Seawater transpoprt and reaction in upper oceanic basaltic basement: chemical data from continuous monitoring of sealed boreholes in a ridge flank environment

摘要

Osmotically pumped fluid samplers were deployed in four deep-sea boreholes that were drilled during Ocean Drilling Program (ODP) Leg 168 on the eastern flank of the Juan de Fuca Ridge. Samplers were recovered from ODP Sites 1024 and 1027 and aliquots were analyzed for a variety of dissolved ions. Results from both of the samplers show a drastic change in the major ion composition within the first 20-40 days after the borehole was sealed at the seafloor followed by a more gradual change in composition. This gradual change ceased after 820 days at Site 1024 but continued throughout the 3-year depolymen at Site 1027. We modeled this change in composition to estimate the flux of formation fluid through the open borehole. The rapid early change requires a flow of ～1500 kg of formation fluid per day. The more gradual later change requires flow rates of 38 kg/day at Site 1024 and 17.5 kg/day at Site 1027. The latter fluxes require a minimum average specific discharge of meters to hundreds of meters per year through the surrounding basaltic matrix. Trace element data show surprisingly little contamination given the presence of steel casing, Li-organic-rich grease at each joint, cement, and drilling muds. Observed changes in trace element concentrations relative to those of bottom seawater provide a measure for the global significance of cool (23 ℃; ODP SIte 1024) ridge flank hydrothermal systems relative to warm (64 ℃; Baby Bare and ODP Site 1027) hydrothermal systems and illustrate the importance of these cooler systems to global geochemical budgets.