How to date a sedimentary serie: Different approaches to better constrain the diversification of early eukaryotes in Central Africa (Mbuji-Mayi Supergroup, Proterozoic, DRCongo)

Peer reviewe

Chronometers and ages in early solar system materials

International audienc

Etude des météorites à l'aide du système isotopique Hf-W (contraintes sur les évènements du système solaire primitif)

Les évènements précoces du système solaire tels que l'accrétion et la différenciation des planétésimaux peuvent être datés en utilisant des isotopes radioactifs naturels présents dans les météorites. Nous nous intéréssons ici aux processus de différenciation planétaire à l'aide du système 182Hf - 182W, basé sur la radioactivité éteinte de 182Hf (T1/2 = 9 Ma). L'hafniumétant fortement lithophile et le tungstène modérément sidérophile, ces deux éléments fractionnent au cours des processus de ségrégation du métal et de fusion partielle des silicates. Une nouvelle méthode de mesure isotopique du tungstène par NTIMS - 100 fois plus sensible que les méthodes existantes a été mise au point, permettant d'analyser avec une grande précision de faibles quantités de matériaux pauvres en tungstène. L'isochrone Hf-W de roches totales obtenue pour les eucrites indique qu'elles se sont formées 11,1 Ma après les chondrites ordinaires. Par ailleurs, la différenciation du corps parent des eucrites et la fusion partielle du manteau qui a donné naissance aux liquides basaltiques sont des processus simultanés d'après les isotopes du tungstène. Cependant, aucune météorite de fer connue ni aucune pallasite ne correspond à la phase métallique complémentaire des eucrites au moment de leur différenciation, tandis que le métal de quelques mésosidérites présente la signature isotopique voulue. La composition isotopique en W est très variable d'un échantillon à l'autre parmi les phases métalliques des mésosidérites et les pallasites. Ceci semble dû à des processus de diffusion (échange isotopique) entre les silicates et le métal. Pour les mésosidérites, d'autres causes telles que la présence d'inclusion contenant de l'hafnium ou des métaux provenant de différents réservoirs peuvent aussi être envisagés. Une utilisation différente des systèmes Hf-W et Mn-Cr - comme traceurs de sources - permet également de déterminer la nature chondrique de l'impacteur qui a heurté la Terre il y a 65 Ma, au moment de la limite Crétacé-TertiairePARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

The nature of the KT impactor. A 54Cr reappraisal

International audienceAccording to the variations observed in the various meteorite classes, 54Cr represents an isotopic tool for planetary body discrimination. In the search for the nature of the Cretaceous-Tertiary (KT) impactor, Cr isotopic measurements were already performed at KT boundary. The current work is aimed to modernize Cr isotopic data with high-precision measurements to confirm a carbonaceous chondrite type infall, the contribution of which is also estimated. Isotopic signatures of two marine clays (Caravaca, Stevens Klint) exhibit an isotopic ratio which would represent a mixing of a carbonaceous chondrite of CM2 type with terrestrial material in a ratio 6% to 19%. A single impactor may account for both marine and continental Cr isotopic signatures

How to date a sedimentary serie: Different approaches to better constrain the diversification of early eukaryotes in Central Africa (Mbuji-Mayi Supergroup, Proterozoic, DRCongo)

peer reviewe

Timing of metal–silicate differentiation in the Eagle Station pallasite parent body

International audiencehe time of the metal–silicate differentiation of the Eagle Station pallasite (ESP) parent body was investigated using the 26Al–26Mg short-lived chronometer (half-life of 0.72 Myr). The Mg isotope ratios were measured in ESP olivines by both MC–SIMS and HR-MC–ICPMS, allowing us to check the consistency between the results given by two different analytical protocols and data reduction processes. Results show that the two datasets are consistent, with a (δ26Mg*)av. value of –0.003 (± 0.005)‰ (2 s.e., n = 89). Such a value, associated with data from the 182Hf–182W short-lived systematics (half-life of 8.9 Myr), indicates an ESP parent body metal–silicate differentiation occurring most likely at least at ∼ 2 Ma, but possibly 4 Ma, after CAI formation. From the 27Al/24Mg ratios measured in ESP olivines using MC–SIMS, the duration of the olivine crystallization process was inferred to have lasted over ∼ 275 kyr if the core has differentiated as early as 2 Ma after CAIs, while in the case of a core differentiation occurring 4 Ma after CAIs, the silicate–silicate differentiation should have lasted for another 4 Myr

