Nucleation theory applied to the development of contrasting garnet crystal densities

Rocks that contain numerous small garnets (high crystal density or HiCD) and few large garnets (low crystal density or LoCD) have been examined from three localities in New England to constrain the degree of overstepping for garnet nucleation. Garnet crystal densities have been measured in nine samples and range from a few crystals/cm 3 to over 100 × 10 6 crystals/cm 3 . Quartz-in-garnet (QuiG) barometry reveals that the quartz inclusion isomekes in HiCD samples are within error the same pressure as quartz inclusion isomekes in LoCD samples, suggesting similar P–T conditions of nucleation. Temperature of nucleation is more difficult to constrain but several lines of reasoning suggest that both HiCD and LoCD samples nucleated garnet at similar P–T conditions, which are, in all cases, significantly above the calculated equilibrium garnet-in reaction. Affinities for garnet nucleation calculated using the Maximum Driving Force (MDF or parallel tangent) approach range from 0.15 to 0.5 kJ/mol-O in the LoCD samples to 1.4–4.3 J/mol-O in the HiCD samples using the SPaC thermodynamic dataset and 0.65–1.8 kJ/mol-O in the LoCD samples to 3.0–6.1 J/mol-O in the HiCD samples using the HP11 thermodynamic dataset. Application of classical nucleation theory permits constraining the surface energy at the time of nucleation to approximately 0.022–0.045 J/m 2 depending on the thermodynamic dataset used and places limits on the pre-exponential constant in the rate equation. The question of why proximal samples should accumulate such different amounts of affinity before garnet nucleates is unanswered, but it is clear that some factor other than just the amount of chemical affinity must be important.