| Abstract |
A new exploration phase was carried out in the Lamentin plain (Martinique, French West Indies) in 2001. On the three bore-holes drilled by CFG in this area, only the bore-hole located more at north (Californie bore-hole) and near the old LA-101 bore-hole, indicated the presence of inflows of hot fluid close to 90?C starting from 400 m of depth. The main inflow was observed at about this depth. A similar geothermal fluid was also found at a depth of about 400 m in the CarrËre bore-hole, situated near thermal springs, but with a lower temperature (50?C). The geochemical monitoring carried out by BRGM made it possible to find some surface inflows of cold fluid with often a probable marine influence in the three bore-holes and to characterise the chemical and isotopic composition of the low temperature geothermal fluid. According to the geochemical data, this Na-Cl geothermal fluid (TDS ª 10-11 g/l; pH ª 6.2-6.3) is a mixing of 20-25% sea water and 80-75% fresh water, which could mainly come from the massif of the Pitons du Carbet, following a general structural NW-SE direction. This mixing has reacted with the surrounding volcanic rocks and with magmatic CO2 emanations (minimum CO2 partial pressure estimated to 1 bar). Relative to a diluted sea water, it is enriched in dissolved Ca, HCO3, SiO2, Sr, Ba, B, As, Fe, Mn, Li, Rb, Cs and Ge and depleted in SO4 and Mg. Additional chemical and isotopic analyses performed on the fluids of neighbouring thermal springs confirm these results and indicate that the geothermal fluid sampled in the bore-holes has been slightly diluted probably by the drilling fluid. When applied on all these fluids, numerous chemical geothermometers yield concordant values of deep temperature around 90-130?C, which are close to the measured temperatures and to those estimated using the gas geothermometers. The relatively high Mg and SO4 concentrations of these waters and their absence of a 18O enrichment from the rocks are rather in agreement with the moderate temperature estimated using the geothermometers. |