Abstract:
Omphacites with varying Na/K ratio were synthesized in the system CaMgSi2O6-NaAlSi2O6-KAlSi 2O6 at pressures of 5, 6, and 7 GPa within the temperature interval 1000-1300°C. It was found that both K and Na contents of omphacites increase irregularly with pressure, and there is a distinct negative correlation between K and Na in clinopyroxene at each pressure. The K content of omphacite increases more intensely within a pressure interval of 6-7 GPa. The isomorphism NaM2 ⇄ KM2 is predominant, whereas the substitution [CaM2MgM1] ⇄ [K M2AlM1] increases with decreasing Jadeite content. At the P-T parameters studied, potassium-bearing omphacites coexist with melt containing 15-21 wt % K2O, 54-63 wt % SiO2, and 19-22 wt % Al2O3 at low Na2O concentration independent of the initial Na2O content in starting mixtures. The partition coefficients Kp(K2O) = K2O (in KCpx)/K 2O (in melt) and Kp(Na2O) = Na2O (in KCpx)/Na2O (in melt) increase with pressure, and K p(Na2O) increases more significantly by comparison to Kp(K2O). Thus, omphacite crystallization promotes melt enrichment in potassium. As a result, ultrapotassic mantle melts could be generated. This is exemplified by potassium-bearing omphacite inclusions in diamonds from kimberlite and lamproite pipes.