%0 Journal Article
%T The K2CO3每CaCO3每MgCO3 System at 6 GPa: Implications for Diamond Forming Carbonatitic Melts
%A Anton Shatskiy
%A Anton V. Arefiev
%A Ivan V. Podborodnikov
%A Konstantin D. Litasov
%J Minerals | An Open Access Journal from MDPI
%D 2019
%R https://doi.org/10.3390/min9090558
%X Carbonate micro inclusions with abnormally high K 2O appear in diamonds worldwide. However, the precise determination of their chemical and phase compositions is complicated due to their sub-micron size. The K 2CO 3每CaCO 3每MgCO 3 is the simplest system that can be used as a basis for the reconstruction of the phase composition and P每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 K 2CO 3每CaCO 3每MgCO 3 system established in Kawai-type multianvil experiments at 6 GPa and 900每1300 ∼C. At 900 ∼C, the system has three intermediate compounds K 2Ca 3(CO 3) 4 (Ca# ≡ 97), K 2Ca(CO 3) 2 (Ca# ≡ 58), and K 2Mg(CO 3) 2 (Ca# ≒ 10), where Ca# = 100Ca/(Ca + Mg). Miscibility gap between K 2Ca(CO 3) 2 and K 2Mg(CO 3) 2 suggest that their crystal structures differ at 6 GPa. Mg-bearing K 2Ca(CO 3) 2 (Ca# ≒ 28) disappear above 1000 ∼C to produce K 2Ca 3(CO 3) 4 + K 8Ca 3(CO 3) 7 + K 2Mg(CO 3) 2. The system has two eutectics between 1000 and 1100 ∼C controlled by the following melting reactions: K 2Ca 3(CO 3) 4 + K 8Ca 3(CO 3) 7 + K 2Mg(CO 3) 2 ↙ [40K 2CO 3ˋ60(Ca 0.70Mg 0.30)CO 3] (1st eutectic melt) and K 8Ca 3(CO 3) 7 + K 2CO 3 + K 2Mg(CO 3) 2 ↙ [62K 2CO 3ˋ38(Ca 0.73Mg 0.27)CO 3] (2nd eutectic melt). The projection of the K 2CO 3每CaCO 3每MgCO 3 liquidus surface is divided into the eight primary crystallization fields for magnesite, aragonite, dolomite, Ca-dolomite, K 2Ca 3(CO 3) 4, K 8Ca 3(CO 3) 7, K 2Mg(CO 3) 2, and K 2CO 3. The temperature increase is accompanied by the sequential disappearance of crystalline phases in the following sequence: K 8Ca 3(CO 3) 7 (1220 ∼C) ↙ K 2Mg(CO 3) 2 (1250 ∼C) ↙ K 2Ca 3(CO 3) 4 (1350 ∼C) ↙ K 2CO 3 (1425 ∼C) ↙ dolomite (1450 ∼C) ↙ CaCO 3 (1660 ∼C) ↙ magnesite (1780 ∼C). The high Ca# of about 40 of the K 2(Mg, Ca)(CO 3) 2 compound found as inclusions in diamond suggest (1) its formation and entrapment by diamond under the P每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每Ca每Mg carbonate melts containing 20每40 mol% K 2CO 3 is stable under P每T conditions of 6 GPa and 1100每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,
%U https://www.mdpi.com/2075-163X/9/9/558