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

Abstract

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