Gil Michard

Sodium/lithium ratio in water applied to geothermometry of geothermal reservoirs

We propose here a new geothermometer for natural waters. Analyses from many explored geothermal fields allow us to define two empirical thermometric relationships. One is for waters of low to moderate salinity (Cl− 0·3 M) log NaLi = 1195T + 0·38 These relationships, which at present are not well understood, result mainly from the increase of Li concentrations in waters with temperature. Equation (a) proved to be adequate for spring waters from mostly known geologic origin; this is an important feature in geochemical surveys for geothermal prospecting. Furthermore, when comparison between springs and drillhole chemistry of a given geothermal field is possible, the Na/Li geothermometer gives more reliable temperature estimates from the spring compositions than do classical geothermometers.

Behaviour of major elements and some trace elements (Li, Rb, Cs, Sr, Fe, Mn, W, F) in deep hot waters from granitic areas

Abstract Analyses of selected hot springs in more than sixty areas in granites from Europe are presented. Concentrations of major elements and of some trace elements (Li, Rb, Cs, Sr, Fe, Mn, W, F) in high-pH waters are not significantly affected by processes during uprise from the deep reservoir to the surface (re-equilibrations, mixing with surficial waters). Normalized to K (or Na), concentrations of all other elements are related to the aquifer temperature. The knowledge of aquifer temperature and total electric charge of mobile elements (Cl, S, possibly F) allows us to predict both major elements (within ± 50%) and trace elements (within a factor ±2 or ±3) concentrations in alkaline waters from granites.

Geochemical study of a crater lake: Pavin Lake, France — Identification, location and quantification of the chemical reactions in the lake

Abstract A study of the main geochemical characteristics of the Pavin crater lake was performed in September 1992. Associated with previous results on the hydrochemical budget of the lake, the present study points out that: 1. (1) in the mixolimnion, the major reactions are photosynthesis, removal of silica by diatoms, iron oxidation and alkali and alkaline-earth incorporation in a solid phase; 2. (2) the major reduction reactions occur within the sediment and the products of the reaction are brought to the lake by diffusion; 3. (3) some reactions: silica dissolution, manganese oxide reduction, sulfate-sulfide transformations, iron and phosphate precipitation occur within the monimolimnion. 4. A complete budget of transfers and chemical reactions is presented for some components such as silica.

SUBMARINE THERMAL SPRINGS ASSOCIATED WITH YOUNG VOLCANOES : THE TEAHITIA VENTS, SOCIETY ISLANDS, PACIFIC OCEAN

Abstract Hydrothermal solutions were sampled close to two summits of Teahitia, a submarine volcano from the southern extension of the Society volcanic chain. The temperatures measured within two vents were 30°C. The vent solutions have a pH value around 5.3 and are enriched in Fe, Mn, Si, Ba, Li, and REEs relative to ambiant seawater but show only a small decrease in Mg and SO4 concentrations. The lowest measured 87Sr/86Sr is 0.70867, while the eNd is identical to those of the surrounding alkali basalt (+2.5). The vent solutions from Teahitia have chemical characteristics similar to Peles vents, Loihi Seamount, another mid-plate volcano. The exact temperature of seawater basalt interaction is difficult to constrain. However, the absence of positive Eu anomaly in the fluids, the particles, and in the hydrothermal deposits from Teahitia seems to exclude the participation of a high-temperature component comparable as those involved in mid-ocean ridge-axis vents.

A new optical sensor for PCO2 measurements in seawater

Abstract A knowledge of the distribution of the partial pressure of CO2 at the surface of the ocean is needed to quantify the absorption of atmospheric CO2 by gas exchange. Owing to the large space and time variability of this quantity at the surface of the ocean, an experimental approach aiming to make time-series measurements from unattended platforms must be envisaged. In this context we have developed a PCO2 sensor which meets the requirements for being installed on a buoy for approximately 1 year. The principle of the sensor is based on a colorimetric method. The selected dye is thymol blue. Its spectra and variation of its pK vs. temperature have been determined. A dedicated optical spectrophotometer has been built making absorbance measurements at three wavelengths. The sensor includes a sensitive volume enclosed with two silicone membranes and filled with thymol blue in an ionic medium. It is related to the spectrophotometer with silica fiber optics. Two calibration experiments were made over a fugacity of CO2 (fCO2) range extending from 340 μatm to 590 μatm. They lasted 3 and 4 days, respectively and showed that in the range 340–600 μatm a variation of ± 1 μatm is seen by the sensor and that the standard error of the fit of the two calibration curves is, respectively, ± 3 μatm and ±5 μatm.

