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The samarium-146 ( 146 Sm)–neodymium-142 ( 142 Nd) short-lived decay system (half-life of 103 million years) is a powerful tracer of the early mantle-crust evolution of planetary bodies. However, an increased 142 Nd/ 144 Nd in modern terrestrial rocks relative to chondrite meteorites has been proposed to be caused by nucleosynthetic anomalies, obscuring early Earth’s differentiation history. We use stepwise dissolution of primitive chondrites to quantify nucleosynthetic contributions on the composition of chondrites. After correction for nucleosynthetic anomalies, Earth and the silicate parts of differentiated planetesimals contain resolved excesses of 142 Nd relative to chondrites. We conclude that only collisional erosion of primordial crusts can explain such compositions. This process associated with planetary accretion must have produced substantial loss of incompatible elements, including long-term heat-producing elements such as uranium, thorium, and potassium.

International audience; To date only few analyses of europium (Eu) isotopic composition of terrestrial and extraterrestrial samples have been reported in the literature. This isotopic systematic remains largely understudied and it is still unknown whether the Eu isotopic composition of the Earth's major reservoirs show measurable variations. Here we present a comprehensive chemical protocol to separate Eu from different sample matrices and a method for measuring high-precision Eu isotopic composition by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). We show that the chemical separation of Eu by ion chromatography may induce measurable isotopic fractionation and that careful attention must be taken to limit analytical artefacts. As Eu has only two stable isotopes, we investigated different methods to correct as best as possible for instrumental mass bias. We found that doping the samples with samarium (Sm), a close neighbor of Eu, provided the best precision and trueness. Based on complete duplicate analyses, we evaluated the repeatability standard deviation (2SD) of the whole procedure at about 50 ppm on the 151Eu/153Eu ratio (i.e. 0.5 ε unit), which is two to four times better than previously published procedures. All igneous and sedimentary reference materials analyzed in this study have Eu isotopic compositions similar to the reference standard NIST 3117a within uncertainties (i.e. εEu = 0 ± 0.5).

co-auteur étranger; International audience; Transition metal isotopes are particularly useful for understanding the conditions under which magmatic processes occur. Moreover, those with more than one oxidation state (e.g. Cr2+, Cr3+ and Cr6+) may also provide powerful constraints on the evolution of the redox state of the mantle over time. By investigating the Cr isotopic compositions in both magnesiochromite and silicate melts during experiments performed at 1300 °C and under controlled redox conditions (−12 −6) are much lighter than those of melts equilibrated with magnesiochromite at lower oxygen fugacity. The observed variations can be explained by changes in bonding environment for Cr under oxidised conditions in the silicate melts and/or in the magnesiochromite grains. Similarly, the second set of experiments designed to study fractional crystallisation (series B) suggest that Cr isotope fractionation is larger under oxidising conditions.

International audience; Anastomosing branching and wavy laminae in quartz-rich sandstones of the 3.22 Ga Moodies Group in the Barberton Greenstone Belt (BGB), South Africa, have been extensively described as fossilized microbial mats developed in terrestrial to marine transitional environments. Petrological and geochemical characteristics of a ~ 350 m thick Moodies succession in the Saddleback Syncline in the central BGB were analyzed to reconstruct the post-depositional history of the sediments and to better constrain the origin of the laminae. The studied sandstones are composed mainly of quartz, potassium feldspar, and chert clastic grains with various proportions of microquartz and sericite cements. In coastal floodplain and inter- to supra-tidal environments, quiescent periods with low current velocity allowed the repeated deposition of thin (

The formation and differentiation of planetary bodies are dated using radioactive decay systems, including the short-lived 146Sm-142Nd (T½ = 103 or 68 Ma) and long-lived 147Sm-143Nd (T½ = 106 Ga) radiogenic pairs that provide relative and absolute ages, respectively. However, the initial abundance and half-life of the extinct radioactive isotope 146Sm are still debated, weakening the interpretation of 146Sm-142Nd systematics obtained for early planetary processes. Here, we apply the short-lived 26Al-26Mg, 146Sm-142Nd, and long-lived 147Sm-143Sm chronometers to the oldest known andesitic meteorite, Erg Chech 002 (EC 002), to constrain the Solar System initial abundance of 146Sm. The 26Al-26Mg mineral isochron of EC 002 provides a tightly constrained initial δ26Mg* of −0.009 ± 0.005 ‰ and (26Al/27Al)0 of (8.89 ± 0.09) × 10−6. This initial abundance of 26Al is the highest measured so far in an achondrite and corresponds to a crystallization age of 1.80 ± 0.01 Myr after Solar System formation. The 146Sm-142Nd mineral isochron returns an initial 146Sm/144Sm ratio of 0.00830 ± 0.00032. By combining the Al-Mg crystallization age and initial 146Sm/144Sm ratio of EC 002 with values for refractory inclusions, achondrites, and lunar samples, the best-fit half-life for 146Sm is 102 ± 9 Ma, corresponding to the physically measured value of 103 ± 5 Myr, rather than the latest and lower revised value of 68 ± 7 Ma. Using a half-life of 103 Ma for 146Sm, the 146Sm/144Sm abundance of EC 002 translates into an initial Solar System 146Sm/144Sm ratio of 0.00840 ± 0.00032, which represents the most reliable and precise estimate to date and makes EC 002 an ideal anchor for the 146Sm-142Nd clock.

FM acknowledge funding from ERC grant agreement No. 101001282 (METAL), the UnivEarthS Labex program (numbers: ANR-10-LABX-0023 and ANR-11-IDEX-0005-02), the IPGP multidisciplinary program PARI, the Region île-de-France SESAME Grants no. 12015908, EX047016, and the IdEx Université de Paris grant, ANR-18-IDEX-0001 and the DIM ACAV+. The source of and timing of delivery of the volatile elements to Earth is a question that is fundamental to understanding how our planet evolved. Here, we show that primitive meteorites have resolved mass-independent Zn isotope anomalies from the terrestrial reservoir. Carbonaceous chondrites (CC), likely originating from the outer Solar System are distinct from non-CC, and Earth is intermediate between these two components. Modelling based on these data indicates that around 30% of Earth's budget of Zn and other moderately volatile material derives from the participation of 6% of CC-like materials during Earth's accretion, with the remaining coming from NC meteorites. This implies that, despite the relatively minor mass of Earth thought to derive from CC-like material, the CC component of Earth was relatively and significantly volatile-enriched; this is in line with the observation that the terrestrial elemental abundance pattern of moderately volatile elements could be explained by a carbonaceous source, and with the carbonaceous chondrite-like isotopic budget of more volatile-rich material accreted later in Earth's accretion history (e.g. Hg, Se, N, noble gases). Publisher PDF

International audience; Heterogeneity in isotopic compositions within the protoplanetary disc has been demonstrated for a number of elements measured in extra-terrestrial materials, mostly based on chondrite meteorite analyses. However, precise 182Hf-182W and 26Al-26Mg ages of iron meteorites, achondrites, and chondrules show that chondrites accreted later than achondrites and therefore do not strictly represent the early (145Nd/144Nd and 148Nd/144Nd ratios (3.9 145Nd < 11.0 and 9.1 148Nd < 17.9 in part per million deviation, or μiNd) compared to NC chondrites (2.7 145Nd < 3.3 and 2.2 148Nd < 8.1). Moreover, the three terrestrial planets for which we have samples available (Earth, Mars, and the Moon) as well as the silicate inclusions from the non-magmatic IIE iron meteorite Miles present a systematic deficit in μ145Nd and μ148Nd compared to early-formed NC achondrites. Unlike chondrites, the Nd anomalies in achondrites are not correlated to the heliocentric distance of accretion of their respective parent bodies as inferred from redox conditions. Chronological constraints on planetesimal accretion suggest that Nd (and other elements such as Mo and Zr) nucleosynthetic compositions of the inner part of the protoplanetary disc significantly changed around 1.5 Ma after Solar System formation due to thermal processing of dust in the protoplanetary disc. This relatively late event coincides with the beginning of chondrule formation or at least their preservation. Terrestrial planets formed subsequently by a complex accretion regime during several million years. Therefore, two scenarios are envisioned considering the reported Nd isotope composition of early planetesimals: 1) Terrestrial planets accreted mostly chondritic material similar in composition to enstatite chondrites, or 2) early planetesimals constitute substantial parts of terrestrial planets building blocks mixed with highly thermally processed material enriched in s-process, still unsampled by meteorites.

