When mud volcanoes sleep: Insight from seep geochemistry at the Dashgil mud volcano, Azerbaijan A. Mazzini a, * , H. Svensen a , S. Planke a, b , I. Guliyev c , G.G. Akhmanov d , T. Fallik e , D. Banks f a Physics of Geological Processes, University of Oslo, Box 1048, 0364 Oslo, Norway b Volcanic Basin Petroleum Research, Oslo Research Park, 0349 Oslo, Norway c Geology Institute Azerbaijan, Husein Avenue 29A, Baku, Azerbaijan d Moscow State University, Faculty of Geology, Vorobjevy Gory, Moscow 119992, Russia e Scottish Universities Environmental Research Centre, Rankine Avenue, East Kilbride, Glasgow G75 0QF, Scotland, UK f School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK article info Article history: Received 28 April 2008 Received in revised form 2 November 2008 Accepted 3 November 2008 Available online xxx Keywords: Dashgil mud volcano Azerbaijan Dormant Methane Water geochemistry abstract The worlds >1500 mud volcanoes are normally in a dormant stage due to the short duration of eruptions. Their dormant stage activity is often characterized by vigorous seepage of water, gas, and petroleum. However, the source of the fluids and the fluid–rock interactions within the mud volcano conduit remain poorly understood. In order to investigate this type of activity, we have combined satellite images with fieldwork and extensive sampling of water and gas at seeping gryphons, pools and salsa lakes at the Dashgil mud volcano in Azerbaijan. We find that caldera collapse faults and E–W oriented faults determine the location of the seeps. The seeping gas is dominated by methane (94.9–99.6%), with a d 13 C (& V-PDB) in the 43.9 to 40.4& range, consistent throughout the 12 analysed seeps. Ethane and carbon dioxide occur in minor amounts. Seventeen samples of seeping water show a wide range in solute content and isotopic composition. Pools and salsa lakes have the highest salinities (up to 101,043 ppm Cl) and the lowest d 18 O (& V-SMOW) values (1–4&). The mud-rich gryphons have low salinities (<18,000 ppm Cl) and are enriched in 18 O(d 18 O ¼ 4–6&). The gas geochemistry suggests that the gases migrate to the surface from continuously leaking deep- seated reservoirs underneath the mud volcano, with minimal oxidation during migration. However, variations in gas wetness can be ascribed to molecular fractionation during the gas rise. In contrast, the water shows seasonal variations in isotopic composition and surface evaporation is proposed as a mechanism to explain high water salinities in salsa lakes. By contrast, gryphons have geochemical signals suggesting a deep-seated water source. These results demonstrate that the plumbing system of dormant mud volcanoes is continuously recharged from deeper sedimentary reservoirs and that a branched system of conduits exists in the shallow subsurface. While the gas composition is consistently similar throughout the crater, the large assortment of water present reflects the type of seep (i.e. gryphons versus pools and salsa lakes) and their location within the volcano. Our data highlight the importance of a carefully planned sampling strategy when the target is water geochemistry, whereas the methane content and isotopic composition is relatively independent of the particular seep morphology. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Piercement structures, such as mud volcanoes (MV), are common in many sedimentary basins with compressional settings, like in the South Caspian Basin, Trinidad, Mediterranean ridge and Indonesia (e.g. Jakubov et al., 1971; Barber et al., 1986; Cita et al., 1996; Dia et al., 1999; Isaksen et al., 2007). The activity and behavior of mud volcanoes can be essentially classified as: eruptive (the periodicity and cyclicity of these violent events depend on the overpressure generated at depth), dormant/sleeping (interval in between eruptions that is characterized by no seepage, or microseepage or focused seepage of fluids and sediments), and extinct (no signs of erupted fluids or solid is documented in historic time). Note that we do not consider isolated gryphons as mud volcanoes unless there is a docu- mentation of their association with eruptions. Mud volcano eruptions are mainly driven by release of thermogenic methane generated at depths often greater than 10 km. The eruptions may be triggered by seismic activity and associated pressure waves or fracturing. This * Corresponding author. E-mail address: adriano.mazzini@fys.uio.no (A. Mazzini). Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo ARTICLE IN PRESS 0264-8172/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2008.11.003 Marine and Petroleum Geology xxx (2008) 1–13 Please cite this article in press as: Mazzini, A., et al., When mud volcanoes sleep: Insight from seep geochemistry at the Dashgil mud volcano, Azerbaijan, Marine and Petroleum Geology (2008), doi:10.1016/j.marpetgeo.2008.11.003