Alteration and fluid flow recorded in a 1400 m section of oceanic gabbros beneath a detachment fault at 30 <sup>o</sup>N mid-Atlantic Ridge, (IODP Site U1309)

HR: 0800h
AN: T41D-1335 [Abstracts]
TI: Alteration and fluid flow recorded in a 1400 m section of oceanic gabbros beneath a detachment fault at 30 ^oN mid-Atlantic Ridge, (IODP Site U1309)
AU: * McCaig, A
EM: andrew@earth.leeds.ac.uk
AF: Leeds University, School of Earth and Environment, Leeds, LS2 9JT United Kingdom
AU: Frost, B R
T41D-1335 AF: Univerity of Wyoming, Dept. Geology and Geophysics , Laramie, WY 82701 United States
AU: Beard, J S
T41D-1335 AF: Virginia Museum of Natural History, Va. Mus Nat Hist, Martinsville, VA 24112 United States
AU: Abratis, M
T41D-1335 AF: University of Jena, Inst. for Geosciences , Jena, 07749 Germany
AU: Andreani, M
T41D-1335 AF: Inst. du Physique du Globe de Paris, Lab de Geosciences, Paris, 75252 France
AU: Delacour, A
T41D-1335 AF: ETH-Zurich, Dept. of Earth Sciences, Zurich, x Switzerland
AU: Drouin, M
T41D-1335 AF: Univ. Montpellier, Lab de Tectonophysique, Montpellier, 34095 France
AU: Fryer, P
T41D-1335 AF: Univ of Hawaii, School of Ocean and Earth Science and Technology, Honolulu, HI 96822 United States
AU: Ozaka, T
T41D-1335 AF: Okayama University, Dept of Earth Sciences, Okayama, 700-8530 Japan
AU: Ohara, Y
T41D-1335 AF: Hydrographic and Oceanographic Dept. of Japan, Ocean Research Laboratory, Tokyo, 104-0045 Japan
AB: IODP Hole U1309D was drilled north of the summit of the Atlantis inside corner high at 30 ^oN during Expedition 304/305. The hole is located on a corrugated surface assumed to be a low angle detachment fault. Small samples of talc-tremolite-chlorite schist recovered at the top of the hole are believed to represent fault rock associated with slip on the detachment fault system. 1400m of gabbro with subordinate diabase, troctolite and ultramafic cumulates were penetrated with over 75% recovery. A superb alteration profile is preserved in the core, with pervasive alteration above 380mbsf overprinted by localised metasomatic alteration around veins and breccia zones. Below 380mbsf, most alteration is localised around veins, faults, igneous contacts and late leucocratic dikelets, and may involve magmatic fluids in some cases. A down-temperature sequence of alteration is seen comprising: 1) Narrow granulite-facies shear zones containing Ti-amphiboles. 2) Widespread but minor amphibolite facies hornblende growth. 3) Low amphibolite to upper greenschist facies coronitic reactions between olivine and plagioclase, 4) Coupled serpentinization of olivine and replacement of plagioclase by prehnite and hydrogarnet, 5) Fracture controlled zeolite and clays. The coronitic reaction went to completion in olivine-bearing rocks above 330 mbsf, and the serpentine-prehnite-hydrogarnet assemblage is only common below this depth, indicating continued downward penetration of water during cooling. Zeolite-facies metamorphism is occurring today at the base of the hole, where temperatures exceed 120 ^oC. Permeability in the core was generated both by thermal and tectonic cracking, and by metamorphic reactions. In particular, volume increase in the coronitic reaction is manifested by radial chlorite-filled cracks in plagioclase surrounding pseudomorphs after olivine. In diabase, complex zoning in amphiboles after pyroxene suggests dissolution and precipitation with generation of porosity. In a model of reaction-induced permeability, the maximum permeability is likely to be in a narrow zone close to the reaction front, a zone which would move downwards in the crust with time. In fully reacted rocks, permeability would be reduced by mineral precipitation, and fracturing events would be required to generate new permeable pathways. These results demonstrate the critical role of metamorphic hydration reactions on fluid flow and the thermal evolution of the oceanic crust. Reaction-enhanced permeability provides a mechanism to continually access fresh rocks in the reaction zone of hydrothermal circulation systems, an observation which is consistent with the minor element composition of vent fluids. In addition, the stress state and hence fracturing of the crust may itself be affected by reactions involving volume change.
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 3036 Ocean drilling
DE: 3653 Fluid flow
DE: 3660 Metamorphic petrology
SC: Tectonophysics [T]
MN: Fall Meeting 2005