Clay-Mineral Signatures Of Fossil And Active Hydrothermal Circulations In The Geothermal System Of The Lamentin Plain, Martinique

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Journal Article: Clay-Mineral Signatures Of Fossil And Active Hydrothermal Circulations In The Geothermal System Of The Lamentin Plain, Martinique

Abstract
Whereas the study of clay minerals (using parameters such as structure modifications, crystal chemistry) has proved to be an effective approach in order to mark the circulation zones and fluid temperatures in high-temperature systems, there is an important lack of data dealing with such problems in moderate- to low-temperature systems. This paper proposes to fill this lack by studying the site of the Lamentin Plain (Martinique), characterized by a low-temperature geothermal activity (50°C<T<90°C). Preliminary data acquired on the three studied exploration wells (LA01, LA02 and LA03) show that the current low-temperature geothermal activity is particularly well expressed through the LA03 well. This low-temperature activity is superimposed on evidence of a high-temperature fossil activity, particularly well expressed in the non-productive levels of the LA01 and LA02 wells. After a petrographic characterization, this work focuses on the textural and microstructural properties of the low-temperature hydrothermal clay phases utilizing X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrometry and scanning electron microscope (SEM). The petrographic observations have permitted successive hydrothermal events to be recognized, which express the thermal collapse of the system, as shown by the superposition of ancient episodes, characterized by vein and pervasive alterations of high to moderate temperatures, and recent episodes, characterized by alterations associated with present-day circulations of low-temperature fluids. The fossil activity results from several hydrothermal episodes, with fluid temperatures ranging from 100 to over 200°C. Illitic phases (i.e. I/S (R=1) to illite) appear as the main markers of the fossil solution paths, due to their abundant development close to major veins and fractures, while chloritic phases (chlorite, corrensite) seem to mark a pervasive alteration, better preserved further from the fossil solution channels. On the other hand, the recent hydrothermal alteration is characterized by the massive presence of kaolinite, dioctahedral smectite and siderite. Whereas smectites are only observed abundantly farther from the current solution channels, kaolinite appears to be the dominant clay mineral in these channels, or in the surroundings of sealed fractures which do not currently drain any more hydrothermal fluids. The crystallinity estimations on XRD and FTIR patterns from the <2 μm fraction, along with SEM morphological observations, clearly show that the microstructural and textural properties of the kaolinites can be related to fluid temperatures. Indeed, kaolinites with the best crystallinity, or the best-ordered, are located in the fractures carrying the hottest fluid temperature (90°C). The comparison with the other flow paths, with fluids of lower temperature (50 and 70°C), shows that the kaolinite crystallinity decreases with temperature. As a consequence, while kaolinite appears to be the signatory clay mineral of the present-day fluid circulation in the Lamentin Plain, its crystallinity can be interpreted as a tracer of the fluid temperature.

Authors 
A. Mas, P. Patrier, D. Beaufort and A. Genter








Published Journal 
Journal of Volcanology and Geothermal Research, 2003





DOI 
Not Provided
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Citation

A. Mas,P. Patrier,D. Beaufort,A. Genter. 2003. Clay-Mineral Signatures Of Fossil And Active Hydrothermal Circulations In The Geothermal System Of The Lamentin Plain, Martinique. Journal of Volcanology and Geothermal Research. (!) .