HOW WELL DOES ZONED GARNET PRESERVE PROGRADE METAMORPHIC CONDITIONS?

Preserved prograde zoning patterns in garnet are commonly used to interpret P-T conditions of porphyroblast growth. Intra-crystalline diffusion, however, continually acts to modify growth zoning, posing the question of what do zoned profiles actually record? Do we expect the initial composition of crystal growth (X_i) to be preserved, partially modified, or does the final composition (X_f) bear little relation to that originally grown? Recipes to deal with such differences are clearly important for mineral geothermobarometry. A useful concept is to establish a series of ‘opening to diffusion’ conditions, beyond which sufficient intra-crystalline diffusive modification occurs to significantly change growth composition before peak temperature is reached. Previous modelled examples show that this must depend non-linearly on the maximum temperature achieved, dT/dt, grainsize, and conditions external to the garnet grain (similar to ‘closure temperature’ during cooling). We calibrate such ‘opening to diffusion’ curves for rocks experiencing all possible linear T-t paths to peak temperatures in the range 500 to 850 ˚C (in durations of 1 to 50 Ma). Grainsizes in the range 0.5 mm to 1 cm are examined. This reveals the prograde P-T-X-t conditions required for each of Fe, Mg, Ca and Mn in garnet to (i) record growth compositions, (ii) preserve apparent growth-zoning trends but lose specific growth compositions, and (iii) entirely lose growth zoning through diffusion. The position of the effective ‘opening’ curves can be significantly different for each component. Furthermore, the ‘opening’ point depends strongly on rock composition, which controls both the specific suite of mineral reactions experienced (and thus the magnitude of growth zoning established), and the absolute crystal composition. Finally, we note that different parts of a single crystal effectively ‘open’ to diffusion at different periods of its history. In complex cases this might result in the ‘core’ and ‘near-rim’ parts of a crystal being highly modified from growth compositions, whilst ‘mantle’ overgrowths relatively faithfully record their growth conditions. We attempt to detail which parts of zoned garnets provide the most suitable conditions for P-T-X-t studies.