NUCLEATION OF GARNET PORPHYROBLASTS

Nucleation of new crystals is fundamental to metamorphic crystallization, and inhibition of nucleation is a central cause of disequilibrium in metamorphic rocks, yet knowledge of nucleation mechanisms and kinetics has long remained rudimentary. For garnet porphyroblasts, new insights come from a combination of HRXCT imaging and EBSD measurements that highlights the importance of epitaxial mechanisms, and from numerical simulations that quantify rates of nucleation and their time variation.

Classical nucleation theory identifies key roles for interfacial energies and reaction affinity. The reduction in activation energy expected from epitaxial nucleation argues for its dominance as a mechanism, especially at low reaction affinity, but this has proven difficult to substantiate. Direct confirmation of epitaxial nucleation of porphyroblasts, however, comes from snowball garnets in the Lukmanier region of central Switzerland: EBSD shows that garnet bears a consistent crystallographic orientation to the included fabric at the site of nucleation.

Rates of garnet nucleation as a function of reaction affinity can be extracted from numerical models of porphyroblast nucleation and growth, constrained by quantitative microstructural data from HRXCT and by chemical zoning patterns within crystals. A marked contrast in nucleation behavior exists between two garnetiferous pelites quite similar in overall bulk composition and metamorphic grade. In rocks from the Picuris Range of New Mexico, nucleation spans nearly the entire crystallization interval, and numerical simulations indicate that the nucleation rate rose rapidly to its maximum steady-state value early in the reaction, so most nucleation took place after reaction affinity had reached high values. In rocks from Passo del Sole in the central Swiss Alps, however, nucleation effectively ceased after ~6-15% of garnet crystallization had taken place, implying rapid saturation of sites suitable for epitaxial nucleation. The impediments to nucleation evident in both cases suggest that garnet crystallization may commonly take place at high levels of reaction affinity, which may in turn induce appreciable disequilibrium for metamorphic mineral assemblages and possibly for the compositions of phases within them.