Optical Determination of Topotactic Aragonite-Calcite Growth Kinetics: Metamorphic Implications

Textures produced by the partial recrystallization of aragonite to calcite in the blueschist facies show that heterogeneous, topotactic nucleation is rapid enough to permit growth kinetics to dominate the overall geologic rate of transformation. Both grain boundary velocities, measured microscopically between 375°C and 455°C, and analysis of topotactic relations predict a growth mechanism involving only small atomic displacements near a highly coherent boundary between the two phases. The kinetic equation of Turnbull for interface-controlled growth, based on that mechanism, accurately describes the experimental results. When calculated rates are integrated over geologically plausible pressure-temperature-time paths for convergent plate margins, even the most extreme values lead to the conclusion that aragonite-bearing rocks enroute to the surface must enter the calcite stability field between 125°C and 175°C. The widespread occurrence of aragonite along the Pacific margin of North America, and by contrast, its absence in other blueschist terranes, may thus reflect in part very slight differences in temperatures encountered by the rocks during upward transport.