HR: 1340h
AN: B13A-0206    [Abstracts]
TI: Diking, Magma Lenses, and Location of Hydrothermal Sites at Mid-Ocean Ridges
AU: Sim, Y
AF: School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332 United States
AU: * Germanovich, L N
EM: leonid@ce.gatech.edu
AF: School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332 United States
AU: Lowell, R P
EM: bob.lowell@eas.gatech.edu
AF: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332 United States
AU: Ramondenc, P
EM: pierre.ramondenc@ce.gatech.edu
AF: School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332 United States
AB: Magma chambers beneath fast spreading mid-oceanic ridges appear in the form of thin lenses in the cross- axis sections. Furthermore, the recent data indicate that such a magma lens also lies beneath the intermediate spreading rate Endeavor segment on the Juan de Fuca Ridge. This shape indicates that the pressurization of the magma chamber should result in the stress concentration near the tips of the lens while the rest of the host rock would be in the state of compression. It is likely, therefore, that if an episode of magma replenishment in the magma lens results in diking, the dikes will initiate near the lens tips. The further propagation of dikes can be described by the principles of fracture mechanics. Our calculations suggest that the dikes propagate almost vertically towards the seafloor from the lens tips. Because diking is likely to generate a region of high permeability near its margin in addition to heat, hydrothermal activity may be localized by diking events. This suggests that hydrothermal vent fields may be located above the tips of the magma lenses away from the ridge axis (i.e., in the cross-axis profile of the lens). Alternatively, the diking may result from solidification of the magma lens. Because the density of magma is lower then that for the fresh rock, crystallization leads to the pressure decrease in the lens. Consequently, the stress distribution in the host rock changes and becomes tensile in the middle part of the magma lens and compressive in the tip regions. In this scenario, the dikes are likely to initiate from the central areas and the hydrothermal sites would be more expected to occur above the central part of the magma lens. Comparing observations of the location of the hydrothermal sites with respect to the magma lens location may be useful for constraining the mechanisms of magma lens evolution. For example, the Salty Dawg hydrothermal site on the Endeavor segment appears to be located near the western tip of the seismically imaged magma lens. Our model suggests that the venting activity at this site may be localized by diking triggered by the lens pressurization. The fact that hydrothermal venting at the EPR occurs above the center of the magma lens suggets that the rapid hydrothermal cooling and magma crystallization in the lens may be important in controlling the permeability distribution at that site. As has recently been argued in the literature, the pressurization of the magma lens may be occuring by the dyking originating from the underlying, deep magma chambers (also with a lens shape). Our computations suggest this hypothesis. Furthermore, not only may these dykes pressurize overlying shallow magma lenses, but these can then also lead to the subsequent dyke propagation from the tips of these shallow lenses to the seafloor.
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3035 Midocean ridge processes
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting