Seawater-metasomatism in hydrothermal sediments (Escanaba Trough, northeast Pacific)

Abstract

Hydrothermal sediments from the NESCA vent-field, Escanaba Trough, are enriched in a number of typical “vent-fluid” metals for which fluid data for the same, sediment-hosted, site exhibit anomalous depletion. These results are consistent with cooling of rising fluids and precipitation of various metal sulphide, sulphate and oxide phases in situ within the sediment column, prior to expulsion of the fluids from the seabed. Significant enrichments of Mg and U, elements which are quantitatively removed from high-temperature vent-fluids, are also observed in the Escanaba Trough sediments, however. Enrichments of these elements cannot derive from simple conductive cooling of the rising vent-fluids but, instead, are believed to result from extensive seawater-metasomatism as relatively fresh unreached seawater is heated from below by hot sediments and/or percolating vent-fluids in the upper levels of the circulation system. Calculated seawater/sediment interaction ratios range from ∼5:1 to 2500 l of seawater per gram of sediment. Pb, Sr and Nd isotope distributions for the Escanaba sediments are consistent with combined sediment-fluid and seawater-sediment interactions. Pb isotope ratios for hydrothermal and background sediments coincide with local sulphide deposits but are much more radiogenic than local MORB. This indicates not only that complete isotopic re-equilibration must occur between the hot, rising fluids and the host sediments but, in addition, that sediment composition appears to dominate the Pb isotopic composition of these fluids. Sr and Nd isotope ratios in the hydrothermally altered sediments, which are distinct from background ratios, coincide closely with modern seawater values, providing confirmatory evidence that extensive interactions must also have occurred between these hydrothermal sediments and relatively fresh, unreacted seawater.