Lead isotope evidence for a young formation age of the Earth–Moon system

AbstractA model of a giant impact between two planetary bodies is widely accepted to account for the Earth–Moon system. Despite the importance of this event for understanding early Earth evolution and the inventory of Earth's volatiles critical to life, the timing of the impact is poorly constrained. We explore a data-based, two-stage Pb isotope evolution model in which the timing of the loss of volatile Pb relative to refractory U in the aftermath of the giant impact is faithfully recorded in the Pb isotopes of bulk silicate Earth. Constraining the first stage Pb isotopic evolution permits calculating an age range of 4.426–4.417 Ga for the inflection in the U/Pb ratio related to the giant impact. This model is supported by Pb isotope data for angrite meteorites that we use to demonstrate volatility-driven, planetary-scale Pb loss was an efficient process during the early Solar System. The revised age is ∼100 Myr younger than most current estimates for the age of the Moon but fully consistent with recent ages for lunar ferroan anorthosite and the timing of Earth's first crust inferred from the terrestrial zircon record. The estimated loss of ∼98% of terrestrial Pb relative to the Solar System bulk composition by the end of the Moon-forming process implies that the current inventory of Earth's most volatile elements, including water, arrived during post-impact veneering by volatile-rich bodies

Major element and isotope geochemistry (Sr, Nd and Hf) of mantle derived peridotites, carbonatites and kimberlites from Canada and Greeland; insights into mantle dynamics

Nous avons étudié la composition en éléments majeurs d'une suite de péridotites provenant de la marge nord du craton Archéen de l'ouest du Groenland, ainsi que la composition isotopique (Sr, Nd et Hf) de kimberlites et de carbonatites de l'est du Canada et du sud-ouest du Groenland. Nos résultats indiquent que le manteau lithosphérique sous la région de Sarfartoq de l'Ouest du Groenland a une épaisseur d'environ 225 km et est constitué d'assemblages réfractaires d'harzburgites et de lherzolites (nombre Mg de l'olivine >0.9). Contrairement aux péridotites du craton de Kaapvaal (Afrique du Sud), les péridotites de la région de Sarfartoq ne sont pas enrichies en OPX et montrent donc plus d'affinités avec les péridotites de l'Est du Groenland (Wiedemann Fjord), de l'île de Somerset (arctique canadien) et de Tanzanie. L'étude détaillée de la composition des différentes phases minérales des péridotites suggère que le manteau lithosphérique sous la région de Sarfartoq soit stratifié comme suit: (1) une couche (70-180 km), caractérisée par une stratification interne, composée de harzburgites (avec ou sans grenats) non déformées, et (2) une couche (180-225 km) composée de lherzolites fertiles (avec CPX) porphyroclastiques. La stratification interne observée dans la couche de harzburgites non déformées (70-180 km) est reflétée par une augmentation de fertilité (par exemple diminution de l'abondance d'olivine et contenu inférieur en fostérite) avec la profondeur. La nature abrupte de la stratification semble indiquer que la formation de la racine lithosphérique s'est effectuée en au moins deux stades. La composition isotopique du Sr de cristaux d'apatite et de carbonate provenant de l'intrusion alcaline de Sarfartoq, Groenland, a été mesurée avec l'aide d'un système d'ablation laser couplé à un MC-ICP-MS. Cent sept analyses montrent des rapports 87Sr/86Sr variant de -0.7025 à -0.7031. Cette variation du rapport 87Sr/86Sr (-0.0006) est largement supérieure à la reproductibilité externe de la méthode (-0.000047; 2a). L'ampleur de l'intervalle des valeurs 87Sr/86Sr suggère que l'apatite et le carbonate ont précipité dans des conditions de déséquilibre. Il est possible que les variations isotopiques observées à l'échelle centimétrique reflètent des processus d'ordre régionaux, comme par exemple le mélange de magma carbonate provenant de différentes sources mantelliques ou, alternativement, un domaine mantellique hétérogène. Cette technique analytique fournit une approche rapide et pertinente pour déterminer l'ampleur de l'hétérogénéité isotopique dans un échantillon ou parmi différents minéraux, avec la précision analytique approchant cela obtenu par méthode TIMS conventionnelles. L'analyse des isotopes de l'Hf et les âges U-Pb de carbonatites et kimberlites provenant du Canada et Groenland, y compris la plus ancienne carbonatite connue (3 Ga), indique que ces magmas proviennent d'une source mantellique enrichie. Ce nouveau domaine mantellique, caractérisé par une composition isotopique de l'Hf non radiogénique et préservé dans le manteau profond, peu possiblement boucler le bilan de masse terrestre pour l'Hf et le Nd. Nos résultats indiquent que le manteau terrestre Archéen montrait une hétérogénéité isotopique (surtout pour l'Hf) beaucoup plus importante que le manteau actuel tel qu'échantillonné par les basaltes de types MORB et OIB. Cette hétérogénéité isotopique est semblable à l'hétérogénéité caractérisant les corps planétaires sans convection mantellique comme Mars et la Lune, et peut représenter un régime transitoire hérité de la differentiation initiale de la Terre. Notre étude confirme donc qu'une telle hétérogénéité était toujours présente aux environ de 3 Ga, mais plus élusive dans les roches juvéniles d'âge <2.7 Ga. Les nouvelles données isotopiques de l'Hf ici présentées favorisent une source et origine communes pour les carbonatites, kimberlites et certains basaltes océaniques

