Experimental Study on Liquid Immiscibility in Silicate-Carbonate Systems with Applications to Carbonatites

Citation

Lee, Woh-jer (1996) Experimental Study on Liquid Immiscibility in Silicate-Carbonate Systems with Applications to Carbonatites. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/vpan-h802. https://resolver.caltech.edu/CaltechTHESIS:09192023-212841699

Abstract

Phase equilibrium experiments have been conducted in several silicate-carbonate systems to 2.5 GPa in order to understand the magmatic processes involved in the generation of carbonatite complexes. The studied phase fields were intersected by SiO₂-CaCO₃, NaAlSi₃O₈-CaCO₃, NaAlSiO₄-NaAlSi₃O₈-CaCO₃ and primitive nephelinite­ (Na,Ca,Mg) carbonate, which along with the analyzed liquid and solid compositions were used to define the positions of the silicate-carbonate miscibility gap and liquidus field boundaries on various compositional projections. These boundaries exert controls on the evolution of silicate-CO₂ and carbonatitic magmas, and vary strongly with temperature, composition and pressure. The size of the miscibility gap decreases with increasing temperature and Mg/Ca of liquids. The extent of the Mg-free miscibility gap decreases with decreasing pressure, whereas the magnesian one shows an opposite trend. The immiscible carbonate-rich liquids could dissolve at most ~80 wt% CaCO₃ while still containing significant amounts of silicate and alkalis. The position of the silicate-calcite coprecipitation boundary becomes more carbonate-rich as pressure decreases, and as composition becomes more magnesian and aluminous. Calcite grew remarkably rounded in many experiments. A variety of liquid paths are compared with the field boundaries at different conditions. Partial melting of carbonated peridotite produces dolomitic carbonatites along the coprecipitation boundary, to CO₂-bearing, silica-undersaturated liquids in the primary silicate field. Both types of magmas could potentially ascend to the surface of the earth without much modification. None of them would reach the miscibility gap at mantle conditions. Within the crust, carbonated silicate liquids could either intersect the miscibility gap after substantial crystallization to exsolve alkali-bearing to alkalic carbonatitic liquids, or reach the coprecipitation boundary and evolve towards alkali-enrichment and silicate­ depletion without immiscibility. Alkali-bearing, CaCO₃-rich immiscible liquids, when separating from their silicate parents, first precipitate silicate minerals during cooling until calcite is joined, and the residual liquids become more alkalic by further crystallization of calcite. It appears that most calciocarbonatites are cumulates from liquids along the coprecipitation boundary, whereas the natrocarbonatites at Oldoinyo Lengai are produced directly by immiscibility.

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