Modelling of the chemical composition of alkaline hot waters

Abstract The computer program MEQTP, which calculates the composition of a geothermal water and follows the evolution of the composition during temperature and pressure changes, has been applied to some springs from the south of France. This represents a first attempt at taking pressure changes into account in calculating pressure as well as temperature of the deep reservoirs supplying the thermal springs. The calculated temperature corresponds well with the values obtained by means of classical geothermometers and from previous calculations, in which pressure variations are neglected. Calculation of pressure requires a better understanding of the mechanism (read the mineral) controlling the Mg 2+ concentration in the geothermal waters.

Aluminum hydroxide solubility in aqueous solutions containing fluoride ions at 50°C

Aluminum hydroxide solubility experiments were performed at 50°C using the method of May et al. (1979b). The shift observed by May at 25°C between acidic and alkaline solution was also observed in our experiments. Infrared spectra of solids separated from solutions show that the low solubility values observed in alkaline medium can be related to the formation of a new solid phase: bayerite (or nordstrandite) (pK∗s4 = 13.4 at 50°C) in the presence of initial gibbsite. A second set of experiments was performed in solutions containing fluoride ions in the 10−4−10−3 M concentration range. Fluorocomplexes are important in acidic solution; Al(OH)mFp mixed complexes can be important in the minimum of Al solubility when total fluoride exceeds 10−3 M. We propose values of thermodynamic data for Al(OH)2F2−, Al(OH)F3−, Al(OH)2F0dg and for aluminum trifluoride solid. The difference of aluminum hydroxide solubility between acidic and alkaline medium is still observed.

Etude géochimique de la nappe du Dogger du Bassin Parisien

Abstract Waters from the Dogger nappe of the Paris basin, which are utilized for heating, have a chlorinity in the range 0.07–0.6 mole/litre and a temperature in the range 48–80 °C. We present here a geochemical study of waters sampled in about 25 boreholes. Except chloride and sulfates, elements seem to be controlled by equilibrium with minerals commonly encountered in hydrothermal systems: calcite, disordered dolomite chalcedony, Ca-montmorillonite, disordered kaolinite, albite and adularia. A chalcedony geothermometer indicates temperatures close to the observed temperatures, whereas the Na/K geothermometer gives a mean value close to the mean observed temperature, but Na/K variations do not follow local variations of temperature. The Cl/Br ratio (420) suggests that the chlorinity of Dogger water is related to sea water. Dogger water can be the result of interaction between a mixture of evolved sea water and fresh water and minerals of the reservoir.

Field titrations of dissolved sulfur species in anoxic environments — Geochemistry of Puzzichello waters (Corsica, France)☆

Abstract The use of the Ag/1bAg 2 S membrane electrode in combination with Hg 2+ titrations under a N 2 atmosphere enables a rapid and accurate determination of numerous dissolved S species: total sulfide, polysulfide S, thiosulfate and sulfite. These species have been measured in the waters of Puzzichello (Corsica, France), which are characterized by atmospheric oxidation of an initially sulfide-rich water. In the early stage of the oxidation process mainly polysulfide and thiosulfate are produced; in the final stage, sulfate is the main product. The concentrations of Fe and Cu are influenced by the oxidation process.

Sedimentation rate and early diagenesis of particulate organic nitrogen and carbon in Aydat Lake (Puy de Dôme, France)

Abstract A core of low-compacted sediment from a eutrophic lake (Aydat Lake) was extracted in order to study the early diagenesis of particulate organic carbon and nitrogen. We use a simple model where the advection rate was estimated with the classical 210 Pb method. We show that the 210 Pb data are not sufficient by themselves to provide a correct estimation of this parameter, but a model which includes the sediment compaction leads to a better understanding of the sedimentation process. Compaction is also taken into account in the diagenetic model which allows us to make an estimation of the carbon and nitrogen mineralization rates.