Mineral Growth Controlled By Aperture Of Fluid-filled Cracks In Subduction Zones: An Example From The Sanbagawa Belt, Japan

Sealed cracks in high-pressure metamorphic rocks have been regarded as a direct evidence of the fracture- controlled fluid flow in subduction zones. Although various growth microstructures of vein minerals have been reported, the relationship between microstructures and fluid flow remains unclear. The pelitic schists from Nagatoro area of the Sanbagawa metamorphic belt, Japan contain two types of veins composed of quartz + albite + K-feldspar + chlorite (type I) and quartz + albite + calcite (type II). Both veins cut the foliation and the stretching lineation of the host rocks, indicating the formation at the exhumation stage (about 300 degC). Within type I veins, elongate quartz and albite grains grew from the fractured quartz and albite grains of the vein wall, respectively, and K-feldspar and chlorite form corresponding to muscovite + chlorite-rich layers of host rocks. In contrast, type II veins have euhedral quartz grains with concentric zoning, and the mineral distribution is independent of those of host rocks. The veins systematically change from type I to type II with increasing vein width, and the critical width is about 1 mm. Both veins show the evidences of multiple crack- seal events, but the aperture width of each crack are 0.01 - 0.05 mm for type I, whereas 0.5 - 3.0 mm for type II. Considering the cubic dependence of the permeability on crack aperture, the permeability of type II veins was 103 - 106 times larger than type I veins. The materials for forming vein minerals come from deeper parts of the subduction zones, or from the surrounding host rocks. The mineral distribution of type I veins suggests that material diffused into fluid-filled cracks from the host rocks, and that the effect of fluid advection was very small. In contrast, for type II veins with wide aperture, the upward fluid flow would have brought the high concentration of Si into the crack, that leaded to the homogeneous nucleation of quartz. Although the frequency of type II veins is about 10 % of total veins in the Nagatoro area, this type veins would have played an important roll on fluid and material transport within the subduction zone.