Low-temperature fluid flow through sulfidic sediments from TAG: modification of fluid chemistry and alteration of mineral deposits

Low-temperature fluid flow has been identified as an important component of the heat, mass and chemical flux at sites of active venting. One way to study the movement of water through hydrothermal deposits is to analyse the pore-fluids extracted from sulfidic sediments. The composition of diffuse flow fluids in one sediment core from the south of the active TAG mound reflects the dilution of high-temperature, black smoker fluids, with modification inside the mound during ongoing circulation. Fluid modification includes conductive cooling, removal of Si from solution and entrainment and removal of U from seawater which can represent significant sinks of these elements. Precipitation of clay minerals and Fe-oxides is currently occurring within this sediment core generating ochreous type material subsurface. Extensive lowtemperature alteration of basaltic material at the core-top is apparent in conjunction with supergene enrichment of Cu and Mn. The heterogenous substrate and kinetic limitation of fluid-solid reactions will result in a complex mixture of sedimentary components and the conjugate pore-fluids. The core described here is from the active TAG mound, I I0m to the south of the active black smoker complex. The coring site is in an area of diffuse flow delineated by a sparse anemone population and surficial hydrothermal crusts just north of a steep southwards dipping sedimented escarpment. Temperatures as high as 46øC were measured within the sediment column indicating advection of warm water through the sediment [heat flow site 2598-5. Bec'ker et al., 1996]. Once on board, pore-fluids were extracted using a method adapted from Jahnke [!988]. All pore-fluid and solid phase analyses were carried out after returning to shore.