Инд. авторы:	Arefiev A.V., Shatskiy A., Podborodnikov I.V., Litasov K.D.
Заглавие:	The K2CO3–CaCO3–MgCO3 system at 6 GPa: implications for diamond forming carbonatitic melts
Библ. ссылка:	Arefiev A.V., Shatskiy A., Podborodnikov I.V., Litasov K.D. The K2CO3–CaCO3–MgCO3 system at 6 GPa: implications for diamond forming carbonatitic melts // Minerals . - 2019. - Т.9. - № 9. - Ст.558.
Внешние системы:	DOI: 10.3390/min9090558; WoS: 000488032600026;
Реферат:	eng: 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 degrees C. At 900 degrees 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 degrees C to produce K2Ca3(CO3)(4) + K8Ca3(CO3)(7) + K2Mg(CO3)(2). The system has two eutectics between 1000 and 1100 degrees C controlled by the following melting reactions: K2Ca3(CO3)(4) + K8Ca3(CO3)(7) + K2Mg(CO3)(2) -> [40K(2)CO(3)center dot 60(Ca0.70Mg0.30)CO3] (1st eutectic melt) and K8Ca3(CO3)(7) + K2CO3 + K2Mg(CO3)(2) -> [62K(2)CO(3)center dot 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 degrees C) -> K2Mg(CO3)(2) (1250 degrees C) -> K2Ca3(CO3)(4) (1350 degrees C) -> K2CO3 (1425 degrees C) -> dolomite (1450 degrees C) -> CaCO3 (1660 degrees C) -> magnesite (1780 degrees C). The high Ca# of about 40 of the K-2(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 degrees 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 degrees 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.
Ключевые слова:	CRYSTAL-STRUCTURE; MINERAL INCLUSIONS; LITHOSPHERIC MANTLE; LIQUID IMMISCIBILITY; HIGH-PRESSURE; UDACHNAYA KIMBERLITE; FIBROUS DIAMONDS; FLUID INCLUSIONS; INTERNATSIONALNAYA KIMBERLITE PIPE; continental lithosphere; diamond formation; high-pressure experiment; ultrapotassic carbonatite melt; butschliite; double potassium carbonates; PHASE-RELATIONS;
Издано:	2019
Физ. характеристика:	558