Abstract
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 °C. At 750 and 800 °C, the system has five intermediate compounds: Dolomite, Ca0.8Mg0.2CO3 Ca-dolomite, K2(Ca≥0.84Mg≤0.16)2(CO3)3, K2(Ca≥0.70Mg≤0.30)(CO3)2 bütschliite, and K2(Mg≥0.78Ca≤0.22)(CO3)2. At 850 °C, an additional intermediate compound, K2(Ca≥0.96Mg≤0.04)3CO3)4, appears. The K2Mg(CO3)2 compound disappears near 900 °C via incongruent melting, to produce magnesite and a liquid. K2Ca(CO3)2 bütschliite melts incongruently at 1000 °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 bütschliite, 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 °C and 53K2CO3∙47Ca0.4Mg0.6CO3. The minimum point resembles a eutectic controlled by a four-phase reaction, by which, on cooling, the liquid transforms into three solid phases: K2(Mg0.78Ca0.22)(CO3)2, K2(Ca0.70Mg0.30)(CO3)2 bütschliite, 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 bütschliite. 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 °C), and at a depth of 100 km.
Highlights
Two possible scenarios of the carbonatite melt formation in the mantle were recently suggested.The first one is a partial melting of hydrothermally-altered basaltic crust [1]and overlying marine sediments [2] subducted to 500–600 km depths, and stagnating in the transition zone
We found no difference in the phase composition of identical samples loaded in different cassettes
Two of the three-phase regions are different compared to the others
Summary
Overlying marine sediments (carbonated pelites) [2] subducted to 500–600 km depths, and stagnating in the transition zone Partial melting at this depth occurs, owing to destabilization of Minerals 2019, 9, 296; doi:10.3390/min9050296 www.mdpi.com/journal/minerals. The Na-dolomitic melt could ascend from the mantle transition zone to the continental lithosphere, contributing to kimberlitic magmatism as a precursor melt (e.g., [10]), and in diamond formation as a solvent-catalyst and carbon source. These findings emphasize the importance of studying the Na2 CO3 –CaCO3 –MgCO3 system under mantle pressures, which were performed recently at 3 and 6 GPa [11,12]
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