Co-auteur étranger; International audience; Komatiites and sedimentary rocks sampled during the International Continental Drilling Program (BARB1-2–3-4–5) in the Barberton greenstone belt, South Africa, were analyzed for 146Sm-142Nd systematics. Resolved negative l142Nd values (down to 7.7 ± 2.8) were identified in komatiites from the 3.48 Ga Komati Formation and this signature correlates with low Hf/Sm ratios measured in these samples. The negative l142Nd point to a source with subchondritic Sm/Nd ratio which formed during the Hadean. No analytically resolvable 142Nd anomalies were measured in crustal detritus-rich, Si-rich, Ca-Fe-rich sediments and cherts from the Buck Reef (3.42 Ga) and the Fig Tree Group (3.23–3.28 Ga). Our new measurementsare incorporated into a larger set of 147Sm-143Nd and 176Lu-176Hf data to better understand the 142,143Nd-176Hf isotope signatures in the mantle source at the time of komatiite crystallization. Our calculations show that the 142,143Nd-176Hf isotope signatures and Hf/Sm ratios cannot be produced by recycling into the komatiite source of detrital sediments like those sampled in the Barberton area. Only cherts have the required trace element characteristics – low Hf/Sm, radiogenic e176Hf –but the trace element concentrations in the cherts are so low that unrealistic amounts of chert would need to be added. We propose a four-stage model for the formation of these rocks. Negative l142Nd and low Hf/Sm ratios developed during the crystallization of a deep magma ocean soon after Earth accretion. The material that ultimately became the source of komatiites was a residual liquid produced by 50% crystallization leaving a bridgmanite/ferropericlase/Ca-perovskite cumulate. The Lu-Hf and Sm-Nd isotope systems were decoupled at this stage. After extinction of 146Sm around 4 Ga, parent/daughter ratios fractionated during a melt extraction event. With this model we explain the positive e176Hf and slightly negative e143Nd in these samples. The 3.55 Ga Schapenburg komatiites in another part of the Barberton belt share similar chemical signatures, supporting our model of fractionation in a deep magma ocean early in Earth’s history.

International audience

Co-auteur étranger; International audience; Anastomosing branching and wavy laminae in quartz-rich sandstones of the 3.22 Ga Moodies Group in the Barberton Greenstone Belt (BGB), South Africa, have been extensively described as fossilized microbial mats developed in terrestrial to marine transitional environments. Petrological and geochemical characteristics of a ~ 350 m thick Moodies succession in the Saddleback Syncline in the central BGB were analyzed to reconstruct thepost-depositional history of the sediments and to better constrain the origin of the laminae. The studied sandstones are composed mainly of quartz, potassium feldspar, and chert clastic grains with various proportions of microquartz and sericite cements. In coastal floodplain and inter- to supra-tidal environments, quiescent periods with low current velocity allowed the repeated deposition of thin (

International audience; This study investigates the partitioning of rare earth elements (REE) from La to Gd between molten metal and silicate to evaluate potential fractionation occurring during core-mantle differentiation. We report molten metal-silicate liquid partition coefficients from 24 multi-anvil experiments, extending the range of pressure, previously ranging from 1 to 8 GPa, up to 14 GPa. Experiments were performed at temperatures of between 2300 and 2560 K, and for oxygen fugacities ranging from the IW (Iron-Wüstite buffer) to IW–4. Metal-silicate partition coefficients for the studied REE vary with the oxygen fugacity and S concentration in the metallic phase of the system. These elements were all lithophile during the Earth's accretion. By compiling all existing data on molten metal-silicate liquid partitioning, REE partitioning between the mantle and core during the Earth's accretion can be determined for a wide range of P, T and fo2 conditions representing the early evolution of planetary bodies from planetesimals to planets. REE concentrations of the bulk silicate Earth (BSE) are calculated from accretion scenarios using varying proportions and compositions of chondritic building blocks. The models selected are those that reproduce the Earth's nucleosynthetic isotope signature and the Ni/Co, Th/U and Nb/Ta ratios of the BSE. The BSE refractory element enrichment factor determined from REE data is equal to 2.88 (relative to CI chondrites). This calculation takes into account the depletion in volatile elements in the Earth compared to chondrites. This new estimate is in good agreement with previous determinations based on analysis of the upper mantle rocks, which supports the idea of a chemically homogeneous mantle. We also confirm that the formation of the core, with or without segregation of a sulfide phase, does not fractionate Sm/Nd and cannot be responsible for the 142Nd excess measured in modern terrestrial samples relative to chondrites.

International audience

The 146Sm–142Nd extinct decay scheme (146Sm half-life of 103 My) is a powerful tool to trace early Earth silicate differentiation. Differences in 142Nd abundance measured between different chondrite meteorite groups and the modern Earth challenges the interpretation of the 142Nd isotopic variations found in terrestrial samples because the origin of the Earth and the nature of its building blocks is still an ongoing debate. As bulk meteorites, the enstatite chondrites (EC) have isotope signatures that are the closest to the Earth value with an average small deficit of ∼10 ppm in 142Nd relative to modern terrestrial samples. Here we review all the Nd isotope data measured on EC so far, and present the first measurements on an observed meteorite fall Almahata Sitta containing pristine fragments of an unmetamorphosed enstatite chondrite belonging to the EL3 subgroup. Once 142Nd/144Nd ratios are normalized to a common chondritic evolution, samples from the EC group (both EL and EH) have a deficit in 142Nd but the dispersion is important (µ142 Nd= -10 ± 12 (2SD) ppm). This scatter reflects their unique mineralogy associated to their formation in reduced conditions (low fO2 or high C/O). Rare-earth elements are mainly carried by the sulfide phase oldhamite (CaS) that is more easily altered than silicates by weathering since most of the EC meteorites are desert finds. The EL6 have fractionated rare-earth element patterns with depletion in the most incompatible elements. Deviations in Nd mass independent stable isotope ratios in enstatite chondrites relative to terrestrial standard are not resolved with the level of analytical precision achieved by modern mass spectrometry techniques. Here we show that enstatite chondrites from the EL3 and EL6 subgroups may come from different parent bodies. Samples from the EL3 subgroup have Nd (µ142Nd= -0.8 ± 7.0, 2SD) and Ru isotope ratios undistinguishable from that of the Bulk Silicate Earth. EL3 samples have never been analyzed for Mo isotopes. Because these enstatite chondrites are relatively small in size and number, they are usually not available for destructive isotopic measurements. Average values based on the measurement of EL6 samples should not be considered as representative of the whole EL group because of melting and thermal metamorphism events affecting the Sm/Nd ratios and prolonged open-system history. The EL3 chondrites are the best candidates as the Earth's building blocks. These new results remove the need to change the composition of refractory incompatible elements early in Earth's history.

