Comparison of UV and IR Laser Ablation ICP-MS on Silicate Reference Materials and Implementation of Normalisation Factors for Quantitative Measurements

Direct analysis of geological reference materials was performed by LA-ICP-MS using two Nd:YAG laser systems operating at 266 nm and 1064 nm. The aim of this work was to compare UV and IR laser ablation and to assess the potential of the technique for the quantitative bulk analysis of rocks, sediments and soils. The laser sampling process was investigated and the analytical performance of both systems was compared. The influence of the laser operating conditions and the nature of the matrix on ICP-MS response factors calculated for major, minor and trace elements was evaluated. Under consistent laser settings, the response factors appeared to be matrix dependent. For a given matrix, the response factors were also significantly different for the two lasers. Normalisation with a single matrix element was effective only for matrices with similar mineralogy. When operating at 266 nm instead of 1064 nm, matrix effects could be reduced but not overcome. However, variations of the response factors between the different matrices appeared to be similar within distinct groups of elements, reflecting geochemical associations. When using multiple internal standards, matrix effects but also effects of the laser wavelength, could be fully compensated.     

Modelling metolachlor exports in subsurface drainage water from two structured soils under maize (eastern France)

The study investigated the processes involved in metolachlor transport in two artificially drained, structured soils in eastern France. Measured losses of bromide and metolachlor in drainage water were compared with results simulated by the mechanistic, stochastic AgriFlux model. Simulated drainage water volumes were generally similar to the measured volumes when the spatial variability of the soil water properties was taken into account. When such variability was disregarded, cumulative water volumes of the clay soil were over- or underestimated by more than 20%. Two types of adsorption were tested. For instantaneous, reversible adsorption, using the partition coefficient Koc, metolachlor losses were underestimated in the first drainage water volumes and overestimated for the total study period. The use of slow adsorption and desorption kinetics (ADK) produced an export pattern similar to the observed one. A sensitivity analysis indicated that the simulated results are very sensitive to the values of the ADK rates, especially for the silty loam soil. The effect of ADK on the attenuation of metolachlor exports was more significant than the effect of degradation (2.3 and 6.7 times higher for the clay and silty loam soils, respectively). For the same four-month period, the bromide and metolachlor losses (using ADK) in the clay soil were 2.1 and 1.3 times greater, respectively, if the macroporosity was set at 10% than if it was not simulated. Conversely, macroporosity did not significantly affect these losses in the silty loam. The main factors involved in the metolachlor transport in the studied soils were: (i) the macroporosity, especially in the clay soil because of the low hydraulic conductivity of the matrix and (ii) the sorption kinetics rates which varied according to the soil physico-chemical characteristics.     

The Cenozoic evolution of the Roer Valley Rift System integrated at a European scale

The Roer Valley Rift System (RVRS) is located between the West European rift and the North Sea rift system. During the Cenozoic, the RVRS was characterized by several periods of subsidence and inversion, which are linked to the evolution of the adjacent rift systems. Combination of subsidence analysis and results from the analysis of thickness distributions and fault systems allows the determination of the Cenozoic evolution and quantification of the subsidence. During the Early Paleocene, the RVRS was inverted (Laramide phase). The backstripping method shows that the RVRS was subsequently mainly affected by two periods of subsidence, during the Late Paleocene and the Oligocene–Quaternary time intervals, separated by an inversion phase during the Late Eocene. During the Oligocene and Miocene periods, the thickness of the sediments and the distribution of the active faults reveal a radical rotation of the direction of extension by about 70–80° (counter clockwise). Integration of these results at a European scale indicates that the Late Paleocene subsidence was related to the evolution of the North Sea basins, whereas the Oligocene–Quaternary subsidence is connected to the West European rift evolution. The distribution of the inverted provinces also shows that the Early Paleocene inversion (Laramide phase) has affected the whole European crust, whereas the Late Eocene inversion was restricted to the southern North Sea basins and the Channel area. Finally, comparison of these deformations in the European crust with the evolution of the Alpine chain suggests that the formation of the Alps has controlled the evolution of the European crust since the beginning of the Cenozoic.     

Deformation processes at fast to ultra-fast oceanic spreading axes: mechanical approach

