ON THE INTERPRETATION OF PEAK METAMORPHIC TEMPERATURES IN LIGHT OF GARNET DIFFUSION DURING COOLING

Finite difference models of Fe-Mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet-biotite Fe-Mg exchange thermometry. For rocks cooled from high grades where the garnet was initially homogeneous, the calculated temperature (T(calc)) using garnet core and matrix biotite depends on the size of the garnet, the ratio of garnet to biotite in the rock (V(garnet)/V(biotite)) and the cooling rate. For garnets with radii of 1 mm and V(garnet)/V(biotite) << 1, T(calc) is 633, 700 and 777-degrees-C for cooling rates of 1, 10 and 100-degrees-C/Ma. For V(garnet)/V(biotite) = 1 and 4 and a cooling rate of 10-degrees-C/Ma, T(calc) is approximately 660 and 610-degrees-C, respectively. Smaller and larger garnets have lower and higher T(calc), respectively. These results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below T(calc) of the garnet core, provided that V(garnet)/V(biotite) is sufficiently small (< 0.1) and that the composition of the biotite at the metamorphic peak has not been altered during cooling. Numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550-600-degrees-C generate a 'well' in Fe/(Fe + Mg) near the rim during cooling. Maximum calculated temperatures for the assemblage garnet + chlorite + biotite + muscovite + plagioclase + quartz using the Fe/(Fe + Mg) at the bottom of the 'well' with matrix biotite range from 23-43-degrees-C to 5-12-degrees-C below the peak metamorphic temperature for cooling rates of 1 and 100-degrees-C/Ma, respectively. Maximum calculated temperatures for the assemblage garnet + staurolite + biotite + muscovite + plagioclase + quartz are approximately 70-degrees-C below the peak metamorphic temperature and are not strongly dependent on cooling rate. The results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet-biotite Fe-Mg exchange thermometry on amphibolite facies rocks (T(max) > 550-degrees-C) because the rim composition of the garnet, which is required to calculate the peak temperature, is that most easily destroyed by diffusion.