Journal of Petrology Advance Access published online on March 8, 2007
Journal of Petrology, doi:10.1093/petrology/egm002
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Liquidus Equilibria in the System K2O–Na2O–Al2O3–SiO2–F2O–1–H2O to 100 MPa: II. Differentiation Paths of Fluorosilicic Magmas in Hydrous Systems
*Department of Earth and Planetary Sciences, Mcgill University, Montreal, QC H3A 2A7, Canada
Received October 7, 2005; Revised typescript accepted January 9, 2007
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Abstract |
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We investigated phase equilibria in the six-component system Na2O–K2O–Al2O3–SiO2–F2O–1–H2O at 100 MPa to characterize differentiation paths of natural fluorine-bearing granitic and rhyolitic magmas. Topaz and cryolite are stable saturating solid phases in calcium-poor systems. At 100 MPa the maximum solidus depression and fluorine solubility in evolving silicic melts are controlled by the eutectics haplogranite–cryolite–H2O at 640°C and 
4 wt % F, and haplogranite–topaz–H2O at 640°C and 2 wt % F. Topaz and cryolite form a binary peralkaline eutectic at 660°C, 100 MPa and fluid saturation. The low-temperature nature of this invariant point causes displacement of multiphase eutectics with quartz and alkali feldspar towards the topaz–cryolite join and enables the silicate liquidus and cotectic surfaces to extend to very high fluorine concentrations (more than 30 wt % F) for weakly peraluminous and subaluminous compositions. The differentiation of fluorine-bearing magmas follows two distinct paths of fluorine behavior, depending on whether additional minerals buffer the alkali/alumina ratio in the melt. In systems with micas or aluminosilicates that buffer the activity of alumina, magmatic crystallization will reach either topaz or cryolite saturation and the system solidifies at low fluorine concentration. In leucogranitic suites precipitating quartz and feldspar only, the liquid line of descent will reach topaz or cryolite but fluorine will continue to increase until the quaternary eutectic with two fluorine-bearing solid phases is reached at 540°C, 100 MPa and aqueous-fluid saturation. The maximum water solubility in the haplogranitic melts increases with the fluorine content and reaches 12·5 ± 0·5 wt % H2O at the quartz–cryolite–topaz eutectic composition. A continuous transition between hydrous fluorosilicate melts and solute-rich aqueous fluids is not documented by this study. Our experimental results are applicable to leucocratic fluorosilicic magmas. In multicomponent systems, however, the presence of calcium may severely limit enrichment of fluorine by crystallization of fluorite.
KEY WORDS: granite; rhyolite; topaz; cryolite; magmatic differentiation
*Corresponding author. Present address: Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany. Telephone: +49-(0)921-553718. Fax: +49-(0)921-553769. E-mail: david.dolejs{at}uni-bayreuth.de