Behavior and effects of phosphorus in the system Na2O−K2O−Al2O3−SiO2−P2O5−H2O at 200 MPa(H2O)

The addition of phosphorus to H₂O-saturated and initially subaluminous haplogranitic (Qz–Ab–Or) compositions at 200 MPa(H₂O) promotes expansion of the liquidus field of quartz, a marked decrease of the solidus temperature, increased solubility limits of H₂O in melt at low phosphorus concentrations, and fractionation of melt out of the haplogranite plane (projected along an Or₂₈ isopleth) toward a peralkaline, silica-poor but quartz-saturated minimum composition. The partition coefficient for P₂O₅ between aqueous vapor and melt with an ASI (aluminum saturation index, mol Al/[mol Na+K])=1 is negligible (0.06), and consequently so are the effects of phosphorus on other melt-vapor relations involving major components. Phosphorus becomes more soluble in vapor, however, as the concentration of a NaPO₃ component increases via the fractionation of melt by crystallization of quartz and feldspar. The experimental results here corroborate existing concepts regarding the interaction of phosphorus with alkali aluminosilicate melt: phosphorus has an affinity for alkalis and Al, but not Si. Phosphorus is incorporated into alkali feldspars by the exchange component AlPSi_-2. For subaluminous compositions (ASI=1), the distribution coefficient of phosphorus between alkali feldspar and melt, D[P]Af/m, is 0.3. This value increases to D[P]Af/m=1.0 at a melt ASI value of 1.3. The increase in D[P]Af/m with ASI is expected from the fact that excess Al promotes the AlPSi_-2 exchange. With this experimental data, the P₂O₅ content of feldspars and whole rocks can reveal important facets of crystallization and phosphorus geochemistry in subaluminous to peraluminous granitic systems.