Physical properties of carbonatite magmas inferred from molten salt data, and application to extraction patterns from carbonatite–silicate magma chambers

Abstract

AbstractLittle is known about the physical properties of carbonatite magmas, making it difficult to predict dynamic behaviour in carbonatite-bearing magmatic systems. The viscosity of calcium-rich carbonatite magma is approximately estimated from molten salt data to be 0.1 Pa s at 700–800°C, while density is estimated at 2.3−2.5 × 103 kg m−3. The corresponding values for natrocarbonatite are 0.01 Pa s and 2.0−2.1 × 103 kg m−3. It is thus possible for carbonatite to be negatively buoyant with respect to some silicate magmas. The surface tension in air of carbonatite magmas is estimated at 0.25 and 0.21 N m−1 for Ca-carbonatite and natrocarbonatite respectively. Knowledge of the interfacial tension between carbonatite and silicate liquids is critical before the formation and behaviour of silicate–carbonatite emulsions can be properly understood. Interfacial tension is constrained to < 0.09 N m−1 by the application of multiphase drop theory to experimentally-produced textures, and this value receives some support from geological observations. The mechanics of extraction from layered carbonatite-silicate magma chambers are briefly examined using the recommended density and viscosity values and the equations of Blake & Ivey (1986); the degree of eruptive mingling is dependent on which liquid was uppermost in the chamber.