Probing the Protosolar Disk Using Dust Filtering at Gaps in the Early Solar System

Jupiter and Saturn formed early, before the gas disk dispersed. The presence of gap-opening planets affects the dynamics of the gas and embedded solids and halts the inward drift of grains above a certain size. A drift barrier can explain the absence of calcium aluminium rich inclusions (CAIs) in chondrites originating from parent bodies that accreted in the inner solar system. Employing an interdisciplinary approach, we use a $\mu$-X-Ray-fluorescence scanner to search for large CAIs and a scanning electron microscope to search for small CAIs in the ordinary chondrite NWA 5697. We carry out long-term, two-dimensional simulations including gas, dust, and planets to characterize the transport of grains within the viscous $\alpha$-disk framework exploring the scenarios of a stand-alone Jupiter, Jupiter and Saturn \textit{in situ}, or Jupiter and Saturn in a 3:2 resonance. In each case, we find a critical grain size above which drift is halted as a function of the physical conditions in the disk. From the laboratory search we find four CAIs with a largest size of $\approx$200$\,\mu$m. \Combining models and data, we provide an estimate for the upper limit of the $\alpha$-viscosity and the surface density at the location of Jupiter, using reasonable assumptions about the stellar accretion rate during inward transport of CAIs, and assuming angular momentum transport to happen exclusively through viscous effects. Moreover, we find that the compound gap structure in the presence of Saturn in a 3:2 resonance favors inward transport of grains larger than CAIs currently detected in ordinary chondrites.Comment: 16 pages, 10 figures, updated to match published version in Astrophysical Journa

Anatomy of rocky planets formed by rapid pebble accretion II. Differentiation by accretion energy and thermal blanketing