Details regarding the early evolution of the mantle-crust system are still poorly constrained due to the great scarcity of >3.7 Ga rocks in the geological record. The Napier complex (East Antarctica) is an Eoarchean craton that contains some of Earth’s oldest rocks. This complex recorded Meso- and Neoarchean metamorphism that reached extreme conditions corresponding to granulite facies at 2.5 Ga (1050-1120°C and 7-11 kbar). As a consequence, most samples exhibit disturbed radiogenic isotope systematics (e.g., Rb-Sr, Sm-Nd) and zircon crystals found in such samples are very complex rendering isotopic systematics interpretations challenging. The analytical methods employed in previous studies do not allow these complexities to be understood, which motivated the present contribution.Here we studied two granulitic orthogneisses labelled 78285007 (Mount Sones) and 78285013 (Gage Ridge) that correspond to the oldest available rocks from the Napier Complex. Mount Sones displays typical characteristics of Archean tonalite-trondhjemite-granodiorite (TTG) suites (e.g., high Na2O/K2O, high Sr/Y, fractionated REE patterns with low heavy REE concentrations) with a normative composition intermediate between tonalite and trondhjemite whereas Gage Ridge has a composition closer to that of granite despite a strongly fractionated REE pattern and a pronounced positive Eu anomaly. We have conducted zircon texture assessment using cathodoluminescence and back-scattered electron images in annealed and not annealed crystals. We have subsequently combined U-Pb age profiling by laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) and Lu-Hf isotope systematics measurement by LA-MC-ICP-MS in these zircon crystals. Finally, we analysed 146,147Sm-143,142Nd isotopesystematics in corresponding whole-rock samples to better constrain the early history of their source.Our results reveal that Mount Sones and Gage Ridgeorthogneisses formed at 3794 ± 40 and 3857 ± 39 Ma, respectively, with initial ɛHf of -2.6 ± 1.5 and -3.6 ± 2.5, respectively. Sm-Nd isotope measurements indicate a μ142Nd of -8.7 ± 3.9 and a ɛNd of -2.0 ± 0.3 at 3794 Ma for Mount Sones, whereas Gage Ridge exhibits a μ142Nd of -12.1 ± 6.2 and a disturbed 147Sm-143Nd systematics. Taken altogether our results indicate that the oldest granitoids of the Napier Complex formed by reworking of 4456-4356 Ma mafic protocrust(s). Our inferred petrogenesis is similar to what has been proposed for other Eoarchean terranes worldwide (e.g., Itsaq Gneiss Complex, the Acasta Gneiss Complex, the Nuvvuagittuq Supracrustal Belt, and the North China craton). We propose that Hadean protocrusts were massively reworked in the Eoarchean to form cratons which, in turn, would account for both the absence of Hadean crust in the geological record and its little influence throughout the Archean despite crustal growth models proposing that ≤ 25% of present-day volume of continental crust was formed by the end of the Hadean.

We present the first stable chromium isotopic data for a suite of ocean island basalts (OIB) in order to investigate the Cr isotope fractionation during major igneous processes such as partial melting and fractional crystallisation. Twenty-one basaltic samples from Fangataufa Island (Tuamotu Archipelago, Pacific Ocean) have been analysed for major- and trace-element concentrations, and Sr, Nd and Cr isotopic compositions. They define two distinct series: medium to high-K calc-alcaline and low to medium-K calc-alcaline. The variations in incompatible elements such as La and Yb mostly result from varying degrees of partial melting of a mixed mantle source composed of two lithologies: garnet bearing peridotite and a recycled “fertile” component. The recycled component is also identified with the Sr and Nd isotopic composition of Fangataufa basalts. In contrast, the variations in compatible element contents such as Cr and Ni are governed by fractional crystallisation of a mixture of olivine, clinopyroxene and spinel. The samples analysed in this study are also characterised by small Cr isotope variations from −0.24 to −0.17‰. The Cr-poor samples have on average lighter Cr isotopic compositions compared to Cr-rich ones. The observed variations in the low-K series can be modelled by a Rayleigh fractionation model with a fractionation factor ( Δ 53 Crcrystals-melt) of −0.010 ± 0.005‰. The fractionation is more limited than that observed in lunar basalts and two hypotheses may explain this observation: a change in crystallising phases (cpx + spinel on Earth and spinel on the Moon) and/or the difference in temperature and oxygen fugacity between the crystallisation of lunar and terrestrial basalts. The more primitive basalts from Fangataufa have an average Cr isotopic composition of −0.18 ± 0.01‰, lighter than the Cr isotopic composition of pristine mantle xenoliths. Chromium isotopes are therefore slightly fractionated during partial melting with the melts depleted in heavy Cr isotopes. The difference between silicate melts and mantle xenoliths indicates that partial melting could produce small but resolvable shift in mantle xenoliths Cr isotopic composition of up to 0.05‰. The covariation between δ 53 Cr and e 143 Nd values in Fangataufa basalts can be explained by two processes: (i) lower degrees of partial melting produce lighter Cr isotopic composition in basaltic liquids relative to their sources and (ii) melts produced by low degree of partial melting have also experienced more fractional crystallisation.

International audience; 15 We present new measurements of 138 Ce/ 142 Ce and 143 Nd/ 144 Nd isotopic ratios in terrestrial and extra-16 terrestrial samples. The mean value obtained from nine chondrites defines the 138 Ce/ 142 Ce ratio of the 17 chondritic uniform reservoir (CHUR) as 0.02256577 ± 66 (2sd). MORBs and OIBs define the mantle 18 array in the εNd vs. εCe diagram to be εNd =-7.3 (± 0.5) × εCe + 0.4 (± 0.3). From MORB 19 measurements, we derive the isotopic composition of the depleted MORB mantle (DMM) to be εCe =-20 1.1 ± 0.6 (2sd). Both CHUR and a modelled early-depleted mantle reservoir plot on the mantle array. 21 Thus, the precise determination of the mantle array does not further constrain the La/Ce and Sm/Nd 22 ratios of the bulk silicate Earth (BSE; i.e., primitive mantle). The composition of 1.8 Ga upper 23 continental crust obtained from aeolian sediments is εCe = 1.8 ± 0.3 (2sd; εNd =-11.2), and that of its 24 2.2 Ga equivalent is εCe = 2.3 ± 0.3 (2sd; εNd =-17). Binary mixing models between depleted (DMM) 25 and enriched (upper crust or mafic crust composition) components do not reproduce the linear Ce-Nd 26 mantle array but plots close to the island arc basalt data. When the bulk Ce isotopic composition of the 27 2 continental crust is calculated from the range of accepted Nd isotope values and a mass-balance budget 28 of the BSE, the mixing curves are closer to the mantle array. However the calculated Ce isotopic 29 composition for the bulk crust is always less radiogenic than measurements. Adjusting the Ce-Nd 30 isotopic composition or the Ce/Nd ratio of the end-members to fully linearise the mixing curve leads to 31 unrealistic values never measured in terrestrial samples. We propose a recycling model to reconstruct 32 the mantle array with the participation of both oceanic crust and sediments in the mantle through time. 33 Cerium is a redox sensitive element, making the LaCe and Sm-Nd systematics an ideal combination to 34 investigate sediment recycling through time. In this recycling model, the most extreme EM-like 35 signatures require the involvement of oceanic sediments that formed under reduced conditions before 36 the Great Oxygenation Event at 2.4 Ga, and which are devoid of Ce elemental anomalies. 37 38 Keywords: 138 Ce/ 142 Ce; rare earth elements; chondritic bulk silicate Earth; silicate reservoirs; mantle 39 array; ocean island basalts. 40 41 42

International audience; Precambrian banded iron formations (BIF) are chemical sedimentary deposits whose trace element signatures have been widely used to interrogate the chemical composition and redox state of ancient seawater. Here we investigated trace element signatures in BIF of the 3.22 Ga Moodies Group, Barberton Greenstone Belt (South Africa), which are interbedded with near-shore siliciclastic sedimentary rocks and represent one of the oldest known shallow-water occurrences of BIF. Unusual rare earth element (REE) signatures, notably with pronounced negative Ce anomalies in shale-normalized spectra, have been previously reported for chemical sediments of the Moodies Group, which we confirm here through an expanded dataset for Moodies BIF spanning three different localities. We find negative Ce anomalies as low as 0.2 Ce/Ce* that are associated with unusual enrichment of LREE relative to HREE in the sample set. While total REE abundances and certain REE features appear strongly related to the concentration of detrital indicators (e.g., Zr), and are likely primary, other features, notably LREE enrichment, cannot be explained as a primary feature of the sediment. This is better explained by later addition of REE from a LREE-enriched but Ce-depleted fluid that generated the significant negative Ce anomalies observed in surface samples of Moodies Group BIF. This REE addition event influenced both Sm-Nd and La-Ce isotope systematics, the latter yielding an isochron of 60 +/- 32 Ma, thus constraining the timing of emplacement of the negative Ce anomalies to the past 100 Ma, possibly upon surface exposure of the Barberton Greenstone Belt to wetter conditions during the Cenozoic. Our findings constitute a cautionary tale in that even the most immobile elemental redox proxies may be more sensitive to post-depositional modification than previously thought, and demonstrate the clear advantage offered by paleoredox proxies coupled to radiometric geochronometers to enable the direct dating of ancient signals of Earth surface oxygenation.

