QUIG BAROMETRY AND OVERSTEPPING THE GARNET ISOGRAD

Quartz-in-garnet inclusion barometry (QuiG) provides a means of determining the P–T conditions of garnet growth. Laser Raman spectroscopy is used to measure the wavelength shift of the 464 cm^-1 peak of quartz, which can be used to determine the pressure on an encapsulated quartz crystal to an uncertainty in the encapsulation pressure of ±120 bars. The efficacy of QuiG to recover P–T conditions has been tested experimentally at 800˚C, 20 kbar and found to recover the synthesis pressure to within 120 bars of the nominal pressure.

Measurements from several samples from the garnet and staurolite zones of central New England reveal that garnet cores form at conditions similar to the garnet rims and that these conditions are similar to what is inferred to be the peak metamorphic conditions. This result suggests that rather than growing continuously from the garnet isograd to peak conditions at near-equilibrium, garnet nucleates and grows nearly isothermally and isobarically.

The measured composition of the cores of these garnets, when evaluated assuming equilibrium growth, yield P–T conditions near, but not exactly coincident with, the garnet isograd. Conversely, measured garnet core compositions have been used to calculate the conditions of formation assuming that the initial garnet composition is the one that results in the maximum free energy change from the garnet-free matrix – the overstep model. P–T conditions inferred from the overstep model are much more consistent with the P–T conditions of formation inferred from QuiG, thus supporting the inference that garnet the garnet isograd may be overstepped by as much as 50-70˚C and 2-5 kbar (8-18 kJ/mole garnet).

There are numerous examples in the literature in which equilibrium calculations using garnet core compositions yield P–T estimates for nucleation that lie above the equilibrium garnet isograd. It is suspected that many of these examples are manifestations of significantly overstepped garnet nucleation events. Accordingly, chemical zoning in garnet does not follow equilibrium P–T–X–M relations but rather is governed by grain boundary diffusion or interface kinetics.