Smectite Formation In Metalliferous Sediments At East Pacific Rise 13°N

Mineralogy and element geochemistry of a 43 cm long E271 sediment core collected from the East Pacific Rise?EPR?near 13°N were studied to better understand the differences in the orgin and element occurrence states between near-vent and distal metalliferous sediments in the same hydrothermal fields by comparison with the hydrothermal sediments from the EPR axis.The clay minerals?<2?m?in the metalliferous sediments were analyzed to investigate the smectite formation,and to discuss the origin of Si and the geochemical behavior of the Si isotope and REE during the authigenic process.Sediment samples collected from E271 site,along the ridge flank on west sides of the EPR are distal metalliferous sediments that result from hydrothermal non-buoyant plume fallout.However,samples?Site 17A-EPR-TVG1?from the slope of a hydrothermal mound on the EPR axis are near-vent metalliferous sediments that are formed by mass-wasting and erosion of sulfide edifices,and hydrothermal plume fallout of sulfides.Near-vent metalliferous sediments are richer in Fe,Cu,and Zn than distal metalliferous sediments.In terms of sequential leaching,although most of element occurrence states are basically similar in the two kinds of metalliferous sediments,Fe,Cu,Zn,Mo,Y,and REE are mainly present in the residual fraction of near-vent sediments,they mainly occur in the Fe-Mn oxide fraction of distal sediments.The shale-normalized REE distribution pattern of the sediments shows a peculiar positive Eu anomaly similar to those of hydrothermal fluids,suggesting a hydrothermal origin,but those of distal sediments show Ce depletion,characteristic of seawater.Due to the influence of seafloor hydrothermal activity and close to continent,the sources of clay minerals are complex.Illite,chlorite,and kaolinite are suggested to be transported from either North or Central America by rivers or winds,but smectite is authigenic.Data show that smectite is most likely formed by the reaction of hydrothermal Fe-oxyhydroxide with silica and seawater in metalliferous sediments.The Si O₂ that participates in this reaction may be derived from siliceous microfossils?diatoms or radiolarians?,hydrothermal fluids,or detrital mineral phases.And their?³⁰Si values are higher than those of authigenic smectites,which implies that a Si isotope fractionation occurs during the formation because of the selective absorption of light Si isotopes onto Fe-oxyhydroxides.Sm/Fe mass ratios?a proxy for overall REE/Fe ratio?in E271 clay minerals are lower than those in metalliferous sediments,as well as distal hydrothermal plume particles and the Fe-Mn oxide fraction of the E271 sediments.This result suggests that some REE are lost during the smectite formation,perhaps because their large ionic radii of REE scavenged by Fe-oxyhydroxides preclude substitution in either tetrahedral or octahedral lattice sites of this mineral structure.