The System K2CO3–CaCO3–MgCO3 at 3 GPa: Implications for Carbonatite Melt Compositions in the Shallow Continental Lithosphere

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.

The K2CO3–CaCO3–MgCO3 System at 6 GPa: Implications for Diamond Forming Carbonatitic Melts

“…In situ X-ray diffraction study revealed that K 2 Mg(CO 3 ) 2 retains its structure to 8 GPa upon compression at the ambient temperature where it transforms into a monoclinic polymorph [50]. However, the present results at 6 GPa and the results at 3 GPa [37] show a miscibility gap on the K 2 Ca(CO 3 ) 2 -K 2 Mg(CO 3 ) 2 join (Figure 7a-c). This suggests that crystal structures of K 2 Mg(CO 3 ) 2 and K 2 Ca(CO 3 ) 2 at 3-6 GPa and 750-1000 • C are different.…”

“…In contrast to KCM (Figure 8b), which has two eutectics, NCM has a single eutectic, Na 2 Ca 3 -Na 4 Ca-Na 2 Mg, with a composition of 48Na 2 CO 3 •52(Ca 0.75 Mg 0.25 )CO 3 (Figure 8d). As pressure decreases to 3 GPa, the incipient melting of NCM shifts to 850 • C and controlled by the Na 2 Ca + Na 2 + Na 2 Mg assemblage [45], similar to the K 2 Ca-K 2 -K 2 Mg eutectic established in KCM at 3 GPa and 825 • C [37] (Figure 8a,c). The difference is that the KCM eutectic is Mg-rich, 53K 2 CO 3 •47Ca 0.4 Mg 0.6 CO 3 , while the NCM eutectic is Ca-rich with compositions of 52Na 2 CO 3 •48Ca 0.62 Mg 0.38 CO 3 (Figure 8a,c).…”

“…Examples of BSE images of well-polished samples and the same samples damaged by atmospheric humidity are shown in Figure S1. The procedures of the sample preparation and polishing are described in [31,37].…”

“…Recovered samples were studied using a MIRA 3 LMU scanning electron microscope (Tescan Orsay Holding, Brno, Czech) coupled with an INCA energy-dispersive X-ray microanalysis system 450 equipped with the liquid nitrogen-free Large area EDS X-Max-80 Silicon Drift Detector (Oxford Instruments Nanoanalysis Ltd, Oxford, UK) at IGM SB RAS (see [37] for details).…”

“…Thus, at 6 GPa and 1080 • C, the intermediate compounds are represented by K 2 Mg, K 2 Ca 3 , K 8 Ca 3 , and Dol (Figure 5d). K2Ca2(CO3)2, and K2Ca(CO3)2 bütschliite [37,39,40] (Figure 7a,b), which are also stable at ambient pressure [41][42][43]. At 6 GPa, K2Mg remains stable up to 1250 °C, where it melts congruently [40], while the stability field of K2Ca is limited by ~990 °C for the pure endmember ( Figure 6) [39] and ~1050 °C for the Mg-bearing compound (Figure 5d,c).…”