Study of two alteration systems as natural analogues for radionuclide release and migration

U-deposit hosted in hydrothermally altered tuffs in Mexico, together with weathering profiles from Cameroon were studied as natural analogues of radionuclide release and migration. Using petrological and spectroscopic methods (infrared and electron paramagnetic resonance), we have distinguished successive secondary mineral parageneses and the behaviour of radionuclides.

In the U-deposit, the mineral parageneses show that uranium migration is mainly controlled by the redox potential and silica activity of the altering solutions. The high silica content of the solutions is caused by the intense alteration of volcanic rocks. Two types of secondary clay mineral parageneses are evidenced: a kaolinization, intense where uranium is accumulated in the welded tuffs, and a smectitization mainly developed in the underlying weakly welded tuffs.

Several types of kaolinite have been defined according to their genesis (fillings in fissures and feldspar pseudomorphs), their location relative to a breccia pipe where uranium has accumulated (core and rim of the pipe; surrounding rhyolitic tuffs), and particle morphology, structural order and substitutional Fecontent. It is shown that the variations of the concentration of paramagnetic defect centres, always more than ten times as important than those measured in weathering kaolinites, are only correlated to the location of the kaolinites. The highest values correspond to the breccia pipe kaolinites, e.g. kaolinites located in the uranium accumulation zones. Moreover, one or two main defects centres are detected depending on the intimate association of kaolinites with uranium-bearing minerals. Besides, in weathering kaolinites from U-depleted laterites, defect centre concentrations are correlated to the total Fe203 content in bulk samples. This means that the defect centre acts as a memory of the travel of uranium when this element was sorbed onto iron gels in the first stage of weathering.

It is concluded that paramagnetic defect centres in kaolinites might allow an efficient fingerprint of successive irradiations in the natural analogues under study and could be an useful tool to control radionuclides migration through kaolinite-containing clayey materials such as those used for waste repository.

A better understanding of radiation efficiency as well as accurate dose-ratekaolinite-containing clayey materials such as those used for waste reposit estimation are needed for a quantitative tracing of the migration ofA better understanding of radiation efficiency as well as accurate dose-ratekaolin radionuclide elements. With this aim, a simulation has been undertaken withestimation are needed for a quantitative tracing of the migration of various radiations sources. We have determined for each irradiation the parameters of the paramagnetic centres created in order to understand the way they are forming. The knowledge of the parameters governing the formation and the stability of the radiation centres in kaolinites allow to use this mineral as a natural dosimeter.     

Al-O--Al paramagnetic defects in kaolinite

Trapped holes located on Al-O-Al bonds in kaolinite were studied by electron paramagnetic resonance spectroscopy (EPR) at 9.3 and 35 GHz applied to well-crystallized, X-ray irradiated and oriented samples. The Q-band EPR spectrum is characterized by three clearly separated groups of 11 quasi-equidistant superhyperfine lines centered at g_xx=2.040±0.0005, g_yy=2.020±0.0005 and g_zz=2.002±0.001. In each of these groups, the 11 superhyperfine lines exhibit intensities according to the ratios 12345654321. An angular dependence of the Q-band EPR spectrum with respect to the magnetic field is demonstrated by measurements on oriented films of kaolinite. An appropriate numerical treatment of the EPR spectra is described, which allowed extraction of the SuperHyperfine Structures (SHFS). X-and Q-band spectra have also been simulated. It is concluded from these experiments that only one type of center is present. This center, labelled the B-center in the literature, is very probably a hole trapped on oxygen (O^- center) atoms coupled to two octahedral aluminium.     

Platinum-group elements, S, Se and Cu in highly depleted abyssal peridotites from the Mid-Atlantic Ocean Ridge (ODP Hole 1274A): Influence of hydrothermal and magmatic processes

Highly depleted harzburgites and dunites were recovered from ODP Hole 1274A, near the intersection between the Mid-Atlantic Ocean Ridge and the 15°20′N Fracture Zone. In addition to high degrees of partial melting, these peridotites underwent multiple episodes of melt–rock reaction and intense serpentinization and seawater alteration close to the seafloor. Low concentrations of Se, Cu and platinum-group elements (PGE) in harzburgites drilled at around 35–85 m below seafloor are consistent with the consumption of mantle sulfides after high degrees (>15–20 %) of partial melting and redistribution of chalcophile and siderophile elements into PGE-rich residual microphases. Higher concentrations of Cu, Se, Ru, Rh and Pd in harzburgites from the uppermost and lowest cores testify to late reaction with a sulfide melt. Dunites were formed by percolation of silica- and sulfur-undersaturated melts into low-Se harzburgites. Platinum-group and chalcophile elements were not mobilized during dunite formation and mostly preserve the signature of precursor harzburgites, except for higher Ru and lower Pt contents caused by precipitation and removal of platinum-group minerals. During serpentinization at low temperature (<250 °C) and reducing conditions, mantle sulfides experienced desulfurization to S-poor sulfides (mainly heazlewoodite) and awaruite. Contrary to Se and Cu, sulfur does not record the magmatic evolution of peridotites but was mostly added in hydrothermal sulfides and sulfate from seawater. Platinum-group elements were unaffected by post-magmatic low-temperature processes, except Pt and Pd that may have been slightly remobilized during oxidative seawater alteration.     

