CHEMICAL PROPERTIES OF EVOLVED GRANITIC MAGMAS: THE ROLE OF FLUORINE

Progressive enrichment in volatiles and light incompatible elements, observed in upper-crustal granites during magmatic fractionation, often leads to significant changes in chemical and physical properties of these evolved melts. We have experimentally determined melting equilibria in several fluorine-bearing hydrous haplogranitic systems at 100 MPa in order to assess the effects of fluorine on late-stage evolution of granitic melts, water solubilities, fluid-melt partitioning and the significance of fluoride-silicate liquid immiscibility. Progressive fluoridation by F₂O_-1 of albitic and haplogranitic compositions causes melt depolymerization due to formation of Na-Al-F melt complexes and leads to the crystallization of cryolite and fluorotopaz. In the quaternary system haplogranite-cryolite-topaz-H₂O, fluorine solubility is strongly dependent on the melt alkali/alumina ratio. Saturation levels at 720^oC and 100 MPa increase from peraluminous (1.7 wt % F, fluorotopaz) to peralkaline systems (4.2 wt % F, cryolite), with a maximum near subaluminous composition (>13.1 wt % F). This enables prolonged crystallization of topaz from strongly peraluminous compositions and the convergence of many Ca-poor granitic melts towards a single subaluminous F-rich eutectic at 530^oC and 100 MPa. On the contrary, Ca-rich granitic melts exhibit an early fluorite saturation, which severely limits their fluorine enrichment. No fractionation path intersects the fluoride-silicate liquid miscibility gap, which is located on the cryolite liquidus and occurs at high temperatures (> 800^oC). Melt-vapor partitioning is nearly congruent at low fluorine contents in the hydrous melt (4-5 wt % F), but with increasing fluorine content, the alkali-rich aluminofluorides tend to preferentially dissolve in the aqueous vapor, which eventually leads to incongruent melt decomposition (at 25-30 wt % F): fluorosilicate melt=solute-rich vapor + quartz. Vapor-phase transfer of alkali-rich aluminofluorides provides a viable explanation for alkali metasomatism (cryolitization, albitization), but formation of greisens requires prevailing effect of additional ligands (chlorine) to promote hydrolytic alteration.