Jean-Luc Michelot

Further investigations on optimized tail correction and high-precision measurement of uranium isotopic ratios using multi-collector ICP-MS

In the present paper, we further examine the optimum conditions for rapid, precise and accurate determination of 234U/238U ratios in geological materials by multiple collector (magnetic-sector) inductively coupled plasma mass spectrometry (MC-ICP-MS). In our experiments, isotopic measurements were performed on a Micromass IsoProbe™ instrument, using Faraday collectors in static mode only. Unlike the ion counting system coupled with an energy filter, this technique eliminates the difficulty of proper calibration of the Daly/Faraday gain ratio. On the other hand, since our Micromass instrument has a poor abundance sensitivity (the proportion of the 238U ion beam measured at mass 237 is ∼25 ppm), the major issue to be addressed is the tail correction. For this purpose, we have developed a tail correction method slightly modified from Thirlwall [J. Anal. At. Spectrom. 16 (2001) 1121]. This method is based on correction of the actual tail contribution under each peak, as assessed by the tail shape measurements on mono-isotopic ion beams, instead of the usual half-mass zeroes baseline estimation. Our approach enabled us to correct for the large offset that can be observed on isotopic data when tail correction is done by means of linear interpolation between half-mass zeroes, and showed that this latter tail correction method results in nearly 3% underestimation of 234U/238U ratios on the GEOTOP IsoProbe™ for material at secular equilibrium. A 236U–233U double spike was employed to correct for mass discrimination bias. Using an Aridus™ micro-concentric, desolvating nebulizer sample introducing system, a minimum of 200 ng of sample-U was consumed to carry out a precise 234U/238U analysis, thereby allowing a 234U signal of ∼4–5 mV to be monitored for 50 measurement cycles of 5 s each. This time-consuming, 10-min procedure allowed us to obtain an external reproducibility of 0.8‰ (2σ) for isotopic measurements of the NBL-112a standard solution. Replicate measurements of this reference material yielded a mean δ234U value of −36.42±0.80‰ (2σ, n=19), which is highly consistent with values reported by other laboratories. The total reproducibility, including both chemical separation and spectrometric measurement, was assessed using geological samples (a coral and a carbonate rock); the long-term reproducibility obtained was about 1.3‰ (2σ).

Aqueous sulphates from the Stripa groundwater system

Abstract The isotopic composition of aqueous sulphate in the groundwater in the Stripa pluton reflects the geochemical history of the groundwater as well as the origin of the sulphur or sulphate. In shallow, modern groundwater, sulphate originates from fallout and the oxidation of pyrite or other reduced forms of sulphur. At intermediate depths, sulphate of surface origin is lost through bacterial reduction. The origin of sulphate of the deep and more saline water is not clearly defined but could be derived from a sedimentary brine which was subject to substantial modification through rock-water interaction. Bacterial sulphate reduction also occurs locally at depths below about 800 m.

Isotopes and groundwater management strategies under semi-arid area: case of the Souss upstream basin (Morocco).

This study concerns the Souss upstream basin. The objective is to investigate the characteristics of surface water and groundwater, to assess the impact of artificial recharge as reinforcement of the natural replenishment and assess the renewal of groundwater under semi-arid area. Two major water types are observed: (i) surface waters and upstream springs (least mineralized) and (ii) all groundwater samples (prevailing calcium and magnesium bicarbonate water type). Water isotopes show a low evaporation of precipitations during infiltration. Impoverishment in heavy isotopes is the characteristic of mountain rainfalls, or of a climate colder and wetter than present. Carbon-14 activities (34-94 pmc) indicate a long residence time. The artificial recharge is low compared to the reservoir volume, due to which the renewal rate is also low.

Relation entre nappes superficielles et aquifère profond dans le bassin de Sfax (Tunisie)

Abstract The study of the isotopic composition ( 18 O and 2 H) of groundwater collected in the Sfax basin (Tunisia), helped to understand the behaviour of the different aquifers. It showed that the groundwater in the deep aquifer is old, probably slow moving and recharged under a colder climate than at present. The increasing exploitation of the shallow aquifers probably favoured upward leakage from the deep aquifer. Isotope balance equations allowed us to estimate the contribution of the deep aquifer to the shallow aquifer recharge.