A simulation experiment on the GM estimation for Comet 133P/Elst-Pizarro

In China’s asteroid mission to be launched around 2025,(7968)133 P/Elst-Pizarro(hereafter 133 P)will be the second target,after a visit to asteroid(469219)Kamo’oalewa.This paper describes a simulation of precise orbit determination for the spacecraft around comet 133 P,as well as estimation of its gravitational parameter(GM)value and the solar radiation pressure coefficient Cr for the spacecraft.Different cometocentric distances of 200,150 and 100 km orbits are considered,as well as two tracking modes:exclusive two-way range-rate mode(Earth station to spacecraft)and combinations of two-way range-rate and local spacecraft onboard ranging to the comet.Compared to exclusive two-way range-rate,the introduction of local ranging observables improves the final GM uncertainties by up to one order of magnitude.An ephemeris error in the orbit of 133 P is also considered,and we show that,to obtain a reliable estimate of the GM for 133 P,this error cannot exceed a one km range.     

Description and first results of an explicit electrical scheme in a 3D cloud resolving model

The three-dimensional non-hydrostatic mesoscale model MésoNH of the French community offers the numerical environment to develop a cloud electrification scheme in a consistent way with the original mixed phase microphysical scheme. The charge separation mechanisms are entirely due to non-inductive processes and result from elastic ice–snow, ice–graupel and snow–graupel collisions. The electric charges carried by each of the five hydrometeor categories are transported along the airflow and are exchanged according to the various microphysical mass transfer rates but assuming a power law distribution of the individual charges as a function of the particle size. The electric field is diagnosed at each time step after integrating the electric potential induced by a net charge density in the Poisson equation. Finally, a lightning ash is triggered when the electric field locally steps over a given threshold. It propagates in two opposite directions until the magnitude of the electric field falls below a prescribed value. A fractal branching algorithm is then activated to extend lightning streamers away from the main channel and toward cloudy regions where substantial charge densities are present. Charges are neutralized along the tortuous lightning path with a simple procedure that preserves total charge conservation.The complete electrification scheme tested for an ideal case of vigorous supercellular storm shows an intense electrical activity all along its lifecycle. We show that the model is able to produce a direct tripolar structure of the charges as the result of a temperature charge reversal of − 10 °C and of the different sedimentation rates of the hydrometeors.     

Petrology and Mineral Chemistry of Lower Crustal Intrusions: the Chilas Complex, Kohistan (NW Pakistan)

Mineral major and trace element data are presented for the mainrock units of the Chilas Complex, a series of lower crustalintrusions emplaced during initial rifting within the MesozoicKohistan (paleo)-island arc (NW Pakistan). Detailed field observationsand petrological analysis, together with geochemical data, indicatethat the two principal units, ultramafic rocks and gabbronoritesequences, originate from a common parental magma, but evolvedalong different mineral fractionation trends. Phase petrologyand mineral trace element data indicate that the fractionationsequence of the ultramafic rocks is dominated by the crystallizationof olivine and clinopyroxene prior to plagioclase, whereas plagioclaseprecedes clinopyroxene in the gabbronorites. Clinopyroxene inthe ultramafic rocks (with Mg-number [Mg/(Fe_tot + Mg] up to0·95) displays increasing Al₂O₃ with decreasing Mg-number.The light rare earth element depleted trace element pattern(Ce_N/Gd_N 0·5–0·3) of primitive clinopyroxenesdisplays no Eu anomaly. In contrast, clinopyroxenes from thegabbronorites contain plagioclase inclusions, and the traceelement pattern shows pronounced negative anomalies for Sr,Pb and Eu. Trace element modeling indicates that in situ crystallizationmay account for major and trace element variations in the gabbronoritesequence, whereas the olivine-dominated ultramafic rocks showcovariations between olivine Mg-number and Ni and Mn contents,pointing to the importance of crystal fractionation during theirformation. A modeled parental liquid for the Chilas Complexis explained in terms of mantle- and slab-derived components,where the latter component accounts for 99% of the highly incompatibleelements and between 30 and 80% of the middle rare earth elements.The geochemical characteristics of this component are similarto those of a low percentage melt or supercritical liquid derivedfrom subducted mafic crust. However, elevated Pb/Ce ratios arebest explained by additional involvement of hydrous fluids.In accordance with the crystallization sequence, the subsolidusmetamorphic reactions indicate pressures of 0·5–0·7GPa. Our data support a model of combined flux and decompressionmelting in the back-arc. KEY WORDS: Kohistan; Island arc; gabbro; trace element modelling; lower crustal intrusion     

