Borehole fluid samples have been collected from Hole 504B before drilling on a total of seven expeditions. The chemical compositions have revealed that these fluids can be described as a mixture of unaltered seawater with a single reacted fluid component. In addition, analyses of tritium provide unequivocal evidence that some of the surface seawater, originally placed in the borehole during circulation on the leg before sampling, remains after periods of up to 1233 days. In this manuscript we take a numerical approach to simulate the vertical mixing of the borehole fluids to account for the displacement of surface seawater by mixing with bottom seawater. The simulations demonstrate that the composition of borehole fluids from Hole 504B can result from vertical mixing of seawater with a reacted fluid component present in the rubble at the hole bottom. This implies that formation water is not present in the borehole waters as previously suggested. We show that the mixing required to produce the geochemical observations would not disrupt the temperature profile. Finally, we demonstrate observed geochemical trends would not be observed if borehole mixing is the process responsible for the observed 40% reduction in the temperature gradient below 850 mbsf in Hole 504B.

Date of initial receipt: 12 August 1994
Date of acceptance: 26 January 1995

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