Significance of the depth-related transition montmorillonite-beidellite in the geothermal field of Bouillante (Guadeloupe, Lesser antilles)

Abstract : The crystal structure and crystal chemistry of dioctahedral smectites in high enthalpy geothermal systems was investigated through 2 wells drilled in the Bouillante geothermal area in order to understand the factors which control their vertical variation. Smectites were examined by optical and scanning electron microscopy, electron microprobe analysis, X-ray diffraction (after Ca, Li, alkylammonium saturations), Fourier transform infrared spectrometry (after Li, NH4 + saturations) and oxygen isotope analysis. Smectites predominate within the upper part of the drill holes (up to 260m depth - temperature range : 67-160°C). The XRD, FTIR and punctual chemical analysis clearly demonstrate a transition from montmorillonite to beidellite with increasing depth and temperature which proceeded through interstratification of beidellite-like and montmorillonite-like layers. Montmorillonite predominates at temperatures lower than 100°C while beidellite predominates between 110 and 170°C. However, this transition is not explained by a thermally controlled beidellitization process but seems related to the hydrothermal solutions from which precipitated these smectites. The δ18O values of the equilibrated fluids indicate that beidellitic smectite precipitated from the hot geothermal fluid associated with minor amounts of residual solutions resulting from local boiling and that montmorillonitic smectite precipitated from reacting solutions whose origin has to be searched in the phreatic water table associated with minor amounts of liquids resulting from the condensation of vapors escaped from the boiling zones. The mixing rate of Mg-free geothermal fluid with meteoric waters exerted a major influence on the montmorillonite versus beidellite ratio of the smectite material as underlined by the irregular depth related smectite transition