浸透的な流体移動の素過程

Abstract

A review of the pervasive fluid-migration mechanism is presented with newexperimental results in a quartzite-water system. Microgeometry and connectivity of geological fluids in rocks are governed by the balance of interfacial energy between fluid-crystal and crystalcrystal interfaces; i. e., dihedral angle. Fluid with dihedral angle <-60° can migrate through grain edges as pervasive flow. In rocks having a fluid network, the fluid flow velocity under a fluid pressure gradient is determined by Darcy's law. The minimization of the total interfacial energy drives fluid infiltration and segregation (expulsion). Several experimental and theoretical studies on the interfacial energy-driven fluid migration have been reported, where in the systems considered have been extended from a simple triple junction to polycrystalline rocks with or without two lithologies, bimodal grain size and grain growth. The concepts of MEMF (minimum energy melt fraction), lithological partitioning, grain size effect on fluid fraction, and fluid localization in coarsening rocks, all of whichare required to minimize the total interfacial energy of the system, have been developed. The kinetics of fluid infiltration, segregation and grain growth is derived from the difference in pressure over the interfaces with different curvatures that results in different mineral solubility into the fluids.The interfacial tension, which may be easily overwhelmed with fluid pressure gradient as a direct driving force of Darcy flow, significantly affects Darcy flow by controlling porosity and permeability of the rock. Because the rate of aqueous fluid infiltration into quartzite obtained by the infiltration experiments is much higher than that of Darcy flow velocity under geologically plausible permeability and pressure gradient during metamorphism, interfacial energy-driven fluid infiltration might play a major role in some large-scale fluid/rock systems, although long-term assessment of infiltration rate in natural rocks involves large uncertainty arising from phenomena such as grain faceting, choking of the pore network by accessory minerals and precipitated crystals. More experimental studies to clarify the fluid behavior in geologically realistic systems, as well as direct observation of natural rocks, are needed for better understanding of fluid migration in the Earth's interior.