The system K2CO3–CaCO3 at 3 GPa: link between phase relations and variety of K–Ca double carbonates at ≤ 0.1 and 6 GPa

Arefiev, Anton V.; Shatskiy, Anton; Podborodnikov, Ivan V.; Rashchenko, Sergey V.; Chanyshev, Artem; Litasov, Konstantin D.

doi:10.1007/s00269-018-1000-z

Cited by 28 publications

(29 citation statements)

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“…Although a mutual solubility of K 2 Mg and K 2 Ca compounds is noticeable, a miscibility gap between these phases was observed ( Figures 1i and 5a,b, and Tables S1 and S2). It was previously shown that at 6 GPa, K 2 Ca(CO 3 ) 2 bütschliite decomposes between 950 and 1000 • C according to the reaction [38,39]:…”

Section: Resultsmentioning

confidence: 99%

“…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).…”

Section: Comparison With the Previous Studymentioning

confidence: 99%

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

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Carbonate micro inclusions with abnormally high K2O appear in diamonds worldwide. However, the precise determination of their chemical and phase compositions is complicated due to their sub-micron size. The K2CO3–CaCO3–MgCO3 is the simplest system that can be used as a basis for the reconstruction of the phase composition and P–T conditions of the origin of the K-rich carbonatitic inclusions in diamonds. In this regard, this paper is concerned with the subsolidus and melting phase relations in the K2CO3–CaCO3–MgCO3 system established in Kawai-type multianvil experiments at 6 GPa and 900–1300 °C. At 900 °C, the system has three intermediate compounds K2Ca3(CO3)4 (Ca# ≥ 97), K2Ca(CO3)2 (Ca# ≥ 58), and K2Mg(CO3)2 (Ca# ≤ 10), where Ca# = 100Ca/(Ca + Mg). Miscibility gap between K2Ca(CO3)2 and K2Mg(CO3)2 suggest that their crystal structures differ at 6 GPa. Mg-bearing K2Ca(CO3)2 (Ca# ≤ 28) disappear above 1000 °C to produce K2Ca3(CO3)4 + K8Ca3(CO3)7 + K2Mg(CO3)2. The system has two eutectics between 1000 and 1100 °C controlled by the following melting reactions: K2Ca3(CO3)4 + K8Ca3(CO3)7 + K2Mg(CO3)2 → [40K2CO3∙60(Ca0.70Mg0.30)CO3] (1st eutectic melt) and K8Ca3(CO3)7 + K2CO3 + K2Mg(CO3)2 → [62K2CO3∙38(Ca0.73Mg0.27)CO3] (2nd eutectic melt). The projection of the K2CO3–CaCO3–MgCO3 liquidus surface is divided into the eight primary crystallization fields for magnesite, aragonite, dolomite, Ca-dolomite, K2Ca3(CO3)4, K8Ca3(CO3)7, K2Mg(CO3)2, and K2CO3. The temperature increase is accompanied by the sequential disappearance of crystalline phases in the following sequence: K8Ca3(CO3)7 (1220 °C) → K2Mg(CO3)2 (1250 °C) → K2Ca3(CO3)4 (1350 °C) → K2CO3 (1425 °C) → dolomite (1450 °C) → CaCO3 (1660 °C) → magnesite (1780 °C). The high Ca# of about 40 of the K2(Mg, Ca)(CO3)2 compound found as inclusions in diamond suggest (1) its formation and entrapment by diamond under the P–T conditions of 6 GPa and 1100 °C; (2) its remelting during transport by hot kimberlite magma, and (3) repeated crystallization in inclusion that retained mantle pressure during kimberlite magma emplacement. The obtained results indicate that the K–Ca–Mg carbonate melts containing 20–40 mol% K2CO3 is stable under P–T conditions of 6 GPa and 1100–1200 °C corresponding to the base of the continental lithospheric mantle. It must be emphasized that the high alkali content in the carbonate melt is a necessary condition for its existence under geothermal conditions of the continental lithosphere, otherwise, it will simply freeze.

