Research papers Fe–Si-oxyhydroxide deposits at a slow-spreading centre with thickened oceanic crust: The Lilliput hydrothermal field (9°33′S, Mid-Atlantic Ridge) Vesselin M. Dekov a, ⁎, Sven Petersen b , C.-Dieter Garbe-Schönberg c , George D. Kamenov d , Mirjam Perner e , Ernő Kuzmann f , Mark Schmidt b a Department of Geology and Paleontology, University of Sofia, 15 Tsar Osvoboditel Blvd., 1000 Sofia, Bulgaria b Leibniz-Institut fuer Meereswissenschaften, IFM-GEOMAR, Wischhofstr. 1-3, D-24148 Kiel, Germany c Institut fuer Geowissenschaften, Abt. Geologie, Universitaet Kiel, Olshausenstr. 40, D-24118 Kiel, Germany d Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611, USA e Microbiology and Biotechnology Unit, University of Hamburg, Ohnhorststr. 18, D-22609 Hamburg, Germany f Laboratory of Nuclear Chemistry, Hungarian Academy of Sciences Chemical Research Center, Eötvös University, 1/A Pázmány P., Budapest H-1117, Hungary abstract article info Article history: Received 15 May 2010 Received in revised form 17 September 2010 Accepted 20 September 2010 Editor: J. Fein Keywords: Fe–Si-oxyhydroxides Geochemistry Hydrothermal Lilliput vent field Mid-Atlantic Ridge Diffuse and focused low-temperature fluids emanate at 9°33′S (Mid-Atlantic Ridge) and precipitate Fe–Si- oxyhydroxides that form chimneys, mounds and flat-lying deposits. This extensive vent field, named Lilliput, lies at the axial zone of a spreading segment with a significantly thickened crust (~ 11 km). Theoretically much more heat needs to be removed from a thick-crust spreading center compared to a spreading center with typical thickness of ~ 6 km. Therefore, settings with thickened crust should be favourable for supporting very powerful hydrothermal systems capable of producing large mineral deposits. This is the first report on the composition of seafloor hydrothermal deposits at abnormally thickened oceanic crust due to hotspot–ridge interaction. Our studies revealed that generally the Lilliput hydrothermal deposits are very similar in morphology, structure, composition and lateral extent to other low-temperature hydrothermal deposits of mid-ocean ridges and intraplate volcanoes. Deposits at the Lilliput vent field are composed of Si-containing goethite and ferrihydrite, have very low contents of a number of transition and rare earth elements and show REE distribution patterns with negative Ce and Eu anomalies. The speciation and precipitation of the main deposit-forming elements, Fe and Si, at the hydrothermal field appear to be partially controlled by live microbes and exuded organic compounds. The δ 18 O values of the precipitated silica-containing Fe- oxyhydroxides point to low-temperature formation and Sr–Nd–Pb–isotope variations suggest that the hydrothermal precipitates scavenged metals predominantly from the ambient seawater. These findings are in agreement with the biogeochemical scenario for their precipitation. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Seafloor hydrothermal activity at mid-ocean ridges (MOR), intra- oceanic arcs and hotspots is one of the fundamental processes controlling the transfer of heat and chemical species between the lithosphere and the ocean (Elderfield and Schultz, 1996; German and Von Damm, 2004). Seafloor hydrothermal systems are also of biological importance because they support unique and rich ecosystems. The biology in turn can affect precipitation of minerals. The MOR chain transects all ocean basins on Earth, and the diverse geological conditions of submarine tectonic extension generate a great variety of hydrother- mal vents and deposits. Since their discovery in 1977 (Corliss et al., 1979; Spiess et al., 1980), scientists have investigated more than 200 seafloor vent fields (Hannington et al., 2005). However, the search for hydrothermal sites has largely been constrained to the northern Atlantic and Pacific Oceans. Vast segments of the MOR in the South Atlantic, Indian, and Arctic Oceans remain virtually unexplored for hydrothermal activity. These underexplored segments of the global MOR system largely correspond to spreading centers where the tectonic extension rates are very slow, and where lithospheric extension may be accommodated by stable fault slip as opposed to volcanic accretion (e.g., Dick et al., 2003). Based on seismic investigations some of these slow-spreading centers are known to have thickened crust (~10 km), as a result of high magma production due to hotspot–ridge interaction. Therefore, this type of setting may be expected to support very active and high-power hydrothermal systems, which can generate large mineral deposits as theoretically, much more heat has to be transported away from a thick-crust spreading center than from a spreading center with a typical thickness of ~6 km. On the other hand, a crustal thermal model (Chen, 2003) suggests that the thicker, hotter, more ductile crust associated with hotspot-affected spreading centers has a substantially reduced depth of brittle fracturing and consequent depth of seawater Chemical Geology 278 (2010) 186–200 ⁎ Corresponding author. Tel.: +359 2 9308 276; fax: +359 2 9446 487. E-mail address: dekov@gea.uni-sofia.bg (V.M. Dekov). 0009-2541/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2010.09.012 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo