PHOSPHORUS DISTRIBUTION BETWEEN POTASSIC ALKALI FELDSPAR AND (HAPLO-) GRANITIC LIQUID AT 200 MPA (H₂O): THE EFFECT OF UNDERCOOLING ON CRYSTAL /LIQUID SYSTEMATICS

We have examined the incorporation of P in potassic alkali feldspar at 200 MPa as a function of liquidus undercooling (ΔT) in the range of ΔT=25°-200°C. Experiments utilized cylinders of the nominally metaluminous haplogranitic composition Ab₄₁Or₄₈Qtz₁₁ with 3 wt% P₂O₅ hydrated to H₂O-saturation for 48 hrs at 825°C (~10°C above T_liq) prior to single-step, isobaric cooling to the final condition of crystal nucleation and growth. In experiments with ΔT=25-50°C, crystal morphologies are single to slightly radial clusters of well-filled, tabular prisms. Both crystals and glasses are homogeneous and yield D(P)^Afs/liq ~ 0.4. With undercooling to ΔT=100-200°C, growth occurs in radial or spherulitic clusters of skeletal to cuneiform crystals with marked adjacent compositional boundary layers in glass. Despite enrichment of the boundary layers in P (up to ~ 9 wt% P₂O₅), the value of D(P)^Afs/liq appears to decrease slightly to ~0.2-0.3. Values of D(P)^Afs/liq at high undercooling are somewhat inconsistent because boundary layers in glass vary as functions of crystal orientation and density. Where well-spaced, individual crystallites radiate into glass, boundary layers within and along the edges of skeletal crystals are enriched in Si and P, but depleted in Al. At ΔT=200°C, laterally continuous boundary layers adjacent to dense crystal growth fronts are depleted in all haplogranitic components indicating elevated concentration of a hydrous P₂O₅ component. Na and K appear to homogenize by diffusion. Even in cases involving boundary layer development, no Na or K concentration gradients (relative to Si and Al) were found, though the feldspars produced are much more potassic than the starting bulk composition. Feldspar compositions grown at ΔT=25-50°C indicate that all P is incorporated via the substitution mechanism (P,Al)Si_-2. Feldspars grown at ΔT=100-200°C have insufficient Al to charge balance P; deficiencies in alkalis and Si indicate a (,P)(M+,Si)_-1 substitution that becomes more important with progressive undercooling and seems appropriate for the Al-depleted, but P-enriched boundary layers.