Albedo control of seasonal South Polar cap recession on Mars

Over the last few decades, General Circulation Models (GCM) have been used to simulate the current martian climate. The calibration of these GCMs with the current seasonal cycle is a crucial step in understanding the climate history of Mars. One of the main climatic signals currently used to validate GCMs is the annual atmospheric pressure cycle. It is difficult to use changes in seasonal deposits on the surface of Mars to calibrate the GCMs given the spectral ambiguities between CO₂ and H₂O ice in the visible range. With the OMEGA imaging spectrometer covering the near infra-red range, it is now possible to monitor both types of ice at a spatial resolution of about 1 km. At global scale, we determine the change with time of the Seasonal South Polar Cap (SSPC) crocus line, defining the edge of CO₂ deposits. This crocus line is not symmetric around the geographic South Pole. At local scale, we introduce the snowdrop distance, describing the local structure of the SSPC edge. Crocus line and snowdrop distance changes can now be used to calibrate GCMs. The albedo of the seasonal deposits is usually assumed to be a uniform and constant parameter of the GCMs. In this study, albedo is found to be the main parameter controlling the SSPC recession at both global and local scale. Using a defrost mass balance model (referred to as D-frost) that incorporates the effect of shadowing induced by topography, we show that the global SSPC asymmetry in the crocus line is controlled by albedo variations. At local scale, we show that the snowdrop distance is correlated with the albedo variability. Further GCM improvements should take into account these two results. We propose several possibilities for the origin of the asymmetric albedo control. The next step will be to identify and model the physical processes that create the albedo differences.     

OMEGA long wavelength channel: Data reduction during non-nominal stages

The Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) instrument is a visible and near-infrared imaging spectrometer on board the European Mars Express (MEx) mission. The on-board calibration (OBC) performed at the beginning of observations on each orbit reveals that the photometric response of the C channel (1.0–2.5 μm) has been very stable since orbit insertion in January 2004. On the contrary the L channel (2.5–5.1 μm) response has varied significantly during the mission, and only orbits for which the response is close to nominal could be used with confidence. The spatial coverage of ice-free surfaces in this wavelength range is consequently limited to only ～30%, mainly during northern spring and summer. This paper presents the empirical method used to derive new instrumental transfer functions (ITF) for the non-nominal orbits. This method consists of analyzing the variation of the signal between several observations of a same region acquired at nominal and non-nominal calibration states. In the cases where the mineralogy and the atmospheric conditions between the two observations are the same, the variation in reflectance spectra is only due to the ITF variation, which provides a new ITF. We then associate these new ITFs with their corresponding OBCs to model a relationship between both. The resulting model enables us to provide a new ITF for each orbit for which the OBC is available. The new ITFs derived for the entire dataset have been validated (1) through a comparison of the C and L channel global albedo trends and (2) through a comparison of the surface temperatures derived from the L channel with those calculated from the General Circulation Model (GCM) numerical simulation of the LMD released in the Martian Climate Database. The non-nominal data processed with adapted ITFs for orbits up to 3050 increase the non-icy surface coverage of Mars to ～70% including all seasons.     

Intense fracturing and fracture sealing induced by mineral growth in porous rocks

Mineral precipitation in the pores of a rock may exert a force, which is called crystallization pressure. This process has been studied experimentally and results bring a new look on the way fractures may develop and seal in natural systems. Cylindrical core samples of porous limestone and sandstone were left for several weeks in contact with an aqueous solution saturated with sodium chloride, at 30 or 45 °C, under axial normal stress in the range 0.02–0.26 MPa. The fluid was allowed to rise in the core samples by capillary forces, up to a controlled height where evaporation and precipitation occurred. The uniaxial deformation of the samples was measured using high-resolution displacement sensors. The samples were characterized using computed X-ray tomography, allowing therefore imaging in 3D the intensity and localization of the damage. Two kinds of damage could be observed. Firstly, small rock fragments were peeled from the sample surface. Secondly, and more interestingly, fracture networks developed, by nucleation of microcracks at the interface where evaporation occurred, and propagation to the free surface. Two families of fractures could be identified. A first set of sealed fracture parallel to the evaporation front is directly induced by crystallization pressure. A second fracture network, perpendicular to the evaporation front, accommodates the first set of fractures. An analytical model where fluid flow is coupled to evaporation, vapour transport, and localization of mineral precipitation explains the shape of this fracture network.