J.-P. Gallien

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed (∼180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.

The Pierre Süe Laboratory nuclear microprobe as a multi-disciplinary analysis tool

Abstract The nuclear microprobe at the Pierre Sue Laboratory is a facility exclusively devoted to microanalysis. The microprobe consisting of a single stage Van de Graaff accelerator and two microbeam lines will be described. Simultaneous detection (X-rays, γ-rays, charged particles, etc.) and imaging are routinely performed by a PC-based multi-parameter data acquisition system. Telescope mounting is frequently used for particle identification particularly when performing nuclear reaction analysis (NRA). A unique feature of the Pierre Sue Laboratory nuclear microprobe is the ability to analyze radioactive samples. One of the two beamlines has been specifically designed for that purpose. Radioactive environment requires suited target handling and detection setups. Main application fields of the microprobe are material, earth, planetary, environmental sciences and electronuclear related topics.

Development of “position–charge–time” tagged spectrometry for ion beam microanalysis

During sample analysis in scanning mode, our PC-based data acquisition system (MPA/PCTM) is able to record eight parameter events in a list data file. The system can acquire simultaneously collected energies from up to four detectors (X and gamma ray detectors, charged particle detectors). Each event is tagged with beam position, integrated charge and a time-code. Beam scanning control is currently upgraded. Until now, beam position tagging required a double conversion (digital-analog and analog-digital) generating supplementary dead time. The new system allows direct digital transfer with high-resolution maps and large scanning rate capabilities. RISMIN, an home made software, is used to extract maps, spectra, profiles, etc. from generated list data files. This software is developed under LabVIEWTM and IMAQTM. Beyond the classical elemental mapping capabilities, the software allows any 1D or 2D multi-filtering parameter representations and filtered events list data file generation. Map processing (standard operations and image filtering) is also integrated.

Biogenic nitrogen and carbon in Fe-Mn-oxyhydroxides from an Archean chert, Marble Bar, Western Australia

To quantify and localize nitrogen (N) and carbon (C) in Archean rocks from the Marble Bar formation, Western Australia, and to gain insights on their origin and potential biogenicity, we conducted nuclear reaction analyses (NRA) and carbon and nitrogen isotope ratio measurements on various samples from the 3460-Myr-old Fe-rich Marble Bar chert. The Marble Bar chert formed during the alteration of basaltic volcanoclastic rocks with Fe- and Si-rich hydrothermal fluids, and the subsequent precipitation of magnetite, carbonates, massive silica, and, locally, sulfides. At a later stage, the magnetite, sulfides, and carbonates were replaced by Fe-Mn-oxyhydroxides. Nuclear reaction analyses indicate that most of the N and C resides within these Fe-Mn-oxyhydroxides, but a minor fraction is found in K-feldspars and Ba-mica dispersed in the silica matrix. The N and C isotopic composition of Fe-oxides suggests the presence of a unique biogenic source with δ15NAIR values from +6.0 ± 0.5‰ to 7.3 ± 1.1‰ and a δ13CPDB value of −19.9 ± 0.1‰. The C and N isotope ratios are similar to those observed in Proterozoic and Phanerozoic organic matter. Diffusion-controlled fractionation of N and C released during high combustion temperatures indicates that these two elements are firmly embedded within the iron oxides, with activation energies of 18.7 ± 3.7 kJ/mol for N and 13.0 ± 3.8 kJ/mol for C. We propose that N and C were chemisorbed on iron and were subsequently embedded in the crystals during iron oxidation and crystal growth. The Fe-isotopic composition of the Marble Bar chert (δ56Fe = −0.38 ± 0.02‰) is similar to that measured in iron oxides formed by direct precipitation of iron from hydrothermal plumes in contact with oxygenated waters. To explain the N and C isotopic composition of Marble Bar chert, we propose either (1) a later addition of N and C at the end of Archean when oxygen started to rise or (2) an earlier development of localized oxygenated environments, where biogeochemical cycles similar to modern ones could have developed.

In situ mapping of high-pressure fluids using hydrothermal diamond anvil cells

We present new results combining high pressures and temperatures attainable in a diamond anvil cell with in situ synchrotron radiation induced micro-X-ray fluorescence measurements. Hydrothermal diamond anvil cells experiments have been performed by measuring the partitioning of Pb between aqueous fluids (pure water or NaCl-enriched water) and hydrous silicate melts of haplogranite composition using synchrotron X-ray fluorescence. The in situ measurements were performed in the range 0.3–1.2 GPa and 730–850 °C both in the aqueous fluid and in the silicate melts being in equilibrium. Pb is strongly partitioned into high-pressure–temperature hydrous melts when Cl is present in either the hydrous melt or the aqueous fluid. Moreover, our comparisons of in situ results with post-mortem results show that significant changes take place during rapid quenching especially when samples are small (few hundred of microns in diameter). Water exsolution is induced by the quench in the silicate melt showing the high mobility of Pb which immediately partitions into the water vapor phase during the quench. The current in situ approach offers thus a pertinent complementary method to the classical experimental petrology investigations.

