Tracing fluid–rock reaction and hydrothermal circulation at the Saldanha hydrothermal field Á.S. Dias a, ⁎, R.A. Mills b , I. Ribeiro da Costa a , R. Costa a,c , R.N. Taylor b , M.J. Cooper b , F.J.A.S. Barriga a a CREMINER, Department of Geology, University of Lisbon, Ed. C6, Piso4, Campo Grande, 1749-016 Lisboa, Portugal b School of Ocean and Earth Science, National Oceanography Centre, Southampton, University of Southampton, SO14 3ZH, UK c EMEPC, R. Costa Pinto 165, 2770-070 Paço-de-Arcos, Portugal abstract article info Article history: Received 15 May 2009 Received in revised form 29 December 2009 Accepted 16 February 2010 Editor: B. Bourdon Keywords: Saldanha hydrothermal field Radiogenic isotopes Steatization The Saldanha hydrothermal field is positioned on the top of a seamount located in a non-transform offset (NTO5) on the Mid-Atlantic Ridge (MAR). This hydrothermal system was first described as a low-temperature diffuse field, driven by peridotite–seawater reactions following the detection of high concentrations of CH 4 and H 2 in the water column, and the occurrence of serpentinite outcrops in the vent area. We have studied the geochemistry and isotopic composition of sediment and rock samples collected across the area and show that hydrothermal circulation at Saldanha is complex and spatially variable, comprising areas of low-temperature diffuse flow but also more focused higher-temperature venting zones. While most sediment samples have an isotopic composition that is similar to normal pelagic sediments, one core (SCD7) show significant hydrothermal influence, sulphide mineralization, non-radiogenic Pb and radiogenic Nd isotope ratios and positive Eu anomalies. This is best explained by mineral precipitation from high-temperature hydrothermal fluids that have circulated through mafic rocks. The host rock lithology and alteration is also highly variable and comprises both fresh basalts, serpentinites and hydrothermally altered rocks (metabasalts, metagabbros and steatites). Serpentinites have REE patterns and ε Nd(0) values that fall between seawater and mantle peridotite reference values, resulting from extensive interaction of seawater with the original peridotite. This process was probably favoured by the deeply penetrating and long-lived faults occurring at this NTO. Steatites have a positive Eu anomaly and non- radiogenic Pb isotopic values. These signatures, together with the sulphide mineralisation and the extensive Si input necessary for steatization of serpentinites, imply that higher-temperature hydrothermal fluids reacted with gabbroic intrusions at depth. The more hydrothermally altered sediment and rock samples appear to be associated with the Saldanha fault network that promotes a more focused fluid flow and thus enhances hydrothermal alteration within a region of low-temperature diffuse flow. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Hydrothermal activity associated with ultramafic outcrops at mid- ocean ridges (MOR) is a relatively recent discovery. This type of activity seems to be confined to slow and ultraslow spreading ridges and the ultramafic outcrops are usually associated with detachment faulting, low magma budgets, relatively thin crust and irregular faulting patterns (Cannat et al., 1997; Gràcia et al., 2000; Mevel, 2003). The first evidence for this type of activity on the Mid-Atlantic Ridge (MAR) was the discovery of the Logatchev field (Krasnov et al., 1995) followed by the Rainbow field (Fouquet et al., 1997), both hosting high-temperature hydrothermal activity. Other ultramafic-hosted systems discovered later on the MAR include the low-temperature Saldanha (Barriga et al., 1998) and Lost City fields (Kelley et al., 2001). The detection of H 2 and CH 4 anomalies in the overlying water column, resulting from the serpentinization of abyssal peridotites (e.g. Charlou et al., 2002; Seyfried et al., 2007), suggests that these types of hydrothermal systems are common (Charlou et al., 1993; German et al., 1996; Charlou et al., 2002). The dynamics of the hydrothermal processes driving these systems is still poorly known and in particular their heat source has been the source of some debate. However in low-temperature ultramafic-hosted hydrothermal systems the heat source has been suggested to be derived from the exothermic reactions of serpentinization (e.g. Barriga et al., 1998; Kelley et al., 2001; Lowell and Rona, 2002; Schroeder et al., 2002), through heat balance models suggest that an additional heat source is necessary particularly for higher-temperature fields such as Rainbow and Logatchev (Lowell and Rona, 2002; Allen and Seyfried, 2004). The Saldanha hydrothermal field is hosted in ultramafic and mafic rocks and it is located on a 100 m high semi-circular NNE–SSW seamount (2200–2300 m deep) within a non-transform offset (NTO5), between the Famous and Amar segments (36° 34′N; 33° 26′W) on the Mid-Atlantic Ridge (MAR) (Fig. 1). This site was first visited by the Nautile submersible in 1998 (Barriga et al., 1998). The discovery of Chemical Geology 273 (2010) 168–179 ⁎ Corresponding author. Tel.: +351 217500000. E-mail address: agata.dias@fc.ul.pt (Á.S. Dias). 0009-2541/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2010.02.020 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo