SUBSOLIDUS ISOTHERMAL FRACTIONAL CRYSTALLIZATION

In theory, multicomponent silicate liquids of minimum or eutectic composition should crystallize their solidus phases simultaneously and in their invariant proportions. In reality, the crystallization of natural and synthetic liquids of granitic composition (sensu lato) produces sequential assemblages and normal fractional crystallization of solid solutions when crystallization commences at or along an isotherm well below the solidus of the system. In the granite system, the slopes of the liquidus surfaces for quartz and feldspars are not equivalent. Hence, their metastable extensions below the solidus diverge, to different extents, from the projection of the minimum or eutectic composition to the actual subsolidus temperature of crystallization. The free energies (ΔG) of the reactions µ_i,L → µ_i,Xl, which correspond to the chemical potentials of components in melt and their equivalent crystalline phases, is zero for all phases at the minimum or eutectic, and values of ΔG become increasingly negative below the solidus. Relative to the composition and temperature of undercooling, however, -ΔG is greater for some phases than for others, and for the high-temperature ends of solid solutions. For this reason, the driving force to crystallize phases is not equivalent at a temperature well below the solidus, and the first phase out is the one whose free energy change (-ΔG) for the crystallization reaction is greatest. As a result, crystallization is sequential, even for minimum or eutectic liquid compositions. For natural liquid compositions that contain at least tenths of a weight percent of CaO, feldspars predominate over quartz, plagioclase precedes K-feldspar, and the most calcic plagioclase and mafic minerals, with their higher actual liquidus temperatures, crystallize first -- both in experiments and in nature. The liquidus undercooling that drives subsolidus isothermal fractional crystallization is the single-most important process for the generation of zoned granitic pegmatites. Whereas large liquidus undercooling (ΔT ≈ 200°C) produces zoned pegmatitic textures via subsolidus isothermal fractional crystallization, crystallization closer to the solidus (ΔT ≈ 50°C) forms granites.