Vol 57, No 9 (2019)

Behavior of nitrogen in the deep Earth still remains unrevealed. Geochemical data suggest that substantial amount of nitrogen could be stored in the deep Earth. In this study, reactions between forsterite (Mg₂SiO₄) and molecular nitrogen (N₂) were investigated at high pressure and high temperature using laser-heating diamond anvil cells (DACs). Pressure in the DAC was estimated from Raman spectra of nitrogen before heating and the initial pressure was set at 5 GPa. Pelleted sample of powder forsterite or a single crystal of forsterite was loaded in the DAC with N₂ fluid. A carbon dioxide laser (λ = 10.64 μm, <100 W) and a fiber laser (λ = 1.019 μm, <100 W) were used to heat forsterite in the temperature range from 1300 to 3500 K. An SEM image on the surface of the recovered forsterite crystal after the laser heating showed a stepwise texture which strongly suggests the dissolution of forsterite into the N₂ fluid. The EDS chemical mapping showed that Mg-rich area and Si-poor area overlapping each other, which suggests the preferential dissolution of MgO component and its precipitation from the N₂ fluid. X-ray diffraction patterns of the powder and single crystal forsterite samples after the reaction showed reflections assignable to orthopyroxene (MgSiO₃) and periclase (MgO). The present experimental results indicate that Mg₂SiO₄ incongruently melts into MgSiO₃ and MgO in N₂ fluid. Moreover, N1s XPS spectra collected from a single crystal of forsterite after the reaction with N₂ fluid revealed three components assignable to \({\text{NH}}_{4}^{ + },\) N₂, and N^3–. The present study provides a new clue to the reaction between forsterite and molecular nitrogen under the upper mantle condition.

The data on the composition of microinclusions in diamonds from the Lomonosov deposits are reported for the first time. The studied diamonds include “coated” (n = 5) and cubic (n = 5) crystals. The estimated range of the degree of nitrogen aggregation in diamonds (4–39% B1) does not support their direct links with kimberlite magmatism; however, their short occurrence in the mantle at higher temperatures is probable as well. The composition of melt/fluid microinclusions in these samples varies from essentially carbonatitic to significantly silicate. It is shown that the contents of MgO, CaO, Na₂O, Cl, and P₂O₅ decrease with increasing content of silicates and water. Different mechanisms of the generation and evolution of diamond-forming media are discussed to explain the observed variations.

The study of partial melting of model pyrolite showed that garnets synthesized at 7 GPa within the temperature range of 1400–1800°C were characterized by an excess in Si content (>3 f.u.) and a stable admixture of Cr₂O₃, and thus are pyrope–majorite–knorringite solid solutions. An increase in the Cr/Al ratio in the starting composition results in an increase of Cr/Al in garnet. With increasing temperature, the concentration of Cr₂O₃ in restite from partial melting decreases, whereas the chromium content in the melt increases. This is accompanied by an increase in the Cr/Al ratio in all garnets (from the zones of restite and quenched melt). Estimates of the bulk compositions of restite produced via partial melting of model pyrolite at 2.5 and 3.0 GPa showed that the concentration of chromium in restite was higher than that in the starting material. All minerals from the restite zone are characterized by high chromium concentrations, since partial melting within the spinel-depth facies results in redistribution of chromium into restite. The results we obtained show that the origin of high-Cr garnets is related to the protolith with a high Cr/Al ratio formed as a residue from partial melting in the field of spinel stability and further removed into the garnet-depth facies.

The paper reports results of an experimental study of decarbonation and melting reactions in the MgCO₃–SiO₂ system at pressures up to 32 GPa, using multianvil technique and in-situ X-ray diffraction with synchrotron radiation. 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 associated with the formation of wadsleyite and, at a higher temperature, by the generation of carbonated melt (with Mg/Si ratio close to wadsleyite) stishovite, and CO₂ fluid. At this pressure, which coincides with the stability field of the wadsleyite–stishovite assemblage in the MgSiO₃ phase diagram, the reaction temperature decreases by about 100 K. 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 the carbonate–silicate melt to form stishovite. The composition of the eutectic mixture shifts towards MgCO₃ with increasing pressure. The reaction marks the upper temperature limit for the stability of magnesite and a free SiO₂ phase in the Earth’s mantle and generally coincides with the mantle adiabat at depths of 300–900 km.