Abstract
Hydrothermal fluid circulation within the sea floor profoundly influences
the physical, chemical and biological state of the crust and the
oceans. Circulation within ridge flanks (in crust more than 1 Myr
old) results in greater heat loss(1-3) and fluid flux(4) than that
at ridge crests and persists for millions of years, thereby altering
the composition of the crust and overlying ocean(5,6). Fluid flow
in oceanic crust is, however, limited by the extent and nature of
the rock's permeability(7). Here we demonstrate that the global data
set of borehole permeability measurements in uppermost oceanic crust(7-9)
defines a trend with age that is consistent with changes in seismic
velocity(10,11). This trend-which indicates that fluid flow should
be greatly reduced in crust older than a few million years-would
appear to be inconsistent with heat-flow observations, which on average
indicate significant advective heat loss in crust up to 65 Myr old(3).
But our calculations, based on a lateral flow model, suggest that
regional-scale permeabilities are much higher than have been measured
in boreholes. These results can be reconciled if most of the fluid
flow in the upper crust is channelized through a small volume of
rock, influencing the geometry of convection and the nature of fluid-rock
interaction.
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