International audience; The Moon forming giant impact marks the end of the main stage of Earth’s accretion and sets the stage for thesubsequent evolution of our planet. The giant impact theory has been the accepted model of lunar origin for 40 years,but the parameters of the impact and the mechanisms that led to the formation of the Moon are still hotly debated.Here we review the principal geochemical observations that constrain the timing and parameters of the impact, themechanisms of lunar formation, and the contemporaneous evolution of Earth. We discuss how chemical and isotopicstudies on lunar, terrestrial and meteorite samples relate to physical models and how they can be used to differentiatebetween lunar origin models. In particular, we argue that the efficiency of mixing during the collision is a key test of giantimpact models. A high degree of intra-impact mixing is required to explain the isotopic similarity between the Earthand Moon but, at the same time, the impact did not homogenize the whole terrestrial mantle, as isotopic signatures ofpre-impact heterogeneity are preserved. We summarize the outlook for the field and highlight the key advances in bothmeasurements and modeling needed to advance our understanding of lunar origin.

We present the first complete dataset of partition coefficients of Rare Earth Elements (REE) between oldhamites or molten FeS and silicate melts. Values have been determined at 1300 and 1400 °C from experiments on mixtures of a natural enstatite chondrite and sulphides powders (FeS or CaS) performed in evacuated silica tubes for different f O 2 conditions (from IW-6.9 to IW-4.1). Obtained REE partitioning values are between 0.5 and 5 for oldhamites and between 0.001 and 1 for FeS. In both sulphides, Eu and Yb are preferentially incorporated compared to neighbouring REE. X-ray Absorption Near Edge Structure measurements on Yb and Sm demonstrate the partial reduction to 2+ valence state for both elements, Yb reduction being more pronounced. Therefore, the Yb anomaly in the sulphides is interpreted to be an effect of the presence of Yb2+ in the system and the amplitude of the anomaly increases with decreasing oxygen fugacity. The obtained oldhamite/silicate melt partition coefficients patterns are unlike any of the observed data in natural oldhamites from enstatite chondrites and achondrites. In particular, the low values do not explain the observed enrichments in oldhamite crystals. However, positive Eu and Yb anomalies are observed in some oldhamites from EH chondrites and aubrites. We attribute these anomalies found in meteorites to the sole oldhamite control on REE budget. We conclude that the presence of positive Eu and Yb anomalies in oldhamites is a good indicator of their primordial character and that these oldhamites carry a condensation signature from a highly reduced nebular gas.

International audience; Hot spot lavas show a large diversity of isotope compositions resulting from the recycling ofsurface material into the convective mantle. Amongst the mantle end‐members, EM‐1 (enriched mantle)is widely debated and scenarios involving either old pelagic sediments subducted into the deep mantle orsubcontinental lithospheric material incorporated at shallow depths are commonly evoked. We selected 12lavas from Gough Island (south Atlantic Ocean) for the measurement of 138La‐138Ce, 147Sm‐143Nd, and176Lu‐177Hf isotope systems. Results show limited ranges for ε143Nd, ε138Ce, and ε176Hf values, and Ce/Ce*do not correlate with measured isotope ratios. Cerium isotope compositions allow us to exclude thecontribution of old sedimentary material carrying a negative, elemental cerium anomaly in the mantlesource. Pelagic sediments are indeed characterized by strongly negative, elemental cerium anomalies andalso high La/Ce ratios. Modeling a primitive mantle source contaminated by 0.4% to 2.2% of different 2.5Ga‐old pelagic components is able to reproduce the lowest Ce/Ce* values. However, the cerium isotopemeasurements show ε138Ce values between −0.39 and 0.15, too low to give support to the incorporation ofrecycled pelagic sediments in the mantle source of the lavas. Our results suggest that the incorporation ofsubcontinental lithospheric material at shallow depths during the plume ascent is a more suitable modelto explain the formation of the EM‐1 component. Hafnium and Nd isotopes also support this scenario.Subcontinental lithosphere sampled via kimberlites and lamproites has isotopic compositions that plotgenerally below the mantle array, a signature that is also seen in Gough lavas.

The early evolution of planetesimals and planets can be constrained using variations in the abundance of neodymium-142 ((142)Nd), which arise from the initial distribution of (142)Nd within the protoplanetary disk and the radioactive decay of the short-lived samarium-146 isotope ((146)Sm). The apparent offset in (142)Nd abundance found previously between chondritic meteorites and Earth has been interpreted either as a possible consequence of nucleosynthetic variations within the protoplanetary disk or as a function of the differentiation of Earth very early in its history. Here we report high-precision Sm and Nd stable and radiogenic isotopic compositions of four calcium-aluminium-rich refractory inclusions (CAIs) from three CV-type carbonaceous chondrites, and of three whole-rock samples of unequilibrated enstatite chondrites. The CAIs, which are the first solids formed by condensation from the nebular gas, provide the best constraints for the isotopic evolution of the early Solar System. Using the mineral isochron method for individual CAIs, we find that CAIs without isotopic anomalies in Nd compared to the terrestrial composition share a (146)Sm/(144)Sm-(142)Nd/(144)Nd isotopic evolution with Earth. The average (142)Nd/(144)Nd composition for pristine enstatite chondrites that we calculate coincides with that of the accessible silicate layers of Earth. This relationship between CAIs, enstatite chondrites and Earth can only be a result of Earth having inherited the same initial abundance of (142)Nd and chondritic proportions of Sm and Nd. Consequently, (142)Nd isotopic heterogeneities found in other CAIs and among chondrite groups may arise from extrasolar grains that were present in the disk and incorporated in different proportions into these planetary objects. Our finding supports a chondritic Sm/Nd ratio for the bulk silicate Earth and, as a consequence, chondritic abundances for other refractory elements. It also removes the need for a hidden reservoir or for collisional erosion scenarios to explain the (142)Nd/(144)Nd composition of Earth.

