Retrogressive dissolution of garnet: Effect on garnet-biotite geothermometry

Cation-exchange geothermometry is predicated on the basis of chemical equilibration between phases involved in ion-exchange reactions. If chemical equilibrium is attained between the phases at any temperature and subsequently locked in, temperature determinations should be consistent throughout a given sample. The attainment of chemical equilibrium is commonly inferred where phases used for thermobarometry are in mutual contact or where sharp, straight grain boundaries exist between grains. Thermometric determinations on garnet-biotite pairs in a metapelite from the Cheyenne belt, Wyoming, illustrate the problem of using these criteria to infer chemical equilibrium. In one example, garnet-biotite pairs from different parts of a single zoned garnet grain yield apparent temperatures that range over 150°C and are anomalously higher than documented metamorphic conditions.

Almandine garnets from low- to intermediate-grade pelitic rocks commonly show strong compositional zoning. Retrograde modification coupled with strain-induced dissolution of garnet may result in meaningless temperatures because of nonequilibration of phases used for geothermometry. Apparent textural equilibrium or mutual contact of grains may not be indicative of chemical equilibrium. Dissolution of garnet under conditions of biotite stability can produce garnet-biotite pairs with sharp, straight grain boundaries suggestive of textural equilibrium. In the absence of visible truncation of compositional zones, compositional maps or microprobe traverses both parallel and normal to schistosity are necessary to evaluate the possibility of dissolution. Failure to do so may result in the incorrect interpretation of T-P determinations.