Datation à haute précision par l' Al de l'histoire du disque d'accrétion

Une période intéressante de l'histoire précoce du système solaire est celle du disque, i.e. la période pendant laquelle se déroule la plupart des processus qui vont conduire du mélange de gaz et de poussières nébulaires à des grains et des planétésimaux, qui seront à un stade ultérieur le matériel de départ pour la formation des embryons planétaires et des planètes. Les témoins de cette époque sont les constituants des météorites primitives (chondrites), principalement les inclusions réfractaires (CAIs) et les chondres. Une des questions centrales dans la compréhension des processus à l'origine des CAIs et des chondres est celle du temps. Les travaux récents de Johan Villeneuve ont permis de démontrer que l' Al et les isotopes du Mg étaient distribués dans le disque d'accrétion à un niveau d'homogénéité permettant d'utiliser le système Al- Mg comme le chronomètre le plus précis qui soit des évènements qui se sont déroulés lors des 2 ou 3 premiers millions d'années du disque. Le but de cette thèse a été de reprendre toute l'étude de l'26Al avec des mesures de plus haute précision que les mesures existantes, en associant les mesures in-situ (sonde ionique) et en roche totale (HR-MC-ICPMS). Les développements analytiques mis en place pour mesurer les compositions isotopiques en Mg ont été appliqués à l'étude d'olivines réfractaires et de chondres de la météorite Allende, et d'olivines de la pallasite Eagle Station. L'ensemble des données a permis d'apporter des éléments de réponse sur les âges de formation des chondres et de leurs précurseurs, et sur l'origine des olivines réfractaires riches en Mg et la possibilité que celles-ci figurent parmi les précurseurs des chondresThe disk history is a very interesting period of the early Solar System history, i.e. the period during which occurred most of the processes leading from the mixing of nebular gas and dust to grains, and then planetesimals, which will then constitute the starting material for formation of planetary embryos and terrestrial planets. The witnesses of this period that we have in the laboratory are the chondrite constituents, mostly refractory inclusions (CAIs) and chondrules. One of the central questions regarding the processes leading to the formation of CAIs and chondrules is the timing. Villeneuve et al. (2009, 2011) have shown that Al and Mg isotopes were homogeneously distributed in the accretion disk, at a level allowing the use of the Al- Mg system as the most precise short-lived chronometer to constrain the 2 or 3 first million years of the disk history. My PhD project aimed at reconsidering the Al study with more precise measurements, and by combining in-situ (by MC-SIMS) and bulk (by HR-MC-ICPMS) analyses. The analytical developments we set for Mg-isotope measurements (because of the high precision needed) were then applied to a set of extraterrestrial materials, including Mg-rich isolated olivines and Mg-rich olivines in porphyritic type I chondrules from the Allende CV3 meteorite, as well as chondrules from the same meteorite, and olivines from the Eagle Station pallasite. The whole data set allowed to answer questions such as (i) the origin of chondrule precursor materials and the time of chondrule formation, as well as (ii) the origin of Mg-rich refractory olivines, and the possibility that the latters were part of chondrule precursorsNANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

Complex proterozoic to paleozoic history of the upper mantle recorded in the Urals lherzolite massifs by Re–Os and Sm–Nd systematics

Re-Os and Sm-Nd isotope systematics for the Nurali and the Mindyak lherzolite massifs have been determined in conjunction with their whole-rock major and trace element contents. The data suggest that the peridotites represent residues after the extraction of up to 25% of partial melt from the fertile mantle protolith. Later melt percolation and associated fluid-rock reaction events have modified the Sm/Nd, Re/Os and Pd/Os ratios of peridotites, but didn't affect significantly their major element compositions. The Re-Os and Sm-Nd isotope data strongly suggest that several magmatic events are involved in the evolution of these bodies. The mantle sections of these complexes formed during Proterozoic times, represent the first reported evidence for Precambrian peridotites in the Southern Urals. The oldest Re-Os age of 1250 +/- 80 Ma for the Nurali cumulates records the separation from the convective upper mantle. Multiple partial melting of the peridotites followed by fractional crystallisation produced layered cumulates which were subsequently stored in the sub-continental lithosphere over similar to 0.8 Ga. The age of the Nurali ophiolite coincides with the development of an epicontinental rift basin on the passive margin of the Baltica proto-continent. The younger Sm-Nd age of Mindyak peridotites (882 83 Ma) and Re-Os age of associated gabbros (804 +/- 37 Ma) record another tectonic event responsible for the separation of the Mindyak massif from convective mantle. Between similar to 850 and 650 Ma, the margins of the paleo-Asian ocean became the site of island-arc formation. This ophiolite then evolved in an intra-oceanic island-arc setting. The Mindyak lherzolite massif could be the first record of a Neoproterozoic Cadomian are in the Southern Urals. A later island-arc formation event has then affected both massifs in different ways. The Mindyak massif was incorporated into a rifting zone at similar to 500 Ma, producing a second partial melting event of peridotites, cross-cutted by mafic dykes. The Nurali massif has also been cross-cut by gabbro-diorite dykes at Devonian time during the subduction event leading to Urals island-arc formation. The combination of. Re-187-Os-187 and Sm-147-Nd-143 systematics for peridotites, mafic-ultramafic cumulates and mafic dykes reveals the complex history of ophiolite complexes, including isolation from the convective upper mantle at Proterozoic time, storage in sub-continental lithospheric mantle and re-activation during later tectonic events, such as island-arc formation. (c) 2007 Elsevier B.V. All rights reserved