The partitioning of barium and lead between silicate melts and aqueous fluids at high pressures and temperatures

Abstract The origin of subduction-related magmas is still a matter of debate in the Earth Sciences. These magmas are characterised by their distinctive trace element compositions compared to magmas from other tectonic settings, e.g. mid-ocean ridges or rifts. The distinct trace element composition of these magmas is generally attributed to alteration of the source region by a contaminating agent: either a silicate melt or a hydrous fluid, possibly chlorine-enriched. In this study, we have used μPIXE (proton induced X-ray emission) to analyse synthetic samples obtained from a micro-experimental petrology study that aims to determine the partitioning behaviour of two key elements, Ba and Pb, between silicate melt and both pure water and saline fluids. Our experiments were performed at high-pressure (>0.34–1.53 GPa) and high-temperature (697–1082 °C) in a hydrothermal diamond anvil cell, that was used as a transparent rapid quench autoclave. We observed that at high pressure and temperature, in the presence of pure water, Ba and Pb are not strongly fractionated into one phase or the other. The partition coefficient of Pb is ranging from 0.46 to 1.28. Results from one experiment performed at 0.83 GPa and 847 °C, in the presence of a saline fluid indicate that the presence of Cl induces strong fractionation of Pb and moderate fractionation of Ba both into the silicate melt. In addition, our data indicate that Cl is strongly partitioned into the fluid phase.

NUCLEAR MICROPROBE APPLICATIONS TO RADIOACTIVE WASTE MANAGEMENT BASIC RESEARCH

Abstract Radioactive waste management is one of the major technical and scientific challenge to be solved by industrialized countries near the beginning of the 21st century. Relevant questions arise about the extrapolation of the long term-behavior of materials from waste package, engineered barriers and near field repository. Whatever the strategical option might be, wet atmosphere or water intrusion through the different barriers constitute the two main remobilization factors for radionuclides in the geosphere and the biosphere. The study of solid alteration processes and elemental sorption phenomena on mineral surfaces is one of the most efficient basic research approaches to assess the long term performance of waste materials. Ion beam analysis and more recently nuclear microprobe techniques have been applied to investigate exchange mechanisms near representative solid/liquid interfaces such as glass/deionized water, uranium dioxide/granitic or clay water or mineral surface/aqueous solution doped with chemical elements analogue to actinide or fission products. This paper intends to describe the different works that have been carried out in Saclay using the nuclear microprobe facility. The coupling of μRBS, μPIXE and μNRA permits to determine the evolution of the surface composition induced by chemical reactions involved. Complementary observation of solid morphology and solution analysis allows to obtain a complete elemental balance on exchange processes.

Parameterization of nuclear reactions cross section using R-matrix theory

Abstract R-matrix theory is used for the determination of tabulated values related to nuclear cross sections of light elements. This is done in order to improve the quantitative study of those elements by ion beam analysis (IBA). We give the fitting procedure adopted for the determination of parameters related to cross section measurements. This has been applied to the 16 O(α, α) 16 O reaction in the energy range 2.9–3.6 MeV.

Nuclear models and microanalysis

Abstract Most of the light elements like d, Li, B, C, N, O, F can be identified and quantified using nuclear reactions induced by protons, deuterons, 3 He or 4 He. Profiles or layers thickness measurements depend on knowledge of the nuclear cross sections (excitation function and angular distribution) in an energy and angular range corresponding to the depth to analyze. Several models have been developed since 30 years by nuclear physicists, to describe the nucleus. The interest was mainly the knowledge of spectroscopic data like energy levels, spin, parity, life time, etc. The knowledge of the cross section in large energy or angular domains was not the aim of the research. Using the nuclear models developed for these fundamental studies it is possible to obtain a complete data base of cross sections, doing only a few measurements to normalize the models in the domain of interest for nuclear analysis. We have performed such measurements for the 12 C(p, p) 12 C reaction in the energy range 0.300–3.5 MeV, and the angular range 110–180°. The data have been analyzed by the R matrix model. Most of the light element reaction cross sections will be measured and analyzed using either the R matrix model for compound nuclear reactions, or a nuclear transfer model for stripping or pick up interactions, depending on the nuclear mechanism of production. Parallel to this, we are developing an analysis model based on the GEANT library developed at CERN. It consists of an events generator, using a Monte Carlo method, and a geometrical description of the experimental set up. It takes into account all the nuclear reactions (particle and gamma-ray emissions) but not the X-ray emission.

Rare metal sequestration and mobility in mineralized black shales from the Zunyi region, South China

The micromineralogical and chemical study of two black mineralized shales from the Zunyi region show that they experimenced a low diagenetic degree still preverving microbial phosphate and sulfides and poorly crystallized phases composed of metals or Al and Si. The two shales differ in diagenetic vein mineralogy. CH11 is carbonate-rich whereas CH25 is sulfate-rich. As CH11 is twice enriched in the redox sensitive elements Ni, Se, As, Mo, Sb, U, Zn, Pb, Cd, a remobilization by CO2 (may be derived from organic matter oxidation) rich fluids is suggested. Seawater characteristic REE element patterns, but Se/S-ratio typical for a volcanogenic environment, favor a fluid mixture during early diagenesis.