GARNET GROWTH AFTER OVERSTEPPING (Invited Presentation)

Overstepping of metamorphic reactions is required to provide the driving force required for porphyroblast nucleation and growth. Forward models of garnet nucleation and growth are presented assuming an affinity for nucleation that corresponds to several tens of degrees or several kilobars of overstepping. The composition of garnet that nucleates is assumed to be that which provides the largest decrease in free energy and the same assumption is made for every increment of growth. With these assumptions, the chemical zoning in garnet following nucleation after overstepping has been calculated. The zoning predicted for a garnet grown under isothermal, isobaric conditions is revealed to be similar to the zoning predicted in garnet grown under continuous near-equilibrium conditions. Thus, distinction between equilibrium and non-equilibrium growth does not appear to be possible through zoning profiles alone. Furthermore, the growth of an initial garnet crystal depletes the affinity for subsequent nucleation by sequestration of Mn into the existing garnet crystal. Progressive nucleation, as documented in numerous studies, therefore requires additional energy input, most likely through changes in pressure and temperature, to replenish the affinity. Prediction of zoning in successively nucleated garnet requires assumption of the rate-limiting step for growth. Assuming either interface controlled, equal surface flux, or sequestering from equal radial shells, the predicted zoning in garnets nucleated following substantial growth of the first nucleated garnet displays a peaked core Mn profile that does not match the typical bell-shaped profile of Rayleigh fraction. This profile would relax by diffusion in relatively short times (less than 1 Ma) and would thus not likely to be preserved except where the duration of peak metamorphism is short. Examples of peaked Mn core zoning have, nevertheless, been observed in garnet-grade rocks in central Vermont. Tectonic interpretations based on P–T paths calculated assuming near-equilibrium nucleation and growth of garnet are thus likely to require reevaluation in light of the unlikelihood that equilibrium was closely realized except, perhaps, near the metamorphic peak.