| Реферат: | eng: To constrain the ternary K2CO3-CaCO3-MgCO3 T-X diagram at 6 GPa and to expand upon the known K-Mg, K-Ca, and Ca-Mg binary systems we have carried out multi-anvil experiments along the K2CO3-Ca0.5Mg0.5CO3.join. The diagram has primary phase fields for K2CO3, K2Mg(CO3)(2), K2Ca0.1-0.5 Mg0.9-0.5(CO3)(2), K4CaMg(CO3)(4), Ca-magnesite, and dolomite. The system has two liquidus minima near 1000 degrees C. At one minimum, a liquid with the composition of 36 K2CO3 center dot 64(Ca0.65Mg0.35)CO3 is in equilibrium with three phases: Ca-magnesite, K2Ca0.1-0.5Mg0.9-0.5(CO3)(2), and K6Ca2(CO3)(5). The other minimum, a liquid with the composition of 62 K2CO3 center dot 38Ca(0.72)Mg(0.28)CO(3) is in equilibrium with K2CO3, K4CaMg(CO3)(4), and K6Ca2(CO3)(5). At 900 degrees C, the ternary diagram contains two- and three-phase regions with Ca-magnesite, aragonite, K2Ca3(CO3)(4), K2Ca(CO3)(2), K6Ca2(CO3)(5), K2CO3, K2Ca0.1-0.5Mg0.9-0.5(CO3)(2) solid solution, K2Mg0.9Ca0.1(CO3)(2), and K4CaMg(CO3)(4). We also expect an existence of primary phase fields for K6Ca2(CO3)(5), K2Ca3(CO3)(4) and aragonite. We suggest that extraction of K from silicate to carbonate components should decrease the minimum melting temperature of dry carbonated mantle rocks up to 1000 degrees C at 6 GPa and yield ultrapotassic Ca-rich dolomite melt containing more than 10 mol% K2CO3. As temperature increases above 1200 degrees C the melt evolves toward an alkali-poor, dolomitic liquid if the bulk molar CaO/MgO ratio >1, or toward K-Mg-rich carbonatite if bulk CaO/MgO < 1. The majority of compositions of carbonatite inclusions in diamonds from around the world fall within the magnesite primary field between the 1300 and 1400 degrees C isotherms. These melts could be formed by partial melting of magnesite-bearing peridotite or eclogite with bulk Ca/Mg < 1 at temperatures <= 1400 degrees C. A few compositions revealed in the Ebelyakh and Udachnaya diamonds (Yakutia) fall within the dolomite primary field close to the 1200 degrees C isotherm. These melts could be formed by partial melting of dolomite-bearing rocks, such as carbonated pelite or eclogite with bulk Ca/Mg >1 at temperatures <= 1200 degrees C.
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