Massive, low-temperature hydrothermal flow from a basaltic outcrop on 23 Ma seafloor of the Cocos Plate: Chemical constraints and implications

Systematic variations in pore water chemical and thermal profiles from sediment gravity cores indicate the presence of a cool'' (10-20 degrees C) ridge-flank hydrothermal system within basement surrounding Dorado outcrop, a small basaltic edifice on 23 Ma seafloor of the Cocos Plate. Dorado outcrop is located within a 14,500-km(2) region of cool seafloor, where 60-90% of the lithospheric heat is removed advectively. Pore water chemical profiles from sediments on and near Dorado outcrop indicate a range of diffusive, advective, and diagenetic influences, including evidence for upward fluid seepage at up to several meters per year. The chemical composition of fluid that discharges from Dorado outcrop is only slightly different from that of bottom seawater. Pore water nitrate and geological constraints suggest a minimum volumetric flux per unit width of basement of 1800 m(3) a(-1) cm(-1) and a total seawater flow through Dorado outcrop of similar to 3000 kg a(-1). This flow rate is orders of magnitude greater than that estimated from Baby Bare outcrop, a similarly sized basement edifice on younger seafloor on the eastern flank of the Juan de Fuca Ridge. The nearest likely basement recharge site is Tengosed Seamount located similar to 20 km away. Calculated rates for the specific discharge at Dorado outcrop are consistent with young C-14 ages, suggesting a residence time in basement no greater than a few hundred years. If the fluid exiting from Dorado outcrop is characteristic of ridge-flank hydrothermal circulation in general (cool, relatively unaltered), these systems can have an important influence on global geochemical budgets for many solutes (e. g., chloride, magnesium, sulfate, potassium, lithium, boron, silica, phosphate, manganese, and iron) because the rate of fluid discharge is so large. Ridge flank fluids having the same composition of fluid exiting Dorado outcrop also may contribute to subseafloor microbial processes within basaltic basement and the overlying sediment, and suggest that oxidation reactions within basaltic crust can continue well beyond 10 Ma in some settings.