Anthropogenic Pb component in hydrothermal ochres from Marsili Seamount (Tyrrhenian Sea)

Abstract

Recent hydrothermal Fe-oxyhydroxide precipitates (ochres) were sampled during two submersible expeditions to the NE part of the Marsili Seamount summit (SE Tyrrhenian Sea). The Marsili ochres belong to type 1 primary sulfide-poor Fe-oxyhydroxides according to the classification of Hekinian et al. [Hekinian, R., Hoffert, M., Larqué, P., Cheminée, J.L., Stoffers, P., Bideau, D., 1993. Hydrothermal Fe and Si oxyhydroxide deposits from south Pacific intraplate volcanoes and East Pacific Rise axial and off-axial regions. Econ. Geol. 88, 2099–2121]. They are X-ray amorphous, highly metalliferous [Al/(Al + Mn + Fe) ≪ 0.4], low in Si, transition metals (Co, Ni, Cu, Zn, Pb) and rare earth elements (REE) and enriched in B, V and Mo. Their chemical composition and REE distribution patterns indicate precipitation from low-temperature hydrothermal fluid mixed with seawater. Measured Pb isotope compositions in the ochres indicate that the Pb is dominantly anthropogenic in origin. By contrast, the Pb isotope ratios in the nearby higher temperature hydrothermal deposits from the Panarea arc volcano show that the Pb is derived mainly from local volcanic rocks. The fact that the Pb isotopic signature of the Marsili ochres is controlled by recent anthropogenic input suggests that the Fe-oxyhydroxides precipitated from fluids depleted in transition metals. The low 206Pb/ 207Pb ratios in the ochres reflect the contribution of Pb from Australian ores, indicating precipitation of the ochres somewhere between present-day and the beginning of Australian Pb import to Europe. The young age of the ochres and the presence of other hydrothermal deposits along the Marsili crest suggest that the magmatic chamber beneath the seamount may still be alive. This study represents the first observation of anthropogenic influence on submarine hydrothermal deposits.