HR: 11:05h
AN: V12A-04    [Abstracts]
TI: Reaction-Enhanced Permeability in Gabbroic Crust, IODP Site 1309, mid Atlantic Ridge
AU: * McCaig, A M
EM: a.mccaig@see.leeds.ac.uk
AF: School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, United Kingdom
AU: Condliffe, E
EM: e.condliffe@see.leeds.ac.uk
AF: School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, United Kingdom
AU: Frost, B R
EM: rfrost@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, United States
AU: Jamtveit, B
EM: bjorn.jamtveit@geo.uio.no
AF: Physics of Geological Processes, University of Oslo, PO Box 1048, Blindern, Oslo, 0316, Norway
AB: Thermal cracking is normally assumed to be the main mechanism of permeability generation in the oceanic crust. Here we present microstructural evidence that metamorphic reactions played a significant role in creating porosity and permeability in gabbroic rocks beneath a detachment fault at 30^o N in the Atlantic ocean. At least two mechanisms for permeability generation have been identified: 1) In basaltic intrusions, euhedral zoning in amphibole replacing clinopyroxene suggests a dissolution/precipitation mechanism similar to textures observed in epidosites from the Troodos ophiolite. The basaltic sills lack macroscopic fractures and alteration patterns suggest a fingering instability allowed fluid to enter the rock. 2) Volume increase reactions in olivine gabbros and troctolites. These generated radial crack networks filled with secondary minerals where olivine was surrounded by other phases, and are seen both in serpentinization reactions and in reaction between olivine and plagioclase forming tremolite-chlorite coronas. Discrete-element modelling shows how isolated olivines can be linked by propagating networks of cracks allowing fluid to access initially impermeable crust. Assuming that most fluid flow occurs along major fractures, reaction permeability provides an effective mechanism for fluid to access intervening unaltered rock, and may be important for scavenging ore-forming components from the crust. The most permeable part of the system is likely to be at the reaction front before precipitating minerals occlude the porosity, and this may promote front-parallel fluid flow.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 3616 Hydrothermal systems (0450, 1034, 3017, 4832, 8135, 8424)
DE: 3625 Petrography, microstructures, and textures
DE: 3653 Fluid flow
DE: 5114 Permeability and porosity
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting