Tectonics of SE China: New insights from the Lushan massif (Jiangxi Province)

International audienceIn south China the Lushan massif forms a topographic high of the South China Block south of the Qinling-Dabie belt. The Lushan massif consists of two main lithotectonic units separated by a major tectonic contact: a Neoproterozoic (upper Sinian)-Paleozoic unit comprising primarily unmetamorphosed sandstones overlies a Paleoproterozoic unit mainly composed of low-pressure, high-temperature gneisses and micaschists. Both units are cut by Cretaceous granitic intrusions. Three primary tectono-metamorphic and magmatic events are recognized. The eastern part of the Lushan massif is cut by a NNE-SSW trending ductile normal fault (D3 deformation) coeval to the emplacement of a 100-110 Ma leucogranite dated by 40Ar/39Ar laserprobe on biotite and muscovite. D2 deformation is responsible for the formation of a decakilometer-scale NE-SW trending upright anticline characterized by NE-SW stretching and NW-SE shortening. The age of this folding event is defined by a 127±1 (2σ) Ma U/Pb titanite date obtained for a syntectonic granodiorite and 40Ar/39Ar ages of 133 Ma for amphibole. This Cretaceous age also corresponds to the 40Ar/39Ar ages of 126 Ma found on syntectonic muscovites at the base of the Sinian unit. An older deformation event, D1, characterized by a top-to-the-NW extensional decollement of the Sinian-Paleozoic series above Proterozoic metamorphic rocks is related to the Triassic tectonics of the Dabieshan. Lastly, in the lower part of Sinian rocks, the occurrence of kyanite cataclased during D1 documents an older, poorly preserved, late Paleozoic-early Mesozoic tectonometamorphic event (Dx) related to a blind thrust in the continental crust of the South China Block in the southern foreland of the Dabieshan

The thermal history of the Acapulco meteorite and its parent body deduced from U/Pb systematics in mineral separates and bulk rock fragments

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Reevaluation of precise lead isotope measurements by thermal ionization mass spectrometry: comparison with determinations by plasma source mass spectrometry

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The major differentiation of the Earth at not, vert, similar 4.45 Ga

International audienceThe revelation of a small 182W terrestrial excess relatively to the 182Hf–182W systematics in meteorites has led some authors to claim that the Earth experienced a rapid accretion and an early differentiation, 30–40 Ma after the birth of the solar system at 4.567 Ga. This interpretation has since been moderated, but the idea of an early segregation of the core is still widely advocated. We challenge this interpretation with quantitative arguments that concern Hf–W, U–Pb and I–Xe systematics on the Earth. The W isotopic composition of the bulk silicate Earth can be explained by an incomplete isotopic re-equilibration between primitive metal and silicate components during the segregation of the Earth's core. We consider that the primitive metal/silicate differentiation in planetesimals and the segregation of cores of planetary bodies occurred during the first million years and that the segregation of the major part of the Earth's core occurred late in respect to the 182Hf decay. Consequently, the non-equilibrated fraction of primitive silicate material is estimated to be small, between 6 and 14%, enough however to « open » the 182Hf–182W chronometer as is presently observed. This significant, but incomplete, metal/silicate re-equilibration only slightly affects the U–Pb chronometer. A reappraisal of the Pb isotope composition of the bulk silicate Earth allows us to define the mean age of the Earth's core's segregation, between 4.46 Ga and 4.38 Ga. This evaluation overlaps the time of outgassing of the atmosphere based on the 129I–129Xe systematics, 4.46–4.43 Ga. We consider that the period around 4.45 Ga relates to the major primitive differentiation of the Earth. This scenario coherently and quantitatively explains the 182Hf–182W, 235, 238U–207, 206Pb, 129I–129Xe and 146Sm–142Nd terrestrial records and it is compatible with the radiometric constraints for the formation of the Moon and coherent with the not, vert, similar 102 Ma time scale for the accretion of the Earth, as evaluated by current numerical simulations for terrestrial planet formation

Open system behaviour andearly chronologies in the Solar System

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Deconvolution of the isotopic drift in LC-MC-ICPMS coupling: a new tool for studding isotope fractionation induced by sample introduction techniques.

International audienceOn-line hyphenated methods between the Multi Collection Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) and a variety of introduction techniques (liquid and gas chromatography, laser ablation…) provide transient signals with specific time-windows. It is now well-known that the isotope ratio drift which is observed during transient signal acquisition is generated from the MC-ICPMS detection system, the introduction techniques, or a combination of both parameters. In this work, the Nd isotope ratios were investigated through a coupling of the MC-ICPMS with Liquid Chromatography (LC). The purpose was to dissociate the isotopic drift coming from the detection system to the drift caused by the introduction technique by using raw isotope data. To this end, the time constants of the MC-ICPMS amplifiers were used, and the isotope drift generated by the detection system was successfully corrected. After this correction, the isotope drift coming exclusively from the LC was highlighted. The use of the Method of Internal Signal Synchronization (MISS) allowed the correction of the chromatographic drift and the calculation of a time lag between the Nd isotopes at 0.0036 s / amu. This is the first time where for an isotope fractionation caused by a specific physicochemical process, a time lag between the isotopes was calculated. We believe that the calculation of time lag values between the isotopes could be a simple and robust method opening up new possibilities for studies of isotope fractionations generated by different introduction techniques directly coupled with the MC-ICPMS

Transient signal isotope analysis using multicollection of ion beams with Faraday cups equipped with 10 12 Ω and 10 11 Ω feedback resistors

International audienceTo improve the precision of isotope analyses of low ion intensities using the Faraday detection system, amplifiers equipped with 10(12) Omega resistors (hereafter 10(12) Omega amplifiers) have been developed. While the behavior of these amplifiers for steady signals has been well investigated, there is no ample evidence regarding the use of 10(12) Omega amplifiers for transient signal acquisition. In this work, we investigated the simultaneous use of amplifiers equipped with 10(12) Omega and 10(11) Omega resistors for transient signal acquisition. Using the equation describing the relationship between the input ion current and the output voltage in the amplifiers, we showed how the transient signal duration influences the accuracy of the isotope ratio measurements. In particular, lead transient signals were investigated using a Neptune Plus MC-ICPMS and Pb-204 and Pb-206 isotopes were measured using 10(12) Omega and 10(11) Omega amplifiers, respectively. The Pb-204/Pb-206 isotope ratio showed an important drift due to a large time lag between 10(12) Omega and 10(11) Omega amplifiers. The time lag was quantified (0.175(3) s) and the isotopic drift was corrected using a method of internal signal synchronization. The Pb-204/Pb-206 drift corrected data obtained from the 10(12)-10(11) Omega amplifier configuration were compared to the data obtained from 10(11)-10(11) Omega amplifiers. Our results point out that for low transient signal intensities (<10(-13) A), the use of 10(12)-10(11) Omega amplifiers is more beneficial in terms of isotope ratio uncertainty, repeatability and trueness, compared to the 10(11)-10(11) Omega amplifier configuration

Automated Analyte separation by Ion Chromatography using a Cobot Applied to Geological Reference Materials for Li Isotope Composition

International audienceThe demand for large and reliable datasets on isotopic composition has increased ingeochemistry and environmental sciences over recent years. We present an automated ionchromatographic separation method using a robotic pipetting arm, termed “ChemCobOne”, toreduce the time of sample separation. Its performance was tested for lithium isotope separationin geological reference materials using a single-step separation with HCl (0.2 mol l-1) and a 2-mlresin volume. This refined lithium purification method does not forfeit precision, accuracy orpurity compared with manual sample processing. In addition, a 7Li value for NASS-6 of 30.99 ±0.50‰ (2s) (95%CI = 0.14‰, n = 44) was determined and the first 7Li values for the granite roc

Comparative study of different methodologies for quantitative rock analysis by Laser-Induced Breakdown Spectroscopy in a simulated Martian atmosphere

International audienceLaser-Induced Breakdown Spectroscopy was selected by NASA as part of the ChemCam instrument package for the Mars Science Laboratory rover to be launched in 2009. ChemCam's Laser-Induced Breakdown Spectroscopy instrument will ablate surface coatings from materials and measure the elemental composition of underlying rocks and soils at distances from 1 up to 10 m. The purpose of our studies is to develop an analytical methodology enabling identification and quantitative analysis of these geological materials in the context of the ChemCam's Laser-Induced Breakdown Spectroscopy instrument performance. The study presented here focuses on several terrestrial rock samples which were analyzed by Laser-Induced Breakdown Spectroscopy at an intermediate stand-off distance (3 m) and in an atmosphere similar to the Martian one (9 mbar CO2). The experimental results highlight the matrix effects and the measurement inaccuracies due to the noise accumulated when low signals are collected with a detector system such as an Echelle spectrometer equipped with an Intensified Charge-Coupled Device camera. Three different methods are evaluated to correct the matrix effects and to obtain quantitative results: by using an external reference sample and normalizing to the sum of all elemental concentrations, by using the internal standardization by oxygen, a major element common to all studied matrices, and by applying the Calibration Free Laser-Induced Breakdown Spectroscopy method. The three tested methods clearly demonstrate that the matrix effects can be corrected merely by taking into account the difference in the amount of vaporized atoms between the rocks, no significant variation in plasma excitation temperatures being observed. The encouraging results obtained by the three methods indicate the possibility of meeting ChemCam project objectives for stand-off quantitative analysis on Mars