Sources of mud volcano fluids in the Gulf of Cadiz—indications for hydrothermal imprint Christian Hensen a, * , Marianne Nuzzo b , Edward Hornibrook b , Luis M. Pinheiro c,d , Barbara Bock a , Vitor H. Magalha ˜es c,d , Warner Bru ¨ ckmann a a Leibniz-Institute of Marine Sciences, IFM-GEOMAR, Wischhofstr. 1–3, D-24148 Kiel, Germany b Bristol Biogeochemistry Research Centre, Department of Earth Sciences, University of Bristol, UK c CESAM and Geosciences Department, Universidade de Aveiro, Aveiro, Portugal d Marine Geology Department, INETI, Alfragide, Portugal Received 16 February 2006; accepted in revised form 20 November 2006 Abstract Mud volcanism in the Gulf of Cadiz occurs over a large area extending from the shelf to more than 3500 m water depth and is trig- gered by compressional stress along the European–African plate boundary, affecting a deeply faulted sedimentary sequence of locally more than 5 km thickness. The investigation of six active sites shows that mud volcano (MV) fluids, on average, are highly enriched in CH 4 , Li, B, and Sr and depleted in Mg, K, and Br. The purity of the fluids is largely controlled by the intensity of upward directed flow. Flow rates could be constrained by numerical modelling and vary between <0.05 and 15 cm yr 1 . Application of dD–d 18 O system- atics identifies clay mineral dehydration, most likely within Mesozoic and Tertiary shales and marls, as the major source of fluids. Hence, Cl and Na in the pore fluids are mostly depleted below seawater values, following a general trend of dilution. However, deviations from this trend occur and are likely caused by the dissolution of halite in evaporitic deposits. Other secondary processes overprinting the ori- ginal fluid composition may occur along the flow path, such as dissolution of anhydrite or gypsum and/or the formation of calcite and dolomite. Different sources of fluids are also indicated by variations in 87 Sr/ 86 Sr, which range from 0.7086 to 0.7099 at the different sites. Dehydration may be induced primarily by overburden and tectonic compression; however, very high concentrations of Li and B, spe- cifically at Captain Arutyunov MV (CAMV) indicate additional leaching at temperatures above 150 °C, which could be explained by the injection of hot fluids along deep penetrating, major E–W strike–slip fault systems. This hypothesis is supported by the occurrence of generally thermogenic, but significantly CH 4 -enriched, light volatile hydrocarbon gases at CAMV which cannot be explained by shallow microbial methanogenesis. Li and Li/B ratios from different types of hot and cold vents are used to infer that high temperature signals seem to be preserved at various cold vent locations and indicate a closer coupling of both systems in continental margin environments than outlined in previous studies. Ó 2006 Elsevier Inc. All rights reserved. 1. Introduction Mud volcanism provides an important, but poorly con- strained transport pathway for fluids along continental margins. Several thousands of mud volcanoes (MVs) may occur in deep-water areas along continental margins (Milkov, 2000; Milkov et al., 2003) so that the expulsion of sedimentary fluids at continental margins may play a sig- nificant role for global fluid and element budgets (Aloisi et al., 2004a). Fluid expulsion is generally coupled to tec- tonic activity controlling episodic fault displacements (Moore and Vrolijk, 1992). Mud volcanoes and other types of cold vent systems are offering a window into otherwise obscured deep structural and diagenetic processes as geo- chemical interactions of the rising fluid with surrounding sediment and rock may significantly alter the chemistry of the pore fluids. Such diagenetic processes may involve 0016-7037/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.gca.2006.11.022 * Corresponding author. Fax: +49 431 6002928. E-mail address: chensen@ifm-geomar.de (C. Hensen). www.elsevier.com/locate/gca Geochimica et Cosmochimica Acta 71 (2007) 1232–1248