GARNET GROWTH AFTER OVERSTEPPING

Abstract

Abstract Overstepping of metamorphic reactions is required to provide the driving force necessary 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 and grows is assumed to be that which provides the largest decrease in free energy. With these assumptions, the zoning predicted for a garnet grown under isothermal, isobaric conditions is revealed to be sufficiently similar to the zoning predicted for garnet grown under continuous near-equilibrium conditions that distinction based on the shape of zoning profiles alone does not appear to be possible. Furthermore, the growth of an initial garnet crystal following nucleation after overstepping depletes the affinity for subsequent nucleation by sequestration of Mn into the existing garnet crystal. Progressive nucleation, therefore, requires additional energy input, most likely through changes in pressure and temperature, to replenish the affinity. The predicted zoning in garnets nucleated following substantial growth of the first nucleated garnet does not match the typical bell-shaped profile of Rayleigh fraction but rather displays distinctly diagnostic zoning depending on the assumed rate limiting step for growth. Diffusion controlled growth results in relatively broader and flattened profiles whereas interface controlled growth results in later garnets having a peaked core Mn profile. This latter profile would relax by diffusion in relatively short times (less than 1 Ma) and would thus not likely to be preserved, but appears to have been observed in garnet zone samples from central Vermont. Calculations for this sample also indicate that garnet ceased growing when chlorite was exhausted, at which point considerable affinity remained indicating that equilibrium was not achieved. 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 magnitude of overstepping needed for nucleation and the possibility that equilibrium was not attained at the metamorphic peak.