TY - JOUR

T1 - The K2CO3-CaCO3-MgCO3 System at 6 GPa

T2 - Implications for diamond forming carbonatitic melts

AU - Arefiev, Anton V.

AU - Shatskiy, Anton

AU - Podborodnikov, Ivan V.

AU - Litasov, Konstantin D.

PY - 2019/9/1

Y1 - 2019/9/1

N2 - 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 betweenK2Ca(CO3)2 andK2Mg(CO3)2 suggest that their crystal structures differ at 6GPa. 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 kimberlitemagma, 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.

AB - 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 betweenK2Ca(CO3)2 andK2Mg(CO3)2 suggest that their crystal structures differ at 6GPa. 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 kimberlitemagma, 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.

KW - Bütschliite

KW - Continental lithosphere

KW - Diamond formation

KW - Double potassium carbonates

KW - High-pressure experiment

KW - Ultrapotassic carbonatite melt

KW - LIQUID IMMISCIBILITY

KW - diamond formation

KW - HIGH-PRESSURE

KW - high-pressure experiment

KW - LITHOSPHERIC MANTLE

KW - CRYSTAL-STRUCTURE

KW - INTERNATSIONALNAYA KIMBERLITE PIPE

KW - butschliite

KW - UDACHNAYA KIMBERLITE

KW - FLUID INCLUSIONS

KW - ultrapotassic carbonatite melt

KW - MINERAL INCLUSIONS

KW - FIBROUS DIAMONDS

KW - continental lithosphere

KW - PHASE-RELATIONS

KW - double potassium carbonates

UR - http://www.scopus.com/inward/record.url?scp=85074570440&partnerID=8YFLogxK

U2 - 10.3390/min9090558

DO - 10.3390/min9090558

M3 - Article

AN - SCOPUS:85074570440

VL - 9

JO - Minerals

JF - Minerals

SN - 2075-163X

IS - 9

M1 - 558

ER -