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Section: Resultsmentioning

confidence: 99%

Section: Comparison With the Previous Studymentioning

confidence: 99%

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

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“…The summary of other findings of double alkaline-alkaline-earth carbonates can be found elsewhere [5]. All these findings gave rise to intensive experimental investigation of Na 2 CO 3 and K 2 CO 3 melting curves in the pressure range of the upper mantle and transition zone of the Earth [19,[23][24][25][26][27][28][29]. The melting curve of Na 2 CO 3 was found to be smooth in the range of 3-18 GPa [23], while on the melting curve of K 2 CO 3 , two kinks, at 5 and 9 GPa, have been found at 2-20 GPa [24,25].…”

Section: Introductionmentioning

confidence: 99%

High-Pressure Phase Diagrams of Na2CO3 and K2CO3

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The phase diagrams of Na 2 CO 3 and K 2 CO 3 have been determined with multianvil (MA) and diamond anvil cell (DAC) techniques. In MA experiments with heating, γ -Na 2 CO 3 is stable up to 12 GPa and above this pressure transforms to P 6 3 /mcm-phase. At 26 GPa, Na 2 CO 3 - P 6 3 /mcm transforms to the new phase with a diffraction pattern similar to that of the theoretically predicted Na 2 CO 3 - P 2 1 /m. On cold compression in DAC experiments, γ -Na 2 CO 3 is stable up to the maximum pressure reached of 25 GPa. K 2 CO 3 shows a more complex sequence of phase transitions. Unlike γ Na 2 CO 3 , γ -K 2 CO 3 has a narrow stability field. At 3 GPa, K 2 CO 3 presents in the form of the new phase, called K 2 CO 3 -III, which transforms into another new phase, K 2 CO 3 -IV, above 9 GPa. In the pressure range of 9–15 GPa, another new phase or the mixture of phases III and IV is observed. The diffraction pattern of K 2 CO 3 -IV has similarities with that of the theoretically predicted K 2 CO 3 - P 2 1 /m and most of the diffraction peaks can be indexed with this structure. Water has a dramatic effect on the phase transitions of K 2 CO 3 . Reconstruction of the diffraction pattern of γ -K 2 CO 3 is observed at pressures of 0.5–3.1 GPa if the DAC is loaded on the air.

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“…The melt field is surrounded by four two-phase fields: Mgs + L (Figure 3d), K2Ca2 + L (Figure 3e), K2Ca + L (Figure 3f), and K2 + L; and by three three-phase fields: Mgs + K2Ca2 + L (Figure 3g), K2Ca2 + K2Ca + L (Figure 3h), and K2Ca + K2 + L (Figure 3i). As temperature increases to 1000 °C, K2Ca(CO3)2 disappears via incongruent melting to produce K2Ca2(CO3)3 and liquid containing 53 mol % K2CO3 [47]. Besides, aragonite transforms to calcite at about 960 °C [47].…”

Section: Resultsmentioning

confidence: 99%

“…Two subsolidus two-phase fields are still remaining: Cal + K2Ca3 (Figure 5i) and K2Ca3 + K2Ca2 (Figure 6f,g, Tables S18-S20). As temperature increases to 1000 • C, K 2 Ca(CO 3 ) 2 disappears via incongruent melting to produce K 2 Ca 2 (CO 3 ) 3 and liquid containing 53 mol % K 2 CO 3 [47]. Besides, aragonite transforms to calcite at about 960 • C [47].…”

Section: Resultsmentioning

confidence: 99%

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

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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.

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The system K2CO3–CaCO3 at 3 GPa: link between phase relations and variety of K–Ca double carbonates at ≤ 0.1 and 6 GPa

Cited by 28 publications

References 48 publications

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

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

High-Pressure Phase Diagrams of Na2CO3 and K2CO3

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

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