Liquidus Equilibria in the System K2O-Na2O-Al2O3-SiO2-F2O-1-H2O to 100 MPa: I. Silicate-Fluoride Liquid Immiscibility in Anhydrous Systems

Abstract

Liquidus relations in the four-component system Na2O–Al2O3–SiO2–F2O−1 were studied at 0· 1 and 100 MPa to define the location of fluoride–silicate liquid immiscibility and outline differentiation paths of fluorine-bearing silicic magmas. The fluoride–silicate liquid immiscibility spans the silica–albite–cryolite and silica–topaz–cryolite ternaries and the haplogranite-cryolite binary at greater than 960°C and 0· 1–100 MPa. With increasing Al2O3 in the system and increasing aluminum/alkali cation ratio, the two-liquid gap contracts and migrates from the silica liquidus to the cryolite liquidus. The gap does not extend to subaluminous and peraluminous melt compositions. For all alkali feldspar–quartz-bearing systems, the miscibility gap remains located on the cryolite liquidus and is thus inaccessible to differentiating granitic and rhyolitic melts. In peralkaline systems, the magmatic differentiation is terminated at the albite–quartz–cryolite eutectic at ∼ 770°C, 100 MPa, ∼5 wt % F and cation Al/Na = 0· 75. The addition of topaz, however, significantly lowers melting temperatures and allows strong fluorine enrichment in subaluminous compositions. At 100 MPa, the binary topaz–cryolite eutectic is located at 770°C, 39 wt % F, cation Al/Na ∼ 0· 95, and the ternary quartz–topaz–cryolite eutectic is found at 740°C, 32 wt % F, 30 wt % SiO2 and cation Al/Na ∼ 0· 95. Such location of both eutectics enables fractionation paths of subaluminous quartz-saturated systems to produce fluorine-rich, SiO2-depleted and nepheline-normative residual liquids.