Abstract
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.
Highlights
Rate-limiting processes and the degree of disequilibrium of a metamorphic reaction are crucial factors in the devolatilization and/or hydration of the Earth’s crust
I examine the diffusional instability of garnet porphyroblasts (Tsukuba metamorphic rocks, central Japan) that grew in a diffusion-controlled regime
Garnet porphyroblasts with an irregular morphology and a diffusional halo of reactants occur in the low pressure–high temperature (LP–HT) Tsukuba metamorphic rocks of central Japan
Summary
Rate-limiting processes and the degree of disequilibrium of a metamorphic reaction are crucial factors in the devolatilization and/or hydration of the Earth’s crust. The growth of a metamorphic mineral at a low degree of disequilibrium (i.e., a system close to equilibrium, implying a small overstep from the equilibrium temperature and a low degree of supersaturation for the metamorphic reaction) may result in an equigranular metamorphic texture because interfacial energy becomes an important control on crystal morphology and size. Growth at a high degree of disequilibrium with a limited number of nuclei may result in a porphyroblastic texture. Diffusion-controlled growth of a mineral at a high degree of disequilibrium is likely to involve diffusional instability and may lead to highly irregular and branched morphologies. I examine the diffusional instability of garnet porphyroblasts (Tsukuba metamorphic rocks, central Japan) that grew in a diffusion-controlled regime
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