Mascle, J., Lohmann, G.P., and Moullade, M. (Eds.), 1998 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 159 49 6. PALEOFLUIDS FROM THE MARGINAL RIDGE OF THE CÔTE D’IVOIRE-GHANA TRANSFORM MARGIN (HOLE 960A) AS THERMAL INDICATORS 1 Marc Lespinasse 2 , Jacques L. Leroy 2 , Jacques Pironon 3 , and Marie-Christine Boiron 3 ABSTRACT Fluid inclusions linked to two quartz veins from the bottom of Hole 960A were studied using microthermometry and Raman microprobe analysis. Results suggest that circulation of hot fluid (160°–170°C) under low pressure (10–20 MPa) occurred within the marginal ridge of the Ivory Coast-Ghana Transform Margin. The fluid composition varies with time, and is either seawater or a mixture of H 2 O-CH 4 -CO 2 fluids. Possible sources of heat for these fluids include circulation in friction zones linked to transform-fault dynamics, either at depth in a thinned continental crust or in the hot oceanic crust south of the sampled area. The passage of the Mid-Atlantic Ridge axis to the south of the sampled zone occurred later than the vein forma- tion and therefore cannot be considered a heat source. INTRODUCTION The inclusions of fluids in minerals are witnesses of fluid circula- tion trapped during (primary inclusions) or after (secondary fluid in- clusions) the crystallization of the host minerals. Their analysis al- lows reconstruction of the characteristics and the history of these presently inactive thermal systems. In the case of a marginal ridge ad- jacent to a margin transform fault, different heat sources are possible and have been proposed. These include sedimentary and/or tectonic burials and thermal flow induced by the friction along the fault. In the case of the Ivory Coast-Ghana Transform Margin, other sources might have existed, especially in relation to the presence and dis- placement of an oceanic lithosphere with a mid-oceanic ridge in con- tact with the sampled zone (Mascle and Blarez, 1987; Basile et al., 1993). The characteristics of the organic matter recovered during Ocean Drilling Program (ODP) Leg 159, the thermal maturation of the clay minerals in these sediments (Shipboard Scientific Party, 1996), and the alteration of the microfauna caused by the temperature increase at the bottom of Hole 960A (Masure et al., Chap. 24, this volume) also suggest the existence of such a thermal anomaly. Fluid inclusion studies (microthermometric and Raman analyses) were therefore undertaken to determine the pressure-volume-temper- ature-composition (PVTX) characteristics of the fluids present in the marginal ridge and to retrace a part of the thermal history of the con- stituent sediments. SAMPLES Only two small quartz veins, sampled at the bottom of Hole 960A could be studied. The first vein, ODP1, is located 442.1 meters below seafloor (mbsf; Sample 159-960A-60R-1, 17–21 cm) and the second, ODP2, is located 446.6 mbsf (Sample 159-960A-61R-1, 41–44 cm), in a silty sandstone attributed to the Albian (Fig. 1). Their orienta- tions are unknown. Calcite veins from Holes 962B and 962D were not studied because the fluid inclusions were too small (<5 μm). The ODP1 vein, which is a subvertical milky quartz vein with mi- croscopic crystals, was brecciated and cemented by secondary quartz. This corresponds to a rapid filling and brittle tectonic activity (syn- or post-filling). The vein borders are rectilinear. The ODP2 vein contains large crystals of an extremely limpid quartz, which indicates a slower crystallization rate. Borders are also rectilinear. The ODP2 sample is not syn- or post-deformed and brecciated. Because of the proximity of these two samples (4.5 m apart), one can conclude that the ODP1 vein was filled before the ODP2 vein. ANALYTICAL METHODS Microthermometric studies of fluid inclusions were performed on wafers (<300 μm in thickness) using a Chaixmeca heating-freezing stage (Poty et al., 1976). The stage was calibrated using melting- 1 Mascle, J., Lohmann, G.P., and Moullade, M. (Eds.), 1998. Proc. ODP, Sci. Results, 159: College Station, TX (Ocean Drilling Program). 2 Université Henri Poincaré Nancy 1, Laboratoire Etude des Systèmes Hydrother- maux, BP 239, 54506 Vandoeuvre-lès-Nancy cedex, France. lespinas@lpesh.u-nancy.fr 3 CREGU and GDR CNRS-CREGU 077, BP 23, 54501 Vandoeuvre-lès-Nancy cedex, France. Pliocene Pleistocene Miocene Eocene Turonian Maastrichtian Aptian - Albian Clay Claystone Glauconitic claystone Sandstone Grainstone Silty sandstone 450 300 200 100 0 ODP1 ODP2 Hole 960A Depth (mbsf) Lithology Figure 1. Simplified stratigraphic column of Hole 960A and locations of the samples ODP1 and ODP2.