Numerous geochemical tools, which are based upon the acquisition of chemical 
and isotopic data obtained from thermal waters, fumaroles or escapes of gases 
collected in surface conditions, or in fluids sampled from deep wells, are 
commonly used to study and better understand the high temperature 
geothermal reservoirs. If it is possible to characterize the fluids of the 
geothermal reservoirs and their deep circulation, estimate their residence time 
and the reservoir temperature (using chemical, isotopic or gas 
geothermometers, for example), no present geochemical tool is available to 
determine the rock nature of these reservoirs and their location without no 
sampling and analysis of reservoir rocks nor direct access by drilling. This study 
shows that the lithium isotope measurements performed in hot waters collected 
from thermal springs (Diamant area, Martinique) and deep production wells 
(Bouillante area) can be very useful to determine these parameters when the 
temperatures of the geothermal reservoirs are known. In the Diamant area, the 
reservoir temperature was estimated to be close to 180-200°C using most of 
the chemical and isotopic geothermometers. In the Bouillante area, it was 
measured at the bottom of the wells (250-260°C) and confirmed by the 
chemical and gas geothermometers. For each area, the lithium isotopic 
signature of the reservoir rocks was estimated using the lithium isotope 
signature of the hot waters and the temperature dependant isotopic 
fractionation equation experimentally determined for basalt-seawater 
interactions. This value close to 7Li= -2.5‰ ± 0.5 suggests that the 
geothermal reservoirs in the Diamant and Bouillante areas are located in the 
transition zone, a zone which marks the contact between volcanic flows and the 
basaltic oceanic crust and is the center of intense fluid mixing and circulation.

Campus SIAF, SP del Monte Volterrano
Localita' Il Cipresso
Volterra, Italy