A hydrochemical and isotopic study of thermal waters on lesbos island (Greece)

Ionic ratios (Br[sup [minus]]/Cl[sup [minus]], Na[sup +]/Cl[sup [minus]], SO[sup 2[minus]][sub 4]/Cl[sup [minus]]) of the samples taken from thermal springs sited at Argenos-Eftalou, Polichnitos-Lisvori, and near Mytilini, on Lesbos Island, are typical of marine solutions. Stable isotope contents of these waters show that they may result from mixing between meteoric and sea water. Virtually no Cl[sup [minus]] is supplied by rock leaching. Balance equations based on oxygen-18, deuterium, and Cl[sup [minus]] contents indicate that marine sources contribute approximately one-third of the water at Argenos, Polichnitos, and Lisvori, and 100% for one of the springs, Thermi, in the Mytilini area. Sulphur-34 contents of dissolved sulfates confirm the participation of marine solutions. Equilibration temperatures based on oxygen-18 contents of dissolved sulfate and water are similar to those given by the chemical geothermometers, except for Thermi spring. The validity of the [Delta][sup 18]O (SO[sup 2[minus]][sub 4] [minus]H[sub 2]O) temperature for the latter is thus questionable.

Fluid transfers at the basement/cover interface: Part I. Subsurface recycling of trace carbonate from granitoid basement rocks (France)

Basement rocks usually contain trace amounts of disseminated carbonate minerals and sometimes also fractures/veins filled with carbonates which may have a variety of origins. Extensive drilling has been performed of a granitoid body (Vienne, western part of the French Massif Central) overlain by a carbonate sedimentary sequence, a situation which is representative of most of the Variscan basement in western Europe. These unique samples provide the opportunity to investigate the behaviour of basement trace carbonate across the basin/basement interface and the mass transfers throughout a series of fluid migration ranging from high (ca. 450 °C) to low (down to 50 °C) temperature conditions. Study of quartz and carbonate found in fracture infillings reveals three major stages of fluid circulation, namely: (1) high to medium temperature late Hercynian fluids which circulated in fractures developed during basement uplift; (2) medium temperature Mesozoic basinal brines migration laterally along the basement/cover contact; (3) late infiltration of diagenetic/marine waters in the upper part of the granitoid body. A majority of analyzed vein carbonates from the three stages displayed a rather restricted range of C isotopic compositions (δ13C between −14‰ and −9‰/PDB) whatever the stage in which they were deposited and despite large variations of O isotopic ratios (δ18O in the range +3‰ to +30‰/SMOW). Carbon in pervasively altered rocks displays the same features and is distributed among the two broad groups defined using O isotopic ratios in veins and fluid inclusions data, namely the Hercynian and the Mesozoic carbonates. Isotopic data argue that carbon was introduced as carbonates in the early stages of the retrograde Hercynian metamorphism and was systematically reworked by subsequent fluids without any significant carbon introduction from external sources. The only exception corresponds to the stage III calcite veins which are located in the upper sections of the cores, at depths lower than 350 m beneath the basement/cover boundary and are the only witnesses of element transfer from the sediments towards the basement. The present data show that trapping of carbon by Ca-rich plutonic rocks during their cooling history confers on these rocks an efficient self-sealing capacity during later fracturing and fluid flow because carbonates possess high solubility and rapid dissolution/precipitation kinetics. Fluid circulation occurred not only in the macroscopic system of fractures but also within some volumes of the granitic “matrix” in order to scavenge and redistribute the early granitic trace carbonate. In the event of poly-cyclic carbonate redistribution, the use of carbon isotopic composition to unravel the fluid origins must be exercised with caution, as early C could be easily redistributed during later fluid migration episodes.

Hydrological implications of deep production of chlorine-36

Abstract Among the possible origins of 36 Cl in ground waters: (1) cosmic radiation in the atmosphere, (2) cosmic in the subsurface, (3) deep underground, (4) marine nuclear tests, the deep component is generally taken as negligible. This could be the case in rocks with low or medium uranium and thorium contents and low content in elements with high cross sections for neutron production. However, under favourable geological conditions (crystalline or metamorphic rocks, sedimentary rocks of detrital and endoreic origin), the deep neutron flux may be significant and thus give rise to an underground production of 36 Cl by reaction on 35 Cl atoms from ground water and possibly from fluid inclusions. This production is easily recognizable when the 36 Cl content per liter of old (tritium free) water is greater than the “modern” 36 Cl content of precipitation corrected for evapotranspiration. A clear example is the deep ground water system of the Stripa granite, in Sweden, where values as high as 7 × 10 8 atoms of 36 Cl per liter are observed. In this particular example, the initial content of the recharge may be neglected and a residence time of the chloride ions may be estimated if the neutron flux is known. Such favourable situations may be encountered when saline intrusions reach the ground water system in uranium and thorium rich rocks. An ambiguity may arise if the deep 36 Cl production corresponds to values of apparent decay of meteoric 36 Cl. This is illustrated by 36 Cl content of deep ground waters from the Sahara (aquifer of the Continental Intercalaire). Values of 6 and 20 × 10 8 atoms per liter may be interpreted in terms either of very high concentration of rain water by evatranspiration during recharge processes, or of deep 36 Cl production. A discussion based on additional paleohydrological information (stable isotopes, noble gas contents) suggests an intraformational generation of 36 Cl. However, in such a case, the differences in activity ratio 36 Cl/Cl tot and in absolute 36 Cl content (atoms per liter) might be interpreted in terms of heterogeneity in the recharge, circulation and mixing processes. No age estimation can be proposed. General limitations of ground water dating with 36 Cl are discussed.