The Isua supracrustal belt (ISB) and the Nuvvuagittuq greenstone belt (NGB) are among the oldest suites of mafic volcanic rocks preserved on Earth and are the best candidates for representing its early crust. Despite the possible 500 Ma age difference between the belts, these mantle-derived rocks show compositional similarities, with features resembling rocks formed in subduction initiation environments. With the addition of new 142Nd data for the Garbenschiefer unit of the ISB reported here, high precision 142Nd data are now available for all the mafic lithologies from both belts. Mantle-derived rocks from both the ISB and NGB belts exhibit a range of 142Nd/144Nd ratios. The datasets for the two belts, however, are significantly different, suggesting a different origin for their 142Nd anomalies. Nearly all ISB samples have excesses in 142Nd, including the newly analyzed Garbenschiefer boninitic amphibolites (mean of + 12 ppm ). Excesses in 142Nd/144Nd compared to the Nd standard for all the ISB rocks range between +8 and + 20 ppm , with a near Gaussian distribution around + 12 ppm . This distribution could simply reflect the analytical error (±5 ppm) around a single 142Nd/144Nd ratio indicating that the samples formed after the extinction of 146Sm from a source with a nearly uniform 142Nd/144Nd ratio. In contrast, the NGB shows a range of 142Nd/144Nd ratios from +8 to − 18 ppm relative to the modern Nd standard and displays a flat distribution of 142Nd/144Nd ratios. The ISB samples show no significant correlation between their 142Nd/144Nd and Sm/Nd ratios, consistent with their formation in the Eoarchean via melting of a Hadean depleted mantle. In contrast, all NGB samples display a 142Nd/144Nd vs. Sm/Nd correlation, consistent with their crystallization in the Hadean. The mantle sources for both the ISB and NGB mantle-derived rocks have a similar 142Nd/144Nd ratio at the possible age of formation of the NGB (∼4.3 Ga) suggesting the derivation of ISB and NGB rocks from a common early-formed depleted mantle source formed between 4.47 and 4.42 Ga with a 147Sm/144Nd ratio ∼0.218. This mantle appears to have been an important source component involved in the formation of the primitive crust during most of the Hadean and Eoarchean eons.

International audience; Negative Cerium (Ce) anomalies are observed in chondrite-normalized Rare Earth Element patterns from various volcanic arc suites. These anomalies are well defined in volcanic rocks from the Mariana arc and have been interpreted as the result of addition of subducted sediments to the arc magma sources. This study combines 143Nd/144Nd and 138Ce/142Ce isotope measurements in Mariana volcanic rocks that have Ce anomalies ranging from 0.97 to 0.90. The dataset includes sediments sampled immediately before subduction at the Mariana Trench (Sites 801 and 802 of ODP Leg 129) and primitive basalts from the Southern Mariana Trough (back-arc basin). Binary mixing models between the local depleted mantle and an enriched end-member using both types of sediment (biosiliceous and volcaniclastic) found in the sedimentary column in front of the arc are calculated. Marianas arc lavas have Ce and Nd isotopic compositions that require

Niobium (Nb) K -edge and tantalum (Ta) L III-edge XANES spectra were acquired at the part-per-million concentration level in silicate glasses quenched from chondritic melts equilibrated at 5 GPa and under moderately to highly reducing conditions (IW-1, IW-4.5, IW-7.9). Standard materials have also been analyzed for Nb and Ta, and the data were used to construct the calibration curves of E (threshold energy) vs. valence. Under moderately reducing conditions our results are consistent with niobium and tantalum being mainly pentavalent in the silicate melts as also suggested by previous studies. We do not exclude that at IW-1, a small fraction of Nb and Ta could be reduced, leading to a mean formal valence slightly lower than five. At IW-4.5, Ta is mainly in the form Ta3+, and at IW-7.9, Ta appears to be Ta1+, whereas Nb is divalent (Nb2+). The possibility for Nb and Ta to be present in reduced forms has implications for the behavior of the two elements during the processes of differentiation on planetary bodies formed in the reduced parts of the early Solar System. Element partitioning is a function of size and valence, and our results show that high field strength elements could be reduced, which could change their chemical affinity. This may also be important for the Earth and Moon formation and early differentiation, as exemplified by the “Nb paradox.”

co-auteur étranger; International audience; Taking the example of Nd, we present a method based on a 4-mass-step acquisition scheme to measure all isotope ratios dynamically by thermal ionization mass spectrometry (TIMS); the aim being to minimize the dependency of all mass fractionation-corrected ratios on collector efficiencies and amplifier gains. The performance of the method was evaluated from unprocessed JNdi-1 Nd standards analyzed in multiple sessions on three different instruments over a period of ~ 1.5 years (n = 61), as well as from standards (18 JNdi-1 and 19 BHVO-2) processed through different chemical purification procedures. The Nd isotopic compositions of standards processed through fine-grained (25–50 μm) Ln-spec resin show a subtle but clear fractionation caused by the nuclear field shift effect. This effect contributes to the inaccuracy of Nd isotope measurements at the ppm level of precision.Following a comprehensive evaluation of the mass spectrometer runs, we suggest several criteria to assess the quality of data acquired by TIMS, in particular to see whether the measurements were affected by domain mixing effects on the filaments. We define maximum tolerable Ce and Sm interference corrections and the minimum number of ratios to acquire to ensure the best possible accuracy and precision for all Nd isotope ratios. Changes in fractionation of Nd isotope ratios in between acquisition steps can result in significant inaccuracy and bias dynamic μ142 values by > 15 ppm. To correct for these effects, we developed a systematic drift-correction based on the monitoring of Nd isotope ratios through time. The residual components of scatter in the JNdi-1 and BHVO-2 datasets were further investigated in binary isotopic plots in which we modeled the theoretical effects of domain mixing on filaments, nuclear field shift and correlated errors from counting statistics using Monte-Carlo simulations. These plots indicate that the 4-step method returns precisions limited by counting errors only for drift-corrected dynamic Nd isotope ratios. Data acquired on three different TIMS instruments suggest the following composition for the JNdi-1 reference standard: 142Nd/144Nd = 1.141832 ± 0.000006 (2s), 143Nd/144Nd = 0.512099 ± 0.000005 (2s), 145Nd/144Nd = 0.348403 ± 0.000003 (2s), 148Nd/144Nd = 0.241581 ± 0.000003 (2s), and 150Nd/144Nd = 0.236452 ± 0.000006 (2s) when normalized to 146Nd/144Nd = 0.7219. Measurements performed on different instruments (Triton™ vs. Triton Plus™) show resolvable differences of about 10 ppm for absolute 143Nd/144Nd, 145Nd/144Nd and 148Nd/144Nd ratios. The different criteria and corrections developed in this study could be applied to other isotopic systematics to improve and better evaluate the quality of high-precision data acquired by TIMS.

This study investigates the impact of Earth's core formation on the metal–silicate partitioning of Sm and Nd, two rare-earth elements assumed to be strictly lithophile although they are widely carried by the sulphide phases in reducing material (e.g. enstatite chondrites). The partition coefficients of Sm and Nd ( D Sm and D Nd ) between molten CI and EH chondrites model compositions and various Fe-rich alloys (in the Fe–Ni–C–Si–S system) have been determined in a multi-anvil between 3 and 26 GPa at various temperatures between 2073 and 2440 K, and at an oxygen fugacity ranging from 1 to 5 log units below the iron–wustite (IW) buffer. The chemical compositions of the run products and trace concentrations in Sm and Nd elements were determined using electron microprobe and laser ablation inductively coupled plasma-mass spectrometry. Our results demonstrate the non-fractionation of Sm and Nd during the segregation of the metallic phases: the initial Sm/Nd ratio of about 1 in the starting materials yields precisely the same ratio in the recovered silicate phases after the equilibration with the metal phases at all conditions investigated in this study. In addition, D Sm and D Nd values range between 10 − 3 and 10 − 5 representing a low solubility in the metal. An increase of the partition coefficients is observed with decreasing the oxygen fugacity, or with an increase of S content of the metallic phase at constant oxygen fugacity. Thus, based on the actual Sm and Nd concentrations in the bulk Earth, the core should contain less than 0.4 ppb for Sm and less than 1 ppb for Nd. These estimates are three orders of magnitude lower than what would be required to explain the reported 142Nd excess in terrestrial samples relative to the mean chondritic value, using the core as a Sm–Nd complementary reservoir. In other words, the core formation processes cannot be responsible for the increase of the Sm/Nd ratio in the mantle early in Earth history.