Automated separation of Sr from natural water samples or carbonate rocks by high performance ion chromatography

An automated separation of strontium prior to isotopic analysis from liquid samples (river water, seawater or solutions resulting from natural carbonate dissolution) was developed using High Performance Ion Chromatography (HPIC) technique. The experimental set up consists of an automatic sampler, an ion chromatograph and a fraction collector. To evaluate the performance of the set up, we compare the separation of strontium performed by HPIC with routine techniques which are manually operated using Sr-Spec resin ([Birck, J.-L.,1986. Precision K–Rb–Sr isotopic analysis; application to Rb–Sr chronology. Chemical Geology, 56, 73–83; Horwitch et al., 1990; Pin, C. and Bassin, C., 1992. Evaluation of a strontium-specific extraction chromatographic method for isotopic analysis in geological materials. Analytica Chemica Acta, 269, 249–255]) for 18 river-waters with very different Sr concentrations and Ca / Sr ratios, 5 seawater samples, and two solutions obtained by partial dissolution of Cenozoic high carbonate content marine sediments. The 87Sr / 86Sr isotopic ratios were measured by TIMS. Results agree within error at acceptable statistical scatter that can mostly be accounted for by the uncertainty of the mass spectrometry. Blank values average at 127 pg, but contamination is more than 90% introduced by the eluent and can be reduced if necessary. We show also that the conductivity cell detector after eluent suppression is sensitive enough to accurately measure these very low concentrations and thus that blank values can be evaluate directly by the HPIC set up. The major advantage of this technique is the saved handling time. Moreover, this technique benefits also from a number of on-line controls. During the procedure, Sr and Ca concentrations in the sample are determined. Within the same analytical procedure, others major elements such as Na, K, or Mg concentration can also be evaluated. In addition, the efficiency and the purity of the separation can easily be monitored by determination of Rb, Ca, and Sr content of the collected fraction. We show that the HPIC-separation technique has a great potential for dating marine carbonate sediments and for other high-resolutions studies. Extension to rare earth and lead separations is foreseen in the near future as well as an extension to Mg isotopic analysis. Silicate analysis is anticipated to be a more difficult step

Northern Hemisphere climate control of the Bengali rivers discharge during the past 4 Ma

International audienceNd isotopes are useful tracers for paleoceanography due to the short Nd residence time in seawater and the large differences between the isotopic signatures of various geological reservoirs. Therefore, var epsilonNd variations reflect the geological history of individual oceanic basins. Using a differential dissolution technique, which extracts Nd isotopes of seawater trapped in MnO2 coatings and carbonates in marine sediment, we measured almost two hundred samples from ODP Sites 758 and 757 in the Northern Bay of Bengal covering the last 4 Ma. For the first time, we have shown a covariation between var epsilonNd and δ18O over at least the last 800 ka. We also show that from 4 Ma to 2.6 Ma, var epsilonNd is almost constant and starts to fluctuate at 2.6 Ma when northern glaciations increased. From 2.6 Ma to 1 Ma the fluctuation period is close to 40 ka while from 1 Ma to present it is dominantly 100 ka. We attribute these findings to mixing between Himalayan river water (that ultimately originates as Indian summer monsoon rain) and normal Bay of Bengal seawater. Previous studies on seawater, using var epsilonNd, δ18O analyzed on planktonic foraminifera and sedimentary data, can be integrated into this model. A simple quantitative binary mixing model suggests that the summer monsoon rain was more intense during interglacial than glacial periods. During last glacial episode, the monsoon trajectory was deviated to the east. At a large scale, the Indian monsoon is fully controlled by the variations in Northern Hemisphere climate but with a complex response function to this forcing. Our study clearly establishes the large potential of Nd isotope data to evaluate the hydrological river regime during the Quaternary and its relationship with climate fluctuations, particularly when the sediment archive is sampled close to sediment sources