Massive fluid flow through oceanic ridge flanks is responsible for a large part of the heat loss of the oceanic crust and likely plays a major role in the geochemical cycling of chemical elements (Wolery and Sleep, 1976; Stein et al., 1995; Kadko et al., 1995; Elderfield and Schultz, 1995). Leg 168 was designed to study the characteristics of fluid circulation in the eastern flank of the Juan de Fuca Ridge (north-east Pacific). A series of ten holes were drilled through the sediments and into the upper basaltic basement along an east-west transect (Fig. 1). Data collected during Leg 168 provide evidence that fluid circulation is still active 100 km from the ridge axis (Davis, Fisher, Firth, et al., 1997).
Both diffuse and focused fluid discharge to the deep ocean have been found on the eastern flank of the Juan de Fuca Ridge. A diffuse vertical upwelling of basement fluid occurs at Sites 1030 and 1031 where the sediment layer is only 40-50 m thick. Diffuse flow in this area was documented from heat-flow data and pore-water compositions from shallow cores (Wheat and Mottl, 1994). Fluid discharge from basement took place during drilling at Site 1026 after penetration of the sediment layers and showed that basement at this location is overpressured, but the sediments were thick enough (250 m) to prevent seepage at the seafloor (Davis, Fisher, Firth, et al., 1997; Fisher et al., 1997). A submarine spring producing warm (25°C) waters was discovered during Alvin dives in August 1995 at the top of an isolated basaltic outcrop called Baby Bare (Mottl et al., 1998). Site 1026 is located a few kilometers north of Baby Bare.
The estimate of chemical fluxes originating from fluid circulation in oceanic ridge flanks is difficult because of the paucity of the data and the large range of crustal conditions (Mottl and Wheat, 1994; Elderfield and Schultz, 1995; Kadko et al., 1995). Sansone et al. (1998) evaluated the contribution of the low-temperature hydrothermal alteration of the oceanic crust to the global carbon budget from the composition of the Baby Bare spring waters. In this paper we investigate the stability of calcium carbonates in the sediments of the eastern flank of the Juan de Fuca Ridge from the pore-water compositions using simple thermodynamic calculations.