A laboratory experiment for determining both the hydraulic and diffusive properties and the initial pore-water composition of an argillaceous rock sample: A test with the Opalinus clay (Mont Terri, Switzerland)

Argillaceous formations are thought to be suitable natural barriers to the release of radionuclides from a radioactive waste repository. However, the safety assessment of a waste repository hosted by an argillaceous rock requires knowledge of several properties of the host rock such as the hydraulic conductivity, diffusion properties and the pore water composition. This paper presents an experimental design that allows the determination of these three types of parameters on the same cylindrical rock sample. The reliability of this method was evaluated using a core sample from a well-investigated indurated argillaceous formation, the Opalinus Clay from the Mont Terri Underground Research Laboratory (URL) (Switzerland). In this test, deuterium- and oxygen-18-depleted water, bromide and caesium were injected as tracer pulses in a reservoir drilled in the centre of a cylindrical core sample. The evolution of these tracers was monitored by means of samplers included in a circulation circuit for a period of 204 days. Then, a hydraulic test (pulse-test type) was performed. Finally, the core sample was dismantled and analysed to determine tracer profiles. Diffusion parameters determined for the four tracers are consistent with those previously obtained from laboratory through-diffusion and in-situ diffusion experiments. The reconstructed initial pore-water composition (chloride and water stable-isotope concentrations) was also consistent with those previously reported. In addition, the hydraulic test led to an estimate of hydraulic conductivity in good agreement with that obtained from in-situ tests.

Origin and Recharge Altitude of the Thermo-Mineral Waters of the Eastern Pyrenees

Abstract The thermo-mineral waters of the axial zone of the Eastern Pyrenees form a geochemically homogeneous group. They emerge in granite or orthogneiss and all have a sodium sulphide chemistry. Principal component analysis of their physico-chemical parameters has distinguished three types of fluid, 1) hot water that has evolved in a closed system and whose chemistry may reflect that of deep water, 2) water that is also of unmixed origin, but whose chemical composition has been modified during cooling by conduction, and 3) water cooled by mixing with surface water. Stable isotope (18O, 2H) contents indicate that all the waters are of meteoric origin (from oceanic and/or Mediterranean precipitation). No heavy isotope enrichment has been found that would indicate evaporation or a geothermal effect between water and the host rock. The differences in isotope contents between surface and thermo-mineral waters are attributed to a difference in recharge altitude; altitude gradients in 18O and 2H, estimated by ...

Isotopic and hydrochemical approach to the functioning of an aquifer system in the region of Marrakech (Morocco).

A geochemical and isotopic (water and dissolved sulphate) study was performed on groundwater in the region of Marrakech, Morocco, with the aim of better understanding the regional hydrogeological system in order to improve water resources management. Significant differences in stable isotope contents and chemical compositions were observed between groundwater collected in the northern part of the region (Jbilets massif), where the basement schists outcrop, and that sampled in the southern part (Haouz basin), where the basement schists are overlaid by Plio-Quaternary deposits. The stable isotope composition of the groundwater showed that in the southern part the aquifer is mainly recharged from high-altitude precipitation over the High-Atlas Mountains, which may reach 600 mm per year or more, whereas, in the northern part, it is only recharged by lower-altitude local precipitation, which does not exceed 240 mm per year. Because of this limited supply, the groundwater flux in the northern compartment is much lower than in the southern compartment. This affects the water-rock interaction and the modalities of groundwater mineralization: the schist alteration is more developed in the southern compartment than in the northern one, leading to different behaviours of the conservative elements dissolved in groundwater. The observed geochemical and isotopic zonation of the studied area corresponds to a hydrogeological compartmentalisation, where the wells located in the north of the area generally produce less water than those located in the south of the area.