| Реферат: | eng: Potassic dolomitic melts are believed to be responsible for the metasomatic alteration of the shallow continental lithosphere. However, the temperature stability and range of compositions of these melts are poorly understood. In this regard, we performed experiments on phase relationships in the system K2CO3-CaCO3-MgCO3 at 3 GPa and at 750-1100 degrees C. At 750 and 800 degrees C, the system has five intermediate compounds: Dolomite, Ca0.8Mg0.2CO3 Ca-dolomite, K-2(Ca >= 0.84Mg <= 0.16)(2)(CO3)(3), K-2(Ca >= 0.70Mg <= 0.30)(CO3)(2) butschliite, and K-2(Mg >= 0.78Ca <= 0.22)(CO3)(2). At 850 degrees C, an additional intermediate compound, K-2(Ca >= 0.96Mg <= 0.04)(3)CO3)(4), appears. The K2Mg(CO3)(2) compound disappears near 900 degrees C via incongruent melting, to produce magnesite and a liquid. K2Ca(CO3)(2) butschliite melts incongruently at 1000 degrees C to produce K2Ca2(CO3)(3) and a liquid. K2Ca2(CO3)(3) and K2Ca3(CO3)(4) remain stable in the whole studied temperature range. The liquidus projection of the studied ternary system is divided into nine regions representing equilibrium between the liquid and one of the primary solid phases, including magnesite, dolomite, Ca-dolomite, calcite-dolomite solid solutions, K2Ca3(CO3)(4), K2Ca2(CO3)(3), K2Ca(CO3)(2) butschliite, K2Mg(CO3)(2), and K2CO3 solid solutions containing up to 24 mol % CaCO3 and less than 2 mol % MgCO3. The system has six ternary peritectic reaction points and one minimum on the liquidus at 825 +/- 25 degrees C and 53K(2)CO(3)center dot 47Ca(0.4)Mg(0.6)CO(3). The minimum point resembles a eutectic controlled by a four-phase reaction, by which, on cooling, the liquid transforms into three solid phases: K-2(Mg0.78Ca0.22)(CO3)(2), K-2(Ca0.70Mg0.30)(CO3)(2) butschliite, and a K1.70Ca0.23Mg0.07CO3 solid solution. Since, at 3 GPa, the system has a single eutectic, there is no thermal barrier for liquid fractionation from alkali-poor toward K-rich dolomitic compositions, more alkaline than butschliite. Based on the present results we suggest that the K-Ca-Mg carbonate melt containing 45 mol % K2CO3 with a ratio Ca/(Ca + Mg) = 0.3-0.4 is thermodynamically stable at thermal conditions of the continental lithosphere (850 degrees C), and at a depth of 100 km.
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