We explore the heating and differentiation of rocky planets that grow by rapid pebble accretion. Our terrestrial planets grow outside of the ice line and initially accrete 28\% water ice by mass. The accretion of water stops after the protoplanet reaches a mass of $0.01\,M_{\rm E}$ where the gas envelope becomes hot enough to sublimate the ice and transport the vapour back to the protoplanetary disc by recycling flows. The energy released by the decay of $^{26}$Al melts the accreted ice to form clay (phyllosilicates), oxidized iron (FeO), and a water surface layer with ten times the mass of Earth's modern oceans. The ocean--atmosphere system undergoes a run-away greenhouse effect after the effective accretion temperature crosses a threshold of around 300 K. The run-away greenhouse process vaporizes the water layer, thereby trapping the accretion heat and heating the surface to more than 6,000 K. This causes the upper part of the mantle to melt and form a global magma ocean. Metal melt separates from silicate melt and sediments towards the bottom of the magma ocean; the gravitational energy released by the sedimentation leads to positive feedback where the beginning differentiation of the planet causes the whole mantle to melt and differentiate. All rocky planets thus naturally experience a magma ocean stage. We demonstrate that Earth's small excess of $^{182}$W (the decay product of $^{182}$Hf) relative to the chondrites is consistent with such rapid core formation within 5 Myr followed by equilibration of the W reservoir in Earth's mantle with $^{182}$W-poor material from the core of a planetary-mass impactor, provided that the equilibration degree is at least 25%-50%, depending on the initial Hf/W ratio. The planetary collision must have occurred at least 35 Myr after the main accretion phase of the terrestrial planets.Comment: Version accepted for Astronomy & Astrophysic

Tracing metal–silicate segregation and late veneer in the Earth and the ureilite parent body with palladium stable isotopes

International audienceStable isotope studies of highly siderophile elements (HSE) have the potential to yield valuable insights into a range of geological processes. In particular, the strong partitioning of these elements into metal over silicates may lead to stable isotope fractionation during metal-silicate segregation, making them sensitive tracers of planetary differentiation processes. We present the first techniques for the precise determination of palladium stable isotopes by MC-ICPMS using a 106Pd-110Pd double-spike to correct for instrumental mass fractionation. Results are expressed as the per mil (‰) difference in the 106Pd/105Pd ratio (δ106Pd) relative to an in-house solution standard (Pd_IPGP) in the absence of a certified Pd isotopic standard. Repeated analyses of the Pd isotopic composition of the chondrite Allende demonstrate the external reproducibility of the technique of ±0.032‰ on δ106Pd. Using these techniques, we have analysed Pd stable isotopes from a range of terrestrial and extraterrestrial samples. We find that chondrites define a mean δ106Pdchondrite = -0.19 ± 0.05‰. Ureilites reveal a weak trend towards heavier δ106Pd with decreasing Pd content, similar to recent findings based on Pt stable isotopes (Creech et al., 2017), although fractionation of Pd isotopes is significantly less than for Pt, possibly related to its weaker metal-silicate partitioning behaviour and the limited field shift effect. Terrestrial mantle samples have a mean δ106Pdmantle = -0.182 ± 0.130‰, which is consistent with a late-veneer of chondritic material after core formation

Anatomy of rocky planets formed by rapid pebble accretion I. How icy pebbles determine the core fraction and FeO contents

We present a series of papers dedicated to modelling the accretion and differentiation of rocky planets that form by pebble accretion within the lifetime of the protoplanetary disc. In this first paper, we focus on how the accreted ice determines the distribution of iron between the mantle (oxidized FeO and FeO$_{1.5}$) and the core (metallic Fe and FeS). We find that an initial primitive composition of ice-rich material leads, upon heating by the decay of $^{26}$Al, to extensive water flow and the formation of clay minerals inside planetesimals. Metallic iron dissolves in liquid water and precipitates as oxidized magnetite Fe$_3$O$_4$. Further heating by $^{26}$Al destabilizes the clay at a temperature of around 900 K. The released supercritical water ejects the entire water content from the planetesimal. Upon reaching the silicate melting temperature of 1,700 K, planetesimals further differentiate into a core (made mainly of iron sulfide FeS) and a mantle with a high fraction of oxidized iron. We propose that the asteroid Vesta's significant FeO fraction in the mantle is a testimony of its original ice content. We consider Vesta to be a surviving member of the population of protoplanets from which Mars, Earth, and Venus grew by pebble accretion. We show that the increase in the core mass fraction and decrease in FeO contents with increasing planetary mass (in the sequence Vesta -- Mars -- Earth) is naturally explained by the growth of terrestrial planets outside of the water ice line through accretion of pebbles containing iron that was dominantly in metallic form with an intrinsically low oxidation degree.Comment: Version accepted for Astronomy