Fusion and solidification of Al and Ag samples, as well as Fe₉₃–Al₃–C₄, Fe₅₆–Co₃₇–Al₃–C₄, and Fe_57.5–Co₃₈–Al₁–Pb_0.5–C₃ alloys (in wt%), have been investigated at 6.3 GPa. Heater power jumps due to heat consumption and release on metal fusion and solidification, respectively, were used to calibrate the thermal electromotive force of the thermocouple against the melting points (mp) for Ag and Al. Thus, obtained corrections are +100°C (for sample periphery) and +65°C (center) within the 1070–1320°C range. For small samples positioned randomly in the low-gradient zone of a high pressure cell, the corrections should be +80°C and +84°C at the temperatures 1070°C and 1320°C, respectively. The temperature contrast recorded in the low-gradient cell zone gives an error about ±17°C. The method has been applied to identify the mp of the systems, which is especially important for temperature-gradient growth of large type IIa synthetic diamonds.

Deep carbon and nitrogen cycles played a critical role in the evolution of the Earth. Here we report on successful studying of speciation in C-O-H-N systems with low nitrogen contents at 6.3 GPa and 1100 to 1400 °C. At fO₂ near Fe–FeO (IW) equilibrium, the synthesised fluids contain more than thirty species. Among them, CH₄, C₂H₆, C₃H₈ and C₄H₁₀ are main carbon species. All carbon species, except for C₁-C₄ alkanes and alcohols, occur in negligible amounts in the fluids generated in systems with low H₂O, but С₁₅-С₁₈ alkanes are slightly higher and oxygenated hydrocarbons are more diverse at higher temperatures and H₂O concentrations. At a higher oxygen fugacity of +2.5 Δlog fO₂ (IW), the fluids almost lack methane and contain about 1 rel.% C₂-C₄ alkanes, as well as fractions of percent of C_15–18 alkanes and notable contents of alcohols and carboxylic acids. Methanimine (CH₃N) is inferred to be the main nitrogen species in N-poor reduced fluids. Therefore, the behaviour of CH₃N may control the nitrogen cycle in N-poor peridotitic mantle. Oxidation of fluids strongly reduces the concentration of CH₄ and bulk carbon. However, higher alkanes, alcohols, and carboxylic acids can resist oxidation and should remain stable in mantle hydrous magmas.

Melting and multiple saturation experiments with systems simulating primary kimberlite magma compositions at 5.5–6.5 GPa provide constraints on magma generation conditions. The liquidus of model kimberlitic systems exceeds the hottest temperatures of lithospheric mantle (1400 °C) but is 150–200 °C lower in systems with lower CO2/(CO2 + H2O) ratios. The high melting points require additional heat sources for the generation of kimberlite magmas. Multiple saturation of experimental melts and the stability of individual near-liquidus phases depend on both major-element contents and XCO2 (as the CO2/(CO2 + H2O) molar ratio) in the starting composition. Generally, olivine-bearing assemblages are stable at XCO2 < 0.5, while an increase in MgO/CaO from 1.8 to > 4.0 leads to progressive changes in the equilibrium assemblages: Ol + Grt + Cpx → Ol + Grt + Opx + Cpx → Ol + Grt + Opx. The results of geochemical reconstructions and multiple saturation experiments indicate partial or complete wehrlitization of the kimberlitic source regions. Most of primary magmas with XCO2 < 0.5 may have been derived from carbonated garnet lherzolite. Some highly calcic (MgO/CaO < 2) magmas with XCO2 < 0.5 likely originated from carbonated garnet wehrlite. A probable scenario is that melts and/or fluids repeatedly metasomatized and oxidized the protolith (caused its carbonation and phlogopitization) and thus provided conditions for buffering CO2 and H2O fugacities in the forming kimberlitic magma, at least early in the melting history. During later magma generation, water was, likely, extracted from nominally anhydrous minerals having hydrated (OH) defects in the structure.