The system CaO-SiO_2-CO_2-H_2O—III. Second critical end-point on the melting curve

Abstract

Melting relationships in the system CaO-SiO_2-CO_2-H_2O have been extended to 40 kbar pressure. Dellaite, dicalcium silicate, spurrite and ealciochondrodite appear as the silicate phases coexisting with vapor-saturated liquids and fluids at high pressures. Contrary to earlier predictions, wollastonite does not appear on the vapor-saturated liquidus, at least to pressures below 10 kbar. The solidus terminates at a second critical end-point at about 32.5 kbar and 515°C. To provide a model for our results in this quaternary system, phase relationships in a hypothetical ternary system with critical end-points are developed. An extension of these relationships into the system CaO-SiO_2-CO_2-H_2O shows that a critical curve limiting the miscibility gap between liquids and vapors moves through the system with changing pressure, terminating a series of univariant reactions at critical end-points. The results are applicable to the origin of carbonatites and associated alkalic rocks and kimberlites and they illustrate that fractional crystallization in this system will produce carbonatitic melts only from liquids originally undersaturated in silica, at least for a pressure range prevailing within the earth's crust.

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