Beryllium 10 in hydrothermal vent deposits from the East Pacific Ridges: Role of sediments in the hydrothermal processes

Abstract

Beryllium 10 concentrations were determined for 31 handpicked hydrothermal sulfides, six oxyhydroxides, seven basalts, and five sediments collected from the hydrothermally active areas of the East Pacific ridges. The samples include specimens from the East Pacific Rise (EPR) at 21°N and 13°N, the Galapagos Rift, the Guaymas Basin, and the Gorda and the Juan de Fuca ridges. Additional samples from massive sulfides associated with the Oman ophiolites were studied. In all samples, we obtained values ranging from 0.04 × 106 atoms/g to 125 × 106 atoms/g, with the lowest values being very close to our blank (0.015 × 106 atoms/g). The data show systematic variations with sample location and type. The 10Be concentrations measured for the mid‐ocean basalts are of the order of 0.3 × 10 atoms/g and reach 3800 × 106 atoms/g for the pelagic deep‐sea sediments collected near the EPR 21°N. Based on their 10Be concentrations, we can clearly distinguish two categories of sulfides: sulfides containing low 10Be concentration (<106 atoms/g) sitting directly on the mid‐ocean basalt (EPR or Juan de Fuca), and sulfides with high 10Be concentration (>106 atoms/g) located atop of a thick pile of young sediments (Guaymas Basin or Gorda Ridge). The oxy‐hydroxides, collected on some of the Juan de Fuca chimneys, are always very rich in 10Be with concentrations reaching 125 × 106 atoms/g. We propose three mechanisms to explain these data: (1) mechanical entrapment of sediments in the hydrothermal chimneys as observed in the Gorda Ridge sulfides, (2) leaching of 10Be from the ocean crust sediments, transport by hydrothermal fluids, and coprecipitation with heavy metals as hypothesized to explain the Be concentrations observed in the Guaymas Basin, and (3) passive adsorption of 10Be from seawater, preferred hypothesis oxy‐hydroxides of the areas with no sediment cover.