EVALUATION OF WATER-RICH BOUNDARY LAYERS FORMED DURING PARTIAL CRYSTALLIZATION OF SYNTHETIC PEGMATITE MELTS

Pegmatites are intrusive magmatic rocks with unusually large crystals, graphic, skeletal, or spherulitic textures, and internal zoning, thought to crystallize inwardly, sequentially, at large values of undercooling (London, Pegmatites, 2008). Boundary layers (BL’s) enriched in incompatible components may form ahead of crystallization fronts if the crystal growth rate exceeds the component diffusion rate. Using confocal Raman spectroscopy we have documented the extent of water enrichments developed in the melt surrounding graphic alkali-feldspar – quartz, petalite, and virgilite crystals in partially crystallized H₂O-B-Li-haplogranite.

Water concentration in glass was determined by calibrating the integrated OH stretching band between 3012 and 4000 cm^-1 using synthetic glasses independently analyzed by Karl-Fisher titration. Compositional traverses and 2D maps were acquired across crystallization fronts in samples produced in unseeded, isothermal runs lasting from 3 to 30 days, at temperatures ranging from 500 to 650°C and 300 MPa, corresponding to variable degrees of undercooling between liquidus and glass transition. Initial H₂O contents ranged from 3 to 8 mass%.

BL’s develop at high degrees of undercooling coupled with maximum crystal growth rates. For example feldspar-quartz spherulites of 1000 µm radius, grown in 16 days at 500°C in a haplogranite melt (composition C1) with 1% Li₂O, 2.3% B₂O₃, and 6.5% H₂O were surrounded by 110 µm wide BL’s enriched by as much as 2.5% H₂O. However, crystallization of melt C1 at 600°C led to negligible BL’s. Petalite-virgilite assemblage crystallized at 500°C in a more fluxed melt (composition C2) with 2% Li₂O, 4.6% B₂O₃, and 8 % H₂O also lead to minor BL’s after 30 days whereas skeletal virgilite in C2 containing only 3% H₂O developed 75 µm BL’s with 0.8% H₂O enrichment after only 7 days.

Our preliminary characterization of water-rich BL’s in an initially viscous, moderately fluxed silicate melt suggests that significant enrichments can occur during rapid, disequilibrium crystallization at optimal degrees of undercooling. The BL’s may further affect the texture development in the crystallizing pegmatite liquids because of the lowered viscosity and increased diffusion rates in the crystallization front.