鉱物脈組織から読み解く地殻流体流動

抄録

  Fluids in the Earth's crust play crucial roles in various geological phenomena including earthquakes, volcanism and formation of ore deposits. Mineral-filling veins are fossils of fluid-filled fractures, but it is not easy to extract information on nature of fluid flow during vein formation. Mineral veins in the metamorphic rocks show systematic variation in internal textures (grain shape, mineral distribution, crystallographic orientation, relation to the host rock) as a function of aperture size and host rock. Hydrothermal experiments for silica precipitation have revealed that the occurrences and mineralogy of silica minerals are strongly controlled by solution chemistry (supersaturation, minor elements), rock substrates (quartz distribution, grain size), fracture geometry (aperture distribution, surface roughness) as well as temperature, which results in a variation in vein texture. Combining modeling and analyses of natural and synthetic veins, vein texture is a potentially useful indicator of hydrological properties (flow velocity, flow direction, degree of supersaturation, duration of sealing etc.) in the crust.