Experimental verification for the magmatic origin of carbonatites

Abstract

Many petrologists believe carbonatites were emplaced as liquid magmas, but others maintain that this is unlikely because the melting temperature of calcite is much higher than the low temperature of emplacement indicated by field relations. Low temperature liquids have been discovered in the system CaO-CO_2-H_2O At 1000 bars pressure a liquid of composition 68CaO, 19CO_2, 13H_2O (weight per cent) coexists with calcite, portlandite, and lime at 683°C, and a liquid 65CaO, 19CO_2, 16H_2O coexists with calcite, portlandite, and vapor (almost pure water) at 675°C. In the pressure range from 27 to 4000 bars the minimum liquidus temperature varies between 685°C and 640°C. Addition of MgO to the ternary system lowers the minimum liquidus temperature at 1000 bars from 675°C to 625° C. The liquids have low viscosity as indicated by the rapid attainment of equilibrium and the observation that crystal settling takes place in 15-minute experiments. It is thus probable that differentiation occurs in multi-component magmas by separation of the successive liquid fractions produced by crystallization of calcite, dolomite, and siderite. The experimental discovery that simplified carbonatite liquids exist at moderate temperatures through a wide pressure range leaves little reason to doubt a magmatic origin for carbonatites.

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