CONCLUSIONS

1. A type of microscopic vein is found in olivine gabbros recovered from Ocean Drilling Program Hole 735B on the Southwest Indian Ridge during Leg 176. These veins are composed of high-temperature minerals (i.e., clinopyroxene, orthopyroxene, brown amphibole, and plagioclase) and yield important information about the magmatic/hydrothermal transition beneath the Southwest Indian Ridge.
2. An important feature of these veins is the along-vein variation in mineralogy, which is strongly correlated with the primary magmatic phase, in which the veins penetrate. Within grains of magmatic plagioclase, the veins are composed of less calcic plagioclase. In grains of olivine, the veins are composed of orthopyroxene + brown amphibole + plagioclase. In clinopyroxene grains, the veins consist of plagioclase and brown amphibole and are accompanied by intergrowths of brown amphibole + orthopyroxene. The occurrence of the intergrowths is completely symmetrical relative to the veins, indicating an intimate genetic relationship between the intergrowth and the microscopic vein. Very similar intergrowths are observed in clinopyroxene grains with the interfingering textures. Similarity between the petrographical features and the chemical compositions of the minerals in the two kinds of intergrowths indicates that they were formed under essentially the same conditions and by the same process.
3. Although calculated temperatures of the minerals from the veins and nearby intergrowths (up to 1000°C) are lower than those of magmatic clinopyroxenes (1200°-800°C), the difference is gradational without any substantial gap between them. This suggests that the formation of the veins and the interfingering textures occurred during the latest stage of (or just after) magmatic crystallization of olivine gabbros. Petrographic evidence also suggests that the penetration of the veins and the formation of the interfingering texture predated complete solidification of the gabbroic crystal mush as well as the ductile deformation.
4. Although the veins are composed of silicate minerals, the discontinuous and irregular occurrence and the along-vein variation of the veins cannot be explained if these veins were crystallized from silicate melts. Consequently, these veins and nearby intergrowths are most likely to be formed by the reaction of magmatic minerals with fluids under the condition of low fluid/rock ratios.
5. In general, it is accepted that the penetration of seawater into Layer 3 gabbros does not predate ductile deformation due to lithospheric stretching beneath slow-spreading ridges (Mével and Cannat, 1991). Therefore, the origin of the fluid phase that appears to predate ductile deformation cannot be attributed to the penetration of seawater. It is likely that the fluids originated from evolved magma by exsolution. We propose that the microscopic veins yield mineralogical evidence for the exsolution and percolation of high-temperature fluid derived from latest-stage magma beneath the Southwest Indian Ridge.
6. The exsolution of magmatic fluids at high-temperature conditions is not restricted to slow-spreading environments. The discrepancy between the hydration temperatures of ~800°-600°C observed for the gabbros from fast-spreading ridges, and 500°C assumed for the penetration of seawater at fast-spreading ridges by a simple cracking front model, can be explained, if the exsolution of magmatic fluids occurred at the higher temperatures.