Co-auteur étranger; International audience; Exogenic contamination is of primary concern for geochemical and biological clean laboratories working with sample sizes at the nanogram or even subpicogram level. Here, we determined sixty trace elements in fifteen different types of gloves from major suppliers world-wide to evaluate whether gloves could be potential sources of contamination for routine trace element and isotope measurements. We found that all gloves contain some trace elements that can be easily mobilised in significant amounts. In weak acid at room temperature, the tested gloves released up to 17 mg of Zn, more than 1 μg of Mg, Ti, Mn, Fe, Rb, Sr, Zr, Sn, Hf and Pb and between 100 and 1000 ng of Li, Sc, V, Cr, Ni, Cu, Ga, As, Se, Y, Ag, Ba, La, Ce, Nd, Tl and Th. Vinyl gloves released lower quantities of biologically and geologically important elements, with the exception of In and Sn. Isotopic analyses indicate that all gloves share roughly the same Zn isotopic composition (average δ66Zn = +0.10 ± 0.32‰ (2s)). A single contact between glove and labware releases an average of ~ 6 ng of Zn and hence can significantly shift δ66Zn above the precision level when the amount of Zn determined is below 500 ng.

Co-auteur étranger; International audience; Accretion from primordial material and its subsequent differentiation into a planet with core and mantle are fundamental problems in terrestrial and solar system. Many of the questions about the processes, although well developed as model scenarios over the last few decades, are still open and much debated. In the early Earth, during its formation and differentiation into rocky mantle and iron-rich core, it is likely that silicate melts played an important part in shaping the Earth's main reservoirs as we know them today. Here, we review several recent results in a deep magma ocean scenario that give tight constraints on the early evolution of our planet. These results include the behaviour of some siderophile elements (Ni and Fe), lithophile elements (Nb and Ta) and one volatile element (Helium) during Earth's core formation. We will also discuss the melting and crystallization of an early magma ocean, and the implications on the general feature of core-mantle separation and the depth of the magma ocean. The incorporation of Fe2 + and Fe3 + in bridgmanite during magma ocean crystallization is also discussed. All the examples presented here highlight the importance of the prevailing conditions during the earliest time of Earth's history in determining the composition and dynamic history of our planet.

International audience; Mineral inventories of enstatite chondrites; (EH and EL) are strictly dictated by combined parameters mainly very low dual oxygen (fO2) and sulfur (fS2) fugacities. They are best preserved in the Almahata Sitta MS-17, MS-177 fragments, and the ALHA 77295 and MAC 88136 Antarctic meteorites. These conditions induce a stark change of the geochemical behavior of nominally lithophile elements to chalcophile or even siderophile and changes in the elemental partitioning thus leading to formation of unusual mineral assemblages with high abundance of exotic sulfide species and enrichment in the metallic alloys, for example, silicides and phosphides. Origin and mode of formation of these exotic chondrites, and their parental source regions could be best scrutinized by multitask research experiments of the most primitive members covering mineralogical, petrological, cosmochemical, and indispensably short-lived isotopic chronology. The magnitude of temperature and pressure prevailed during their formation in their source regions could eventually be reasonably estimated: pre- and postaccretionary could eventually be deduced. The dual low fugacities are regulated by the carbon to oxygen ratios estimated to be >0.83 and

The Earth, which is characterized by an active mantle convection coupled to plate tectonics, is unique in the Solar System. Such mecanisms obscure our understanding of the Earth’s early history 4.5 Ga ago. The evolution of our planet depends on the nature of the parent bodies that have accreted to form the Earth. Then the Earth has differentiated to form the crust, mantle and the core, as well as its outer envelopes (hydrosphere and atmosphere). It is the interactions between these different envelopes that have led and controlled the emergence of life, but also its evolution to the present day. In this contribution we will retrace the history of the Earth from its formation to the development of the first environments favorable to life. This story is told through research conducted at the Laboratoire Magmas et Volcans of the “Université Blaise Pascal” and the CNRS (Centre National de la Recherche Scientifique)., La Terre se distingue des autres corps planétaires par une dynamique mantellique active couplée à une tectonique des plaques. Ces processus rendent particulièrement difficile l’étude des premiers stades d’évolution de notre planète formée il y a plus de 4,5 milliards d’années. L’évolution de notre planète dépend de la nature des corps qui se sont accrétés pour la former puis des processus qui ont permis sa structuration chimique. La Terre s’est rapidement différenciée pour former la croûte en surface, le manteau et le noyau au centre, ainsi que ses enveloppes externes (hydrosphère et atmosphère). Ce sont les interactions entre ces différentes enveloppes qui ont conduit et contrôlé l’émergence de la vie, mais aussi son maintien et son évolution jusqu’à nos jours. Dans cette contribution nous allons retracer l’histoire de la Terre depuis sa formation jusqu’au développement des premiers environnements favorables à la vie. Cette histoire est racontée via les recherches menées au Laboratoire Magmas et Volcans de l’université Clermont Auvergne et du CNRS (Centre National de la Recherche Scientifique).

We report the results of rare earth elements (REEs) and U–Th inventory of individual minerals (oldhamite, enstatite and niningerite) in two of the most unequilibrated and primitive EH3 known so far, ALHA77295 and Sahara 97072. Under the highly reducing condition that prevailed during the formation of enstatite chondrites, REEs are mainly chalcophile and concentrated in oldhamite. The study is guided by detailed petrographic investigations of the individual minerals in chondrules, complex sulfide-metal clasts and enstatite-dominated matrices. We developed two textural parameters in order to resolve the evolution of oldhamite condensates and their residence in the solar gas prior to their accretion in the individual objects or in matrices and relate these textural features to the measured REE patterns of the individual oldhamite crystals. These textural parameters are the crystal habit of oldhamite grains (idiomorphic or anhedral) and their host assemblages. REE concentrations were measured by SIMS and LA-ICPMS. Oldhamite grains display REE enrichments (10–100 × CI). Four types of REE patterns are encountered in oldhamite in ALHA77295. In general the REE distributions cannot be assigned to a specific oldhamite-bearing assemblage. The most represented REE pattern is characterized by both slight to large positive Eu and Yb anomalies and is enriched in light REEs relative to heavy REEs. This pattern is present in 97% of oldhamite in Sahara 97072, suggesting a different source region in the reduced part of the nebula or different parental EH asteroids for the two EH3 chondrites. Different parental asteroids are also supported by MgS–FeS zoning profiles in niningerite grains adjacent to troilite revealing both normal and reverse zoning trends and different MnS contents. The observed homogeneity of REE distribution in oldhamite grains in Sahara 97072 is not related to the mild metamorphic event identified in this meteorite that caused breakdown of the major K- and Rb-bearing sulfide (djerfisherite). REE concentrations in enstatite range between 0.2 and 8 × CI. Hence, enstatite is an important REE host next to oldhamite. Most patterns are characterized by negative Eu and Yb anomalies. Niningerites are negligible contributors to bulk EH3 REE inventory. Average positive Eu and Yb anomalies observed in most oldhamite are complimentary to the negative ones in enstatite thus explaining the flat patterns of the bulk meteorites. The condensation calculations based on cosmic abundances predict that the first oldhamite condensates should have flat REE patterns with Eu and Yb depletions since Eu and Yb condense at lower temperature than other REE. However, this pattern is seen in enstatite. Our findings are at odds with the predicted negative Eu and Yb anomalies in oldhamite earliest condensates from a closed system in a reduced solar source. Our petrographic, mineral chemistry and REE abundances of oldhamite, enstatite and niningerite discards an origin of oldhamite by impact melting ( Rubin et al., 2009 ). Our results do not support in first order the scenario of the incorporation of REE in the Earth’s core to explain 142Nd excess in terrestrial samples relative to chondrites because oldhamite is the major REE carrier phase and has super-chondritic Sm/Nd ratios.

