na₂CO₃−CaCO₃−K₂CO₃ system of the Earth’s mantle transition zone. A reaction of CaCO₃ and SiO₂ with the formation of Ca-perovskite CaSiO₃ is also detected. It is shown that the peritectic reaction of ringwoodite and melt with the formation of stishovite physic-chemically controls the fractional ultrabasic-basic evolution of both magmatic and diamond-forming systems of the deep horizons of the transition zone up to its boundary with the Earth’s lower mantle.

The results of the experimental study of the decarbonation and melting reactions in the MgCO₃–SiO₂ system at pressures up to 32 GPa using multi-anvil technique, in situ X-ray diffraction and synchrotron radiation have been reported. At 3–7 GPa and 1400–1700 K, the reaction proceeds with the release of carbon dioxide and the formation of enstatite. At 9–13 GPa and 1850–1930 K, clinoenstatite, carbonate-silicate melt, and CO₂ were found among the reaction products. At 16 GPa and 1825 K, the reaction is accompanied by the formation of wadsleyite and at higher temperature by the formation of a carbonated melt, with a Mg/Si ratio close to wadsleyite, stishovite and CO₂ fluid. At this pressure, which coincides with the wadsleyite-stishovite assemblage stability field in the MgSiO₃ phase diagram, a decrease in the reaction temperature by about 100 K is observed. At higher pressures, the reaction proceeds with the formation of the MgSiO₃ (akimotoite or bridgmanite) + melt assemblage. The reaction temperature at 25–35 GPa does not change and is about 2000 K. With a further increase in temperature to 2100 K, bridgmanite melts incongruently, reacting with a carbonate-silicate melt to form stishovite. The composition of the eutectic mixture shifts towards MgCO₃ with increasing pressure. The studied reaction marks the upper temperature limit of the stability of magnesite and the free phase of SiO₂ in the Earth’s mantle and generally coincides with the mantle adiabat at depths of 300–900 km.