The Origin of Primary Textures in Granitic Pegmatites

Many granitic pegmatites contain two textural domains. Anisotropic textures of the outer zones, including fine-grained units, graphic intergrowths, and unidirectional solidification textures, are dominated by the effects of liquidus undercooling. The coarse, blocky textures of the interior zones result mostly from the accumulation of fluxes that enhance the diffusion of Al and of Si. Those fluxes are likely to become enriched in a boundary-layer liquid that forms at the growth front of crystals. Experiments that entail the dissolution of acid solutions of H3BO3, H3PO4 and HF into haplogranite glass at 800°C and 200 MPa serve as model subsystems for the boundary-layer liquid in granitic pegmatites. When the fluxes are added as acidic components, they promote an increase in the H2O content of granitic melts via a melt-speciation reaction, M + A ( A = anions of B, P, or F) + H+ = H+ A + M +, which shifts to the right as the activity of H+ in the melt increases. The effects of fluxes on the mass transport of Si were measured in experiments wherein quartz dissolved into a peralkaline, persodic, and multicomponent flux-enriched “granitic” melt at 800°C and 200 MPa. That peralkaline and flux-rich hydrosilicate melt transports ~107 times the mass of silica compared to an equivalent volume of aqueous fluid at the same P–T conditions. The flux-rich hydrosilicate melt transports Si ( via “local” diffusion) at a rate that is ~103 times faster than diffusion of Si through simple H2O-saturated granitic melt. “Field” diffusion of other components made possible by a flux-rich melt erases gradients in Si at a rate that is orders of magnitude faster than the “local” diffusion of Si.