Geochemistry of a sediment push-core from the Lucky Strike hydrothermal field, Mid-Atlantic Ridge

摘要

Hydrothermal sediment mineralogy and geochemistry can provide insights into seafloor mineralization processes and changes through time. We report a geochemical investigation of a short (22 cm) near-vent hydrothermal metalliferous sediment core from the Lucky Strike site (LS), on the Mid-Atlantic Ridge (MAR). The sediment was collected from the base of an active white smoker vent and comprises pure hydrothermal precipitates, mainly chalcopyrite, sphalerite, pyrite and barite, with negligible detrital and biogenic inputs. Geochemically, the core is enriched in elements derived from high-temperature hydrothermalism (Fe, Cu, Zn and Ba) and depleted in elements derived from low-temperature hydrothermalism (Mn), and metasomatism (Mg). The U/Fe content ratio is elevated, particularly in the deeper parts of the core, consistent with uptake from seawater associated with sulphide alteration. Rare earth elements (REE) concentrations are low and chondrite-normalized patterns are characteristic of high-temperature vent fluids with an enrichment in light REE and a pronounced positive Eu anomaly. A stronger positive Eu anomaly associated with higher La-n/Sm-n at the core top is controlled by barite precipitation. The hydrothermal influence on the REE decreases downcore with some evidence for a stronger seawater influence at depth. Nd isotopes also exhibit an increased detrital/seawater influence downcore. Pb isotope ratios are uniform and plot on the Northern Hemisphere Reference Line in a small domain defined by LS basalts and exhibit no detrital or seawater influence. Lucky Strike sediments are derived from high-temperature mineralization and are overprinted by a weak seawater-sediment interaction when compared with other Atlantic hydrothermal sites such as TAG. The larger seawater input and/or a larger detrital contribution in deeper layers can be explained by variable hydrothermal activity during sediment formation, suggesting different pulses in activity of the LS hydrothermal system.