The transition from magmatic crystallization to high-temperature metamorphism in deep magma chambers (or lenses) beneath spreading ridges has not been fully described. High-temperature microscopic veins found in olivine gabbros, recovered from Ocean Drilling Program Hole 735B on the Southwest Indian Ridge during Leg 176, yield information on the magmatic-hydrothermal transition beneath spreading ridges. The microscopic veins are composed of high-temperature minerals, (i.e., clinopyroxene, orthopyroxene, brown amphibole, and plagioclase). An important feature of these veins is the "along-vein variation" in mineralogy, which is correlated with the magmatic minerals that they 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 + brown amphibole and are accompanied by an intergrowth of brown amphibole + orthopyroxene. The mode of occurrence of the veins cannot be explained if these veins were crystallized from silicate melts. Consequently, these veins and nearby intergrowths were most likely formed by the reaction of magmatic minerals with fluid phases under the conditions of low fluid/rock ratios. Very similar intergrowths of brown amphibole + orthopyroxene are observed in clinopyroxene grains with "interfingering" textures. It is believed, in general, that the penetration of seawater does not predate the ductile deformation within Layer 3 gabbros of the slow-spreading ridges. If this is the case, the fluid responsible for the veins did not originate from seawater because the formation of the veins and the interfingering textures preceded ductile deformation and, perhaps, complete solidification of the gabbroic crystal mush. It has been proposed, based on fluid inclusion data, that the exsolution of fluid from the latest-stage magma took place at temperatures >700°C in the slow-spreading Mid-Atlantic Ridge at the Kane Fracture Zone (MARK) area. No obvious mineralogical evidence, however, has been found for these magmatic fluids. The calculated temperatures for the veins and nearby intergrowths found in Hole 735B gabbros are up to 1000°C, and these veins are the most plausible candidate for the mineralogical expression of the migrating magmatic fluids.
1Maeda, J., Naslund, H.R., Jang, Y.D., Kikawa, E., Tajima, T., and Blackburn, W.H., 2002. High-temperature fluid migration within oceanic Layer 3 gabbros, Hole 735B, Southwest Indian Ridge: implications for the magmatic-hydrothermal transition at slow-spreading mid-ocean ridges. In Natland, J.H., Dick, H.J.B., Miller, D.J., and Von Herzen, R.P. (Eds.), Proc. ODP, Sci. Results, 176 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/176_SR/chap_04/chap_04.htm>. [Cited YYYY-MM-DD]
2Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8 Kita, Sapporo, Hokkaido 060-0810, Japan. Correspondence author: jinmaeda@ep.sci.hokudai.ac.jp
3Department of Geological Sciences and Environmental Studies, State University of New York, Binghamton NY 13902-6000, USA.
4Global Environmental Laboratory, University of Toyama, 3190 Gofuku, Toyama 930, Japan. Present address: Deep Sea Research Department, Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan.
Initial
receipt: 6 July 2000
Acceptance: 12 September 2001
Web publication: 26 February 2002
Ms 176SR-004