Chemical characteristics of modern deep-sea metalliferous sediments in closed versus open basins, with emphasis on rare-earth elements and Nd isotopes

Metalliferous sediments deposited on and near spreading ridges show contrasting geochemical signatures depending whether deposition occurred in a restricted basin filled with anoxic dense brines, or an open ocean characterized by oxidized well-circulated seawater. of the Atlantis II Deep, which are precipitated from ca. 60 °C display wide range mineralogical chemical facies. The most abundant facies enriched Fe-oxyhydroxides hydrous Fe-silicates metal sulphides; Mn-oxides, carbonates anhydrite locally important. Terrigenous biogenic components minor. Base precious metals notably sulphide-rich By contrast, precipitates open-ocean settings consist Fe-Mn-oxyhydroxides that were dilute hydrothermal plumes, commonly admixed carbonate-rich ooze. On carbonate-free basis, metalliferous much less enrichment trace relative to Deep but have higher contents rare-earth elements (REEs). shale-normalized REE patterns positive Eu anomalies, lack Ce anomalies. Due low content precipitates, even minor aluminosilicate detritus (10%) can affect patterns. ‘baseline’ pattern is determined mainly component sediment, anomaly resulting component. Nd isotopic variations be explained largely mixing provided lesser dissolved derived basaltic-hydrothermal source. These two sources also account for Pb reported previous studies. majority Sr evaporites flank brine deep, smaller contribution underlying basalts. In settings, plume particulates formed above high-temperature vents acquire seawater-type isotope ratios soon after discharge; these features maintained as drifts hundreds few thousand kilometres ridge axis. Departures seawater occur if detrital material (basaltic terrigenous) present sediment. Sr-isotope are, like Nd, dominated Sr. For Pb, basaltic isotopically identified up 1000 km Where low-temperature fluids discharge through flanks, at Galapagos mounds, Fe-rich smectites (nontronite) formed, together Mn-oxides. nontronites very REEs, patterns, although anomalies negative than those deep Pacific Nontronites active intra-plate seamounts mixtures fluid normal basins, Nd-Sr-isotope generally reflect ambient closed (or restricted) basins subject input, Nd-Sr-Pb differ considerably bear interpretation data ancient exhalative deposits including iron formations.