The short-lived 146 Sm– 142 Nd chronometer ( T 1/2 = 103 Ma) is used to constrain the early silicate evolution of planetary bodies. The composition of bulk terrestrial planets is then considered to be similar to that of primitive chondrites that represent the building blocks of rocky planets. However for many elements chondrites preserve small isotope differences. In this case it is not always clear to what extent these variations reflect the isotope heterogeneity of the protosolar nebula rather than being produced by the decay of parent isotopes. Here we present Sm–Nd isotopes data measured in a comprehensive suite of enstatite chondrites (EC). The EC preserve 142 Nd/ 144 Nd ratios that range from those of ordinary chondrites to values similar to terrestrial samples. The EC having terrestrial 142 Nd/ 144 Nd ratios are also characterized by small 144 Sm excesses, which is a pure p-process nuclide. The correlation between 144 Sm and 142 Nd for chondrites may indicate a heterogeneous distribution in the solar nebula of p-process matter synthesized in supernovae. However to explain the difference in 142 Nd/ 144 Nd ratios, 20% of the p-process contribution to 142 Nd is required, at odds with the value of 4% currently proposed in stellar models. This study highlights the necessity of obtaining high-precision 144 Sm measurements to interpret properly measured 142 Nd signatures. Another explanation could be that the chondrites sample material formed in different pulses of the lifetime of asymptotic giant branch stars. Then the isotope signature measured in SiC presolar would not represent the unique s-process signature of the material present in the solar nebula during accretion.

article i nfo Sm- 142 Nd systematics Short-lived 146 Sm- 142 Nd and long-lived 147 Sm- 143 Nd chronometers have been measured in three cumulate eucrites (Binda, Moore County and Moama). The two major mineral phases (plagioclase and pyroxene) present in these achondrites are characterized by a wide range of Sm/Nd ratios that allows well-resolved Sm-Nd isochrons. This group of meteorites thus is suitable to better constrain the initial 146 Sm/ 144 Sm ratio of the solar system. Binda and Moore County give concordant ages of 4544±88 and 4542±85 Ma, respectively, with initial 143 Nd/ 144 Nd ratios slightly higher, to within error, of chondritic. These ages are in agreement with most of the radiometric ages determined on basaltic eucrites. A best estimate for the solar system initial 146 Sm/ 144 Sm ratio is obtained using the five-point regression line determined for Binda. The 146 Sm/ 144

We report new data for W concentrations, stable W isotopic compositions, high-precision 182W/184W ratios, highly siderophile element (HSE) abundances and 187Re–187Os systematics in a suite of 3.8–3.3 Ga mafic and ultramafic rocks from the Isua supracrustal belt, and the Paleoarchean terrane in the northwestern part of the belt. These data are compared with published data for 146Sm–142Nd systematics in the same samples. The samples from the Isua supracrustal belt show well resolved excesses of 182W/184W of up to ∼21 ppm, consistent with previous W isotopic data reported by Willbold et al. (2011) . While there is abundant evidence that W was mobilized in the crust accessed by the Isua supracrustal suite, the isotopic anomalies are interpreted to primarily reflect processes that affected the mantle precursors to these rocks. The origin of the 182W excesses in these rocks remains uncertain. The Isua mantle source could represent a portion of the post-core-formation mantle that was isolated from late accretionary additions (e.g., Willbold et al., 2011 ). However, the combined 182W, Re–Os isotopic systematics and HSE abundances estimated for the source of the Isua basalts are difficult to reconcile with this interpretation. The W isotope variations were more likely produced as a result of fractionation of the Hf/W ratio in the mantle during the lifetime of 182Hf, i.e., during the first 50 Ma of Solar System history. This could have occurred as a result of differentiation in an early magma ocean. The Isua suite examined is also characterized by variable 142Nd/144Nd, but the variations do not correlate with the variations in 182W/184W. Further, samples with ages between 3.8 and 3.3 Ga show gradual diminution of 142Nd anomalies until these are no longer resolved from the modern mantle isotopic composition. By contrast, there is no diminishment of 182W variability with time, suggesting different mechanisms of origin and retention of isotopic variations for these two extinct-radionuclide isotope systems. The presence of 182W isotopic anomalies in rocks as young as 3.3 Ga, implies that early-formed, high Hf/W domains survived for more than 1 Ga in the convective mantle.

Comparative planetary geochemistry provides insight into the origin and evolutionary paths of planetary bodies in the inner solar system. The eucrite and angrite achondrite groups are particularly interesting because they show evidence of early planetary differentiation. We present 147Sm-143Nd and 176Lu-176Hf analyses of eight noncumulate (basaltic) eucrites, two cumulate eucrites, and three angrites, which together place new constraints on the evolution and differentiation histories of the crusts of the eucrite and angrite parent bodies and their mantle mineralogies. The chemical compositions of both eucrites and angrites indicate similar evolutionary paths and petrogenetic models with formation and isolation of differentiated crustal reservoirs associated with segregation of ilmenite. We report a 147Sm-143Nd mineral isochron age for the Moama cumulate eucrite of 4519 ± 34 Ma (MSWD = 1.3). This age indicates protracted magmatism within deep crustal layers of the eucrite parent body lasting up to about 50 Ma after the formation of the solar system. We further demonstrate that the isotopic compositions of constituent minerals are compromised by secondary processes hindering precise determination of mineral isochron ages of basaltic eucrites and angrites. We interpret the changes in geochemistry and, consequently, the erroneous 147Sm-143Nd and 176Lu-176Hf internal mineral isochron ages of basaltic eucrites and angrites as the result of metamorphic events such as impacts (effects from pressure, temperature, and peak shock duration) on the surfaces of the eucrite and angrite parent bodies.

New measurements of 142 Nd/ 144 Nd in kimberlites, carbonatites, komatiites, ocean island basalts from Pitcairn, and mid-ocean ridge basalts from the Pacific and Indian Oceans show no deviation from the terrestrial Nd standard within an external reproducibility of 8 ppm. Measurements of mafic rocks from Isua Greenland confirm previous discoveries of an excess of between 0 and 17 ppm in the 142 Nd/ 144 Nd of these rocks. Since the terrestrial Nd standard has a 142 Nd/ 144 Nd approximately 20 ppm higher than measured in chondrites and eucrites, all terrestrial rocks so far analyzed have superchondritic 142 Nd/ 144 Nd. This requires either that the bulk silicate earth has a superchondritic Sm/Nd ratio or that the Earth experienced a global differentiation event within a couple of hundred million years of planet formation that created complementary incompatible element enriched and depleted mantle reservoirs. If that portion of the mantle involved in the production of continental crust was originally characterized by incompatible element depletion (i.e. high Sm/Nd) rather than chondritic relative abundances of refractory lithophile elements, as is commonly assumed, the mass of mantle affected by continent formation may constitute as much as 80% or more of the mantle. The abundances of K, U and Th in this early-formed depleted mantle are 60–70% of those calculated for a “chondritic” mantle, which removes the inference that half of the 40 Ar produced over Earth history must still reside in the mantle and explains why the mantle component characterized by high 3 He/ 4 He has superchondritic Nd and Hf isotopic composition. If these concentrations characterize the whole mantle, this implies a bulk-earth Urey ratio of ∼ 0.3 and hence that much of the heat escaping the Earth's interior is primordial and not the result of radioactive decay over Earth history. If the bulk silicate earth is presumed to have chondritic abundances of refractory lithophile elements, there must be an incompatible element enriched reservoir buried deep enough in to mantle to preclude its involvement in mantle convection and its contribution to surface volcanism because no terrestrial rock with 142 Nd/ 144 Nd less than chondritic has been identified.

New high-precision samarium-neodymium isotopic data for chondritic meteorites show that their 142 Nd/ 144 Nd ratio is 20 parts per million lower than that of most terrestrial rocks. This difference indicates that most (70 to 95%) of Earth's mantle is compositionally similar to the incompatible element–depleted source of mid-ocean ridge basalts, possibly as a result of a global differentiation 4.53 billion years ago (Ga), within 30 million years of Earth's formation. The complementary enriched reservoir has never been sampled and is probably located at the base of the mantle. These data influence models of Earth's compositional structure and require revision of the timing of global differentiation on Earth's Moon and Mars.

