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Liquid immiscibility between nephelinite and carbonatite from 1.0 to 2.5 GPa compared with mantle melt compositions

Lee, Woh-Jer and Wyllie, Peter J. (1997) Liquid immiscibility between nephelinite and carbonatite from 1.0 to 2.5 GPa compared with mantle melt compositions. Contributions to Mineralogy and Petrology, 127 (1-2). pp. 1-16. ISSN 0010-7999. doi:10.1007/s004100050261.

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In order to define the conditions for the formation of immiscible carbonatite magmas in the lithosphere and in the crust, we have conducted phase equilibrium experiments to determine the effect of pressure and temperature on the silicate-carbonate liquid miscibility gap in bulk compositions appropriate for magmas in the upper mantle. A primitive (magnesian) nephelinite (NEPH) was used as a starting material, mixed with carbonates. Experiments were made with mixtures in the joins NEPH-dolomite-Na_2CO_3 (NEPH-Dol-NC) at 1.0 to 2.5 GPa, and NEPH-calcite (NEPH-CC) at 1.0 GPa. The miscibility gap was intersected by the join NEPH-Dol-NC (liquids with olivine), but not by NEPH-CC. Together with previous results for the Mg-free system (Na_2O-CaO-Al_2O_3-SiO_2-CO_2), it was established that the size of the miscibility gap for magnesian compositions increases with decreasing pressures from depths of ~100 km to ~35 km; it increases further as compositions are changed by decreasing Mg/Ca. The maximum CaCO_3 in liquids associated with the miscibility gap is 50 wt % for Mg-bearing liquids, and 80 wt % for Mg-free liquids. There is no experimental evidence for nearly pure-CaCO_3 immiscible liquids, but abundant evidence for the precipitation of rounded calcite crystals from carbonate-rich liquids. The join NEPH-CC locates a piercing point on the liquidus field boundary for coprecipitation of olivine and calcite at NEPH_(50)CC_(50) (wt %), part of the silicate-carbonate liquidus field boundary which defines the locus of liquids formed from carbonate-peridotites. The miscibility gap results are compared with magmas formed during partial fusion of CO_2-bearing mantle peridotites, and during fractional crystallization of mantle-derived magmas. None of the probable magma paths in mantle processes intersects the miscibility gap. CO_2-bearing mantle-derived alkalic magmas such as nephelinites and melilitites may fractionate during uprise through the mantle and crystallization within the crust. The compositions of these evolved nephelinites and phonolites approach the silicate side of the miscibility gap, confirming the probable generation of immiscible, alkalic carbonate-rich liquids at crustal pressures.

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Additional Information:© 1997 Springer-Verlag. Received: 29 January 1996; Accepted: August 14, 1996. We thank M.B. Baker for making available his 2.5 GPa results given in Table 2, Table 3 and Fig. 8, and incorporated into Fig. 5, and for critical review of an early manuscript draft. Helpful comments from R.J. Sweeney and A.F. Koster van Groos are greatly appreciated. This research was supported by the Earth Science section of the United States National Science Foundation, grant EAR-921886. This is contribution no. 5777 of the Division of Geological and Planetary Sciences, California Institute of Technology.
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Caltech Division of Geological and Planetary Sciences5777
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ID Code:63567
Deposited By: Tony Diaz
Deposited On:12 Jan 2016 22:19
Last Modified:10 Nov 2021 23:18

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