The K 2 CO 3 –CaCO 3 –MgCO 3 System at 6 GPa: Implications for Diamond Forming Carbonatitic Melts

摘要

Carbonate micro inclusions with abnormally high K 2 O appear in diamonds worldwide. However, the precise determination of their chemical and phase compositions is complicated due to their sub-micron size. The K 2 CO 3 –CaCO 3 –MgCO 3 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 K 2 CO 3 –CaCO 3 –MgCO 3 system established in Kawai-type multianvil experiments at 6 GPa and 900–1300 °C. At 900 °C, the system has three intermediate compounds K 2 Ca 3 (CO 3 ) 4 (Ca# ≥ 97), K 2 Ca(CO 3 ) 2 (Ca# ≥ 58), and K 2 Mg(CO 3 ) 2 (Ca# ≤ 10), where Ca# = 100Ca/(Ca + Mg). Miscibility gap between K 2 Ca(CO 3 ) 2 and K 2 Mg(CO 3 ) 2 suggest that their crystal structures differ at 6 GPa. Mg-bearing K 2 Ca(CO 3 ) 2 (Ca# ≤ 28) disappear above 1000 °C to produce K 2 Ca 3 (CO 3 ) 4 + K 8 Ca 3 (CO 3 ) 7 + K 2 Mg(CO 3 ) 2 . The system has two eutectics between 1000 and 1100 °C controlled by the following melting reactions: K 2 Ca 3 (CO 3 ) 4 + K 8 Ca 3 (CO 3 ) 7 + K 2 Mg(CO 3 ) 2 → [40K 2 CO 3 ?60(Ca 0.70 Mg 0.30 )CO 3 ] (1st eutectic melt) and K 8 Ca 3 (CO 3 ) 7 + K 2 CO 3 + K 2 Mg(CO 3 ) 2 → [62K 2 CO 3 ?38(Ca 0.73 Mg 0.27 )CO 3 ] (2nd eutectic melt). The projection of the K 2 CO 3 –CaCO 3 –MgCO 3 liquidus surface is divided into the eight primary crystallization fields for magnesite, aragonite, dolomite, Ca-dolomite, K 2 Ca 3 (CO 3 ) 4 , K 8 Ca 3 (CO 3 ) 7 , K 2 Mg(CO 3 ) 2 , and K 2 CO 3 . The temperature increase is accompanied by the sequential disappearance of crystalline phases in the following sequence: K 8 Ca 3 (CO 3 ) 7 (1220 °C) → K 2 Mg(CO 3 ) 2 (1250 °C) → K 2 Ca 3 (CO 3 ) 4 (1350 °C) → K 2 CO 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–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% K 2 CO 3 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.