部分的変質帯の幅を規制する要因

抄録

Changes in solute concentration of a fluid phase percolating in rocks are simulated using a mass balance equation. Fluid having given solute concentration is constantly injected into a rock column and flows through it with constant velocity (v). A mono-component system is considered and parameters such as temperature, pressure, porosity, fluid density (ρ), surface area (A) and diffusion coefficient are assumed to be constant. The first order rate constant (k) is used to simulate the rock-fluid interaction. The fluid reacts with the rock, and changes its solute concentration in the reaction zone until the equilibrium is achieved. The concentration profile of the fluid phase does not change in a "steady state", but it advances after the rock is completely reacted at the inlet. The reaction zone, where unreacted mineral remains, corresponds to a "partially altered zone" in nature. The characteristic width of the reaction zone is represented by ρv/Ak. The estimated range of ρv/Ak at 250°C is wide; 10⁴ to 10¹² [cm] for veins, 10^-1 to 10⁷ [cm] for geothermal reservoirs and 10^-4 to 10⁴ [cm] for metamorphic environments. The width of the partially altered zone determined by field observations permits to estimate by means of model calculations.