New techniques of isotopic measurements by a new generation of mass spectrometers equipped with an inductively-coupled-plasma source, a magnetic mass filter, and multiple collection (MC-ICPMS) are quickly developing. These techniques are valuable because of (1) the ability of ICP sources to ionize virtually every element in the periodic table, and (2) the large sample throughout. However, because of the complex trajectories of multiple ion beams produced in the plasma source whether from the same or different elements, the acquisition of precise and accurate isotopic data with this type of instrument still requires a good understanding of instrumental fractionation processes, both mass-dependent and mass-independent. Although physical processes responsible for the instrumental mass bias are still to be understood more fully, we here present a theoretical framework that allows for most of the analytical limitations to high precision and accuracy to be overcome. After a presentation of unifying phenomenological theory for mass-dependent fractionation in mass spectrometers, we show how this theory accounts for the techniques of standard bracketing and of isotopic normalization by a ratio of either the same or a different element, such as the use of Tl to correct mass bias on Pb. Accuracy is discussed with reference to the concept of cup efficiencies. Although these can be simply calibrated by analyzing standards, we derive a straightforward, very general method to calculate accurate isotopic ratios from dynamic measurements. In this study, we successfully applied the dynamic method to Nd and Pb as examples. We confirm that the assumption of identical mass bias for neighboring elements (notably Pb and Tl, and Yb and Lu) is both unnecessary and incorrect. We further discuss the dangers of straightforward standard-sample bracketing when chemical purification of the element to be analyzed is imperfect. Pooling runs to improve precision is acceptable provided the pooled measurements are shown to be part of a single population. Second-order corrections seem to be able to improve the precision on 143 Nd/ 144 Nd measurements. Finally, after discussing a number of potential pitfalls, such as the consequence of peak shape, correlations introduced by counting statistics, and the effect of memory on double-spike methods, we describe an optimal strategy for high-precision and accurate measurements by MC-ICPMS, which involves the repetitive calibration of cup efficiencies and rigorous assessment of mass bias combined with standard-sample bracketing. We suggest that, when these simple guidelines are followed, MC-ICPMS is capable of producing isotopic data precise and accurate to better than 15 ppm.

We measured 142Nd/144Nd in metabasalts and metagabbros from the western and southern parts of the Isua Supracrustal Belt, West Greenland. The samples were selected based on field and thin section criteria so as to minimize metamorphic effects on the isotope systematics. We developed a new multi-stage chemical separation scheme and a new isotope measurement technique using multiple-collector inductively coupled plasma mass spectrometry and, upon replication of measurements, achieved a precision better than 20 ppm. Out of eight samples, three show a positive 142Nd anomaly of 30 ppm consistent with a previous measurement by Harper and Jacobsen [Nature 360 (1992) 728–732]. These samples also define a 147Sm–143Nd isochron age that agrees with the commonly accepted U–Pb zircon age of 3.78 Ga for Isua, while the non-anomalous samples show substantial dispersion. The latter lack of coherence is interpreted as indicating either isotopic heterogeneities in the Isua upper mantle or redistribution of Nd by fluids during late Archean metamorphic events. Scatter among the non-anomalous samples is also observed for the Lu–Hf isotope systematics and confirms the severity of metamorphic remobilization. If the 147Sm–143Nd and 146Sm–142Nd systems are explained by a two-stage model, it is required that the upper mantle went through a major differentiation event within a few tens of millions of years after planetary accretion. If early Archean convection decoupled the two Nd isotope systems, this necessitates that the episode of differentiation took place within no more than 150 Myr of planetary accretion. One interpretation of the high-Sm/Nd reservoir is that it represents the remnants of a stagnant lithospheric lid overlying a magma ocean. Because the lid is shattered by impacts, it is unable to sink. Alternative explanations, such as an ultra-deep magma ocean at the core–mantle boundary or massive subduction of early crust, call for the early burial of enriched material and the ubiquitous presence of a 142Nd anomaly in the early Archean mantle.

International audience; We have measured Sm–Nd systematics, including the short-lived 146Sm–142Nd chronometer, in lunar ferroan anorthositic suite (FAS) whole rocks (15415, 62236, 62255, 65315, 60025). At least some members of the suite are thought to be primary crystallization products formed by plagioclase flotation during crystallization of the lunar magma ocean (LMO). Most of these samples, except 62236, have not been exposed to galactic cosmic rays for a long period and thus require minimal correction to their 142Nd isotope composition. These samples all have measured deficits in 142Nd relative to the JNdi-1 terrestrial standard in the range −45 to −21 ppm. The range is −45 to −15 ppm once the 62236 142Nd/144Nd ratio is corrected from neutron-capture effects. Analyzed FAS samples do not define a single isochron in either 146Sm–142Nd or 147Sm–143Nd systematics, suggesting that they either do not have the same crystallization age, come from different sources, or have suffered isotopic disturbance. Because the age is not known for some samples, we explore the implications of their initial isotopic compositions for crystallization ages in the first 400 Ma of solar system history, a timing interval that covers all the ages determined for the ferroan anorthositic suite whole rocks as well as different estimates for the crystallization of the LMO. 62255 has the largest deficit in initial 142Nd and does not appear to have followed the same differentiation path as the other FAS samples. The large deficit in 142Nd of FAN 62255 may suggest a crystallization age around 60–125 Ma after the beginning of solar system accretion. This result provides essential information about the age of the giant impact forming the Moon. The initial Nd isotopic compositions of FAS samples can be matched either with a bulk-Moon with chondritic Sm/Nd ratio but enstatite-chondrite-like initial 142Nd/144Nd (e.g. 10 ppm below modern terrestrial), or a bulk-Moon with superchondritic Sm/Nd ratio and initial 142Nd/144Nd similar to ordinary chondrites.

The 147Sm–143Nd and 176Lu–176Hf systematics of 21 whole-rock eucrites, including five cumulates, have been investigated by MC-ICP-MS. A statistically significant Sm–Nd isochron was obtained on 18 samples with an age of 4464±75 Ma (MSWD=1.26) and an intercept of 0.50680±0.00010. This age clearly is controlled by the cumulate eucrites. The 21 basaltic and cumulate eucrites together do not form a statistically significant Lu–Hf isochron. Basaltic eucrites, however, form an errorchron with an age of 4604±39 Ma (MSWD=4.52), which becomes an acceptable isochron if the error on the Lu/Hf ratio is doubled with respect to the laboratory estimate. The initial 176Hf/177Hf is 0.27966±0.00002. Three of the cumulate eucrites regressed together further yield a statistically significant age of 4470±22 Ma, indistinguishable from their Sm–Nd age. We therefore conclude that cumulate eucrites are ∼100 Myr younger than basaltic eucrites and basaltic eucrites are close in age to the Solar System. There is no evidence in the present data to support a decay constant of 176Lu significantly different from 1.93 10−11 yr−1 [Sguigna, Can. J. Phys., 60 (1982) 361–364]. The broad range of variation and strong correlation of Lu/Hf and Sm/Nd ratios require that eucrites are partial melts and not residual magmas. The relative Lu/Hf and Sm/Nd fractionation is controlled by the plagioclase to mafic mineral ratio in the residue. The steep correlation between these two ratios is explained by the enhancement of plagioclase saturation in the low-gravity field of the eucrite parent body. Basaltic eucrites probably formed as large-degree melts of a chondritic source. The strong Lu/Hf and Sm/Hf fractionation observed in cumulates clearly reflects the presence of residual ilmenite during melting: our preferred interpretation is that cumulates were impregnated by small-degree melts of ilmenite-bearing gabbros. Since pressure on the eucrite parent body never reaches the garnet stability field, this observation questions the ubiquitous presence of residual garnet in the mantle sources of magmas formed on larger planets such as the Moon, Mars, and possibly Earth.