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Solid earth sciences
Session convener-recommended article JpGU Meeting 2014
Diffusion-controlled growth and degree of disequilibrium of garnet porphyroblasts: is diffusion-controlled growth of porphyroblasts common?
Diffusion-controlled growth, Garnet, Metamorphism, Disequilibrium, Porphyroblast
(Left) Irregular garnet porphyroblast (green) located in the center, and diffusional halo (inside of white broken line) indicated by depletion of reactant biotite (red) (from Figure 8).
(Right) Relationships among diffusional instability, supersaturation and diameter of growing garnet porphyroblast and inferred paths for porphyroblast (path-A) and for non-porphyroblast (path-B) (from Figure 9).
Rate-limiting processes and the degree of disequilibrium during metamorphic mineral growth are key controls on the rate of dehydration and hydration in the Earth’s crust. This paper examines diffusion-controlled growth and the degree of disequilibrium of garnet porphyroblasts in the Tsukuba metamorphic rocks of central Japan. The analyzed porphyroblasts have irregular and branching morphologies with clear diffusional haloes, indicating that they grew in a diffusion-controlled regime. Mathematical analysis shows that the dominant wavelength of the interface of a garnet porphyroblast is dependent on the extent of supersaturation (Δζ), which is an index for the degree of disequilibrium. Using the calculated upper and lower limits of the dominant wavelength, the value of Δζ is estimated to be 0.05 × 10 −1 –0.16, which corresponds to a Gibbs free energy (ΔG r ) overstep of 0.9–27 kJ per mole of garnet (12 oxygen atoms) and a temperature overstep (ΔT) of 1.7–50 °C. Using the average value of the dominant wavelength, the following results are obtained: Δζ = 0.15 × 10 −1 , ΔG r = 2.7 kJ per mole of garnet, and ΔT = 5 °C. These values bring into question the importance of diffusion-controlled growth of garnet porphyroblasts, as highly irregular and branching garnet porphyroblasts are rare in most metamorphic belts. After significant overstepping for the nucleation of garnet, the garnet porphyroblasts grow at a high degree of disequilibrium. However, a high degree of disequilibrium under diffusion-controlled growth would be characterized by diffusional instability. The results indicate that garnet porphyroblasts that lack an irregular and branching morphology may grow at a high degree of disequilibrium under interface-controlled growth, provided they are set in a medium where the diffusion and supply of constituent elements are sufficient, such as a sufficient volume of metamorphic fluid.