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Geophysical Monograph Series

 

Keywords

  • Deep-sea ecology—Congresses
  • Mid-ocean ridges—Congresses

Article

GEOPHYSICAL MONOGRAPH SERIES, VOL. 144, PP. 99-117, 2004

Fluid flow and fluid-rock interaction within ocean crust: Reconciling geochemical, geological, and geophysical observations

W. Bach, S. E. Humphris, and A. T. Fisher

Fluid flow governs heat and mass transport in subseafloor hydrothermal systems and exerts control on the nature and distribution of deep microbial ecosystems in oceanic crust. We have integrated geophysical, petrographical, and geochemical data from key DSDP and ODP drill holes to assess fluid circulation pathways, fluid flow rates, and fluid-rock interaction in the oceanic lithosphere. Fluid ingress into the deep lithosphere is controlled by the development of faults and ductile shear zones, magmatic injection into the shallow crust, and downward propagation of a front of brittle fracturing into hot rock. Seawater circulation in the ridge flanks is governed by the crustal permeability structure, the availability of heat, and the existence of open pathways between the crustal aquifer and the overlying ocean. Rocks from drill holes representing basaltic crust of various ages and geological settings exhibit a uniform first-order sequence of alteration stages. An early stage of oxidative alteration and open seawater circulation is followed by restricted fluid circulation under varying conditions. Zones of high degrees of alteration often correspond to zones of high permeability, indicating that rock alteration and mineral precipitation do not completely seal the large-scale, mostpermeable fluid pathways. On the scale of tens to hundreds of meters, fluid flow is controlled by the distribution of permeable lithologies: breccias, fractured pillow basalts and faults act as aquifers, while massive flows of considerable areal extent may act as fluid flow barriers. Flow boundaries, pillow margins and large cooling cracks are the main fluid conduits at the meter-scale, while at the smallest scale, micro-cracks and grain boundaries control fluid flow. Hence, fluid flow is highly channelized and only a small fraction of basement communicates with the oceans. Fluid-rock interactions away from these main fluid conduits are dominantly diffusion-controlled, making the largest fraction of basement less desirable for microorganisms that rely on supply of nutrients by circulating fluids.

Citation: Bach, W., S. E. Humphris, and A. T. Fisher (2004), Fluid flow and fluid-rock interaction within ocean crust: Reconciling geochemical, geological, and geophysical observations, in The Subseafloor Biosphere at Mid-Ocean Ridges, Geophys. Monogr. Ser., vol. 144, edited by W. S. Wilcock et al., pp. 99–117, AGU, Washington, D. C., doi:10.1029/144GM07.

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