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Processes of Crustal Carbonatite Formation by Liquid Immiscibility and Differentiation, Elucidated by Model Systems

Lee, Woh-Jer and Wyllie, Peter J. (1998) Processes of Crustal Carbonatite Formation by Liquid Immiscibility and Differentiation, Elucidated by Model Systems. Journal of Petrology, 39 (11-12). pp. 2005-2013. ISSN 0022-3530. doi:10.1093/petroj/39.11-12.2005.

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Experimental studies on several silicate–carbonate joins provide a framework in the system CaO–Na_2O–(MgO + FeO)–(SiO_2 + Al_2O_3) (+ CO_2) which illustrates possible processes for the formation of carbonatites. The two key features are the silicate–carbonate liquidus surface, and the miscibility gap liquidus surface. Crystallizing parental carbonated silicate melts may reach a silicate–CO_2 eutectic, a silicate–carbonate field boundary, or a miscibility gap. Some hydrous carbonated silicate melts may bypass the high-temperature miscibility gap and reach the silicate–carbonate field boundary. Immiscible carbonate-rich liquids in model systems simulating magmatic conditions tend to be concentrated near calciocarbonatite compositions (< ∼80% CaCO_3; e.g. nepheline sövite), but may be more alkalic from silicate parents with higher Na/Ca values. An immiscible carbonate-rich liquid separating from the high-temperature parent silicate liquid will cool with the precipitation of silicates only, until it reaches the silicate–carbonate field boundary, where it is capable of precipitating carbonate minerals, which can form carbonatite cumulates. Some parents may reach this boundary by direct crystallization, but most probably traverse the miscibility gap. Along this field boundary, the coprecipitation of calcite drives the liquid toward residual alkali-rich compositions. The carbonate liquidus (>85% CaCO_3) is a ‘forbidden volume’ for magmas. Vapor loss from carbonatite magma can introduce alkalis into country rocks, but this does not cause alkali depletion of magma; calcite precipitates to maintain the magma composition. Hydrous magnesiocarbonatite magmas can precipitate cumulate sövites.

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Additional Information:© 1998 Oxford University Press. Received September 30, 1997. Accepted May 21, 1998. We thank D. R. Baker, D. H. Eggler, B. A. Kjarsgaard and an anonymous reviewer for their critical comments. This research was supported by the Earth Science section of the US National Science Foundation, grant EAR-9218806. This is Contribution 8572 of the Division of Geological and Planetary Sciences, California Institute of Technology.
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Subject Keywords:carbonatite; liquid immiscibility; nephelinite; sövite
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Caltech Division of Geological and Planetary Sciences8572
Issue or Number:11-12
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Official Citation:Woh-Jer Lee and Peter J. Wyllie Processes of Crustal Carbonatite Formation by Liquid Immiscibility and Differentiation, Elucidated by Model Systems J. Petrology (1998) 39 (11-12): 2005-2013 doi:10.1093/petroj/39.11-12.2005
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:63643
Deposited By: Tony Diaz
Deposited On:13 Jan 2016 21:09
Last Modified:10 Nov 2021 23:19

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