GOLDSCHMIDTCONFERENCE TOULOUSE 1998 Deep-sourced fluids and recharge mechanism with surface aquifers: the Trinidad mud volcanoes case A. N. Dia M. Castrec-Rouelle J. Boul~gue D. Bourl~s G~osciences Rennes - CNRS UPR 4661, B~t. 15 - Universit6 Rennes I, Campus Beaulieu, 35042 Rennes Cedex, France Laboratoire de G~ochimie et M~tallog6nie, Universit6 Pierre et Marie Curie, Tour 26 - 16 E5 - 4 Place Jnssieu, 75252 Paris Cedex 05, France CSNSM - Brit. 108, 91405 Orsay Campus, France Mud volcanoes have been widely described along convergent margins. Although their general surface characteristics are well documented, the nature of expelled fluids as well as the mechanism of advection of mud along fault are poorly constrained It has be established that: (1) mud volcanoes do contain mud and over- pressured multiphase pore fluids (water and dissolved gases, mainly methane), (2) the sediment is fluidized by rapid advection of pore fluids up through the sedimentary mass along a conduit fault, (3) mud volcanoes are located mostly above strike- slip faults, (4) when located in subduction zone, mud volcanoes genesis, location and temporal evolution seem closely linked to the volume and the rate of fluid circulation in the subduction zone. Even if chemical data are essential to determine the origin of the expelled fluids. Chemical and isotopic studies of these fluids are very few. Such information would also provide an insight into the deep circulation of fluids feeding mud reservoirs and their geodynamic context. In order to establish the origin of fluids expelled from mud volcanoes in Trinidad, we analysed their major-, trace-element and their isotopic composi- tions (H, O, Sr and Be). The mineralogical, chemical and Sr and Be isotope compositions of associated expelled muds were also determined. Geologic setting and sample location The island of Trinidad is situated on a seaward extension of the South American continent. It consists of two basins separated by the Central Range thrust complex. The oldest formations, low- grade metamorphosed phyllites, quartzites and recrystallized upper Jurassic limestones are located in the Northern Range. The Northern Basin is filled by a thick sequence of upper Miocene and younger age shallow-water sediments. Some samples were collected in the Southern basin filled by Tertiary deposits where clay and sand layers alternate. Older formations and the E-W trending Siparia- Ortoire syncline dip steeply into this basin whose margins are locally complicated by folding and faulting. South of Brighton structure, the Forest-Fyzabad anticline located within middle to lower Miocene sediments, is affected along its margin by a series of westward-pitching folds. These folds host significant oil deposits. The southern margin of Trinidad (Southern Range) is also folded into small, generally anticlinal structures that are separated by numerous faults. This zone contains most of the greatest number of active mud volcanoes found both in the past and during the present field campaign. These mud volcanoes are hosted by several different lower Miocene formations. A major right lateral wrench fault - the Los Bajos fault - which runs across the south-western part of the island in an east-south- easterly direction, is cutting and displacing all pre- existing structures in a right lateral sense. Data and conclusions (1) On the basis of their chemical compositions and their position relative to the Los Bajos fault, the mud volcanoes of Trinidad can be divided into two groups ; the NE and the SW group. The SW group is aligned along the southern anticline. (2) The mud volcano fluids result from a complex mixing between two fluids from two deep reservoirs with fluids from surface aquifers. (3) The major components of the fluids are Na +, C1- and HCO3. Na + and C1- concentrations depend on diagenetic processes and chemical interaction with silicates. 381