The morphology of fast to ultra-fast oceanic spreading ridges such as the East Pacific Rise (EPR) is characterized by an axial dome, 5–10 km wide, culminating at 300–500 m above the surrounding seafloor. This dome is bounded by lateral grabens that develop systematically 2 to 6 km apart from the spreading axis. A large summit trough, 200 m to 2 km wide, locally notches the axial high, only where the dome is inflated, indicative of a time-average robust magma supply. This summit trough is thought to represent an elongated axial summit caldera (ASC) created as a result of the subsidence of the top of the axial magma chamber (AMC). Such subsidence is likely caused by a temporary decrease in melt supply into the shallow magma reservoir suffering continuous regional extension. Analog experiments using small-scale modeling have been performed in order to better constrain the tectonic evolution of the axial region. The experimental apparatus includes an elongated balloon filled with water as an analog of the magma reservoir set in a central groove in a table. It is capped with a silicone layer representing hot rocks below the brittle–ductile transition and is covered by a sand layer representing the brittle crust. The experiments integrate withdrawal of the balloon and extension at the boundary of the model by the mean of two mobile walls. Three experimental setups allowed us to study independently the mechanical parameters controlling the axial tectonic evolution: extension without withdrawal, withdrawal without extension, withdrawal and synchronous extension. We show that the morphology of the EPR axis can be considered as the result of both horizontal and vertical movements. Two symmetrical lateral grabens develop on both sides of a non-deformed axial dome when single extension is applied to a model with a thin silicone layer. Normal faults of the lateral grabens are rooted on two divergent velocity zones (DVZs) located on the edges of the groove. This situation is regarded as an analog of the natural case where the top of the AMC acts as a stress-free boundary that fails to transmit the extensional stresses to the upper brittle layer. An important deflation of the balloon without extension results in the creation of a central collapse trough limited by reverse faults. During synchronous extension and withdrawal, the initiation of the lateral grabens is favored by a balloon deflation, even if such deflation is unable to generate a superficial collapse. This last case is considered as representative of the evolution of EPR segments showing little variations in melt supply into the AMC. Higher deflation rates under continuous extension correspond to EPR segments undergoing strong variations in melt supply. In such experiments, the lateral grabens are created together with a central collapse trough developing in a way similar to that of experiments involving only balloon deflation. Finally, we show that DVZs located at the brittle–ductile boundary are the key mechanical elements which may explain the structural evolution of the axial region of fast to ultra-fast spreading ridges. The distance from axis and the width of the DVZs directly control the location and the distribution of the lateral grabens.     

Origine et évolution du soufre au sein de l'aquifère des Sables infra-molassiques du Bassin aquitainOrigin and evolution of sulphur in the Infra-Molassic Sands Aquifer from the Aquitaine Basin

New data on isotopic composition of sulphur and oxygen in dissolved sulphates are used to determine the origin of sulphate ions (SO₄^2?) in the water of the Infra-Molassic Sands Eocene aquifer. Two very distinct origins appear: one is the gypsum dissolution, from the molasse, and the second is the pyrite oxidation, present at the bottom of and within the aquifer. These isotopic analyses helped, in agreement with hydrogeology, to identify geochemical basins whose properties modify the chemical water composition. They also highlight the existence of bioreduction areas. To cite this article: L. André et al., C. R. Geoscience 334 (2002) 749–756.     

The D/H Ratio in Methane in Titan: Origin and History

We propose a new interpretation of the D/H ratio in CH₄ observed in the atmosphere of Titan. Using a turbulent evolutionary model of the subnebula of Saturn (O. Mousis et al. 2002, Icarus156, 162-175), we show that in contrast to the current scenario, the deuterium enrichment with respect to the solar value observed in Titan cannot have occurred in the subnebula. Instead, we argue that values of the D/H ratio measured in Titan were obtained in the cooling solar nebula by isotopic thermal exchange of hydrogen with CH₃D originating from interstellar methane D-enriched ices that vaporized in the nebula. The rate of the isotopic exchange decreased with temperature and became fully inhibited around 200 K. Methane was subsequently trapped in crystalline ices around 10 AU in the form of clathrate hydrates formed at 60 K, and incorporated into planetesimals that formed the core of Titan. The nitrogen-methane atmosphere was subsequently outgassed from the decomposition of the hydrates (Mousis et al. 2002). By use of a turbulent evolutionary model of the solar nebula (O. Mousis et al. 2000, Icarus148, 513-525), we have reconstructed the entire story of D/H in CH₄, from its high value in the early solar nebula (acquired in the presolar cloud) down to the value measured in Titan's atmosphere today. Considering the two last determinations of the D/H ratio in Titan—D/H=(7.75±2.25)×10⁻⁵ obtained from ground-based observations (Orton 1992, In: Symposium on Titan, ESA SP-338, pp. 81-85), and D/H=(8.75^+3.25_−2.25)×10⁻⁵, obtained from ISO observations (Coustenis et al. 2002, submitted for publication)—we inferred an upper limit of the D/H ratio in methane in the early outer solar nebula of about 3×10⁻⁴. Our approach is consistent with the scenario advocated by several authors in which the atmospheric methane of Titan is continuously replenished from a reservoir of clathrate hydrates of CH₄ at high pressures, located in the interior of Titan. If this scenario is correct, observations of the satellite to be performed by the radar, the imaging system, and other remote sensing instruments aboard the spacecraft of the Cassini-Huygens mission from 2004 to 2008 should reveal local disruptions of the surface and other signatures of the predicted outgassing.     

Exhumation cénozoı̈que des massifs du Canigou et de Mont-Louis (Pyrénées orientales,France)Cainozoic exhumation of the Canigou and Mont-Louis massifs (eastern Pyrenees,France)

The onset of exhumation of the Canigou massif, related to the reactivation of the Têt Fault during the Gulf of Lion opening, is dated at 26–27 Ma, using apatite fission track method and lasts until the Aquitanian, at a rate of $\text{0.29±0.07 }\text{mm}\text{yr}{}^{\mathrm{?1}}$. The subsequent evolution is necessarily followed by a slower rate of exhumation to account for total exhumation. At the hanging-wall of this fault, the granite of Mont-Louis records an older exhumation history, where the crossing of the 110 °C isotherm at 40±4 Ma is related to the Pyrenean thrusting. To cite this article: O. Maurel et al., C. R. Geoscience 334 (2002) 941–948.