岩石学における非平衡過程の速度論的研究

Abstract

Non-equilibrium textures in metamorphic rocks contain a great deal of information on reaction kinetics and mass transfer in metamorphic processes. The reaction zone between chemically incompatible minerals (or rocks) is a good example of such non-equilibrium textures, and hence it is worth studying it in detail. Korzhinskii's chemical potential diagram is a useful method to understand the zonal sequence in the reaction zone. It can explain the variability in the zonal sequence by specifying the corresponding diffusion path on the chemical potential diagram. However, the method cannot tell us why the specific diffusion path is realized in nature. Fisher-Joesten's steady diffusion model can be considered as a quantification of Korzhinskii's chemical potential diagram, which introduces flux-ratio equations corresponding to specification of the diffusion path on the chemical potential diagram. The steady diffusion model successfully describes the stability fields of zonal sequences in the L-ratios (phenomenological coefficient ratios) diagram. Application of the steady diffusion model to an olivine-plagiocalse corona in a norite from the Ryoke Metamorphic Rock is presented. Extension of the steady diffusion model to open system is also discussed for the case of a metabasite-marble reaction zone in the Hirao Limestone and for myrmekites from the Okueyama granite. Experimental studies of the formation of reaction zones in the system dolomite-quartz-H2O reveal temporal change in zonal sequence and mineral species of the reaction zones, showing a transition from an initial transient state to a steady state. Finally, I discuss periodic structures in rocks representing a pattern formation in a mineral-fluid system, taking an orbicular marble from the Hirao Limestone as an example. Although our understanding on such a ‘nonlinear structure’ is insufficient, I believe that there are some unknown physico-chemical processes behind such a structure.