Chemical composition of basement fluids within an oceanic ridge flank: Implications for along-strike and across-strike hydrothermal circulation

Compositions of basement fluids are presented for four sites along a 3.5-m. y.-old, partly buried basement ridge on the eastern flank of the Juan de Fu ca Ridge. This ridge is roughly parallel to the active ridge axis of the En deavor Segment similar to 100 km to the west. From south to north these sit es are Baby Bare Outcrop, Ocean Drilling Program (ODP) Site 1026, and the s outhern and northern sides of Mama Bare Outcrop. The composition of basemen t fluids is determined or estimated from analyses of pore water samples tha t were extracted from sediments at each of these sites, spring waters from Baby Bare, and basement fluids that vented from the open ODP Hole 1026B. Ch emical trends in basement fluids along this transect show increasing altera tion from south to north. A similar trend was observed along an ODP transec t perpendicular to the ridge axis with increasing fluid alteration from wes t to east. Much of the increase in fluid alteration along the ODP transect is explained by greater water-rock exchange with increasing basement temper ature to the east. In contrast, the trend along the 3.5-m.y.-old ridge is b est explained by diffusive exchange with the overlying sediment. The rate o f this exchange is used to constrain hydrologic properties within basaltic basement. Flow within the 3.5-m.y.-old ridge is inferred to occur from sout h to north and lacks significant exchange with basement fluids from the act ive ridge crest to the west. Thus the two flow systems are hydrologically d istinct, and flow paths are likely influenced by the complex distribution o f permeability in basement, the pattern of seafloor morphology, and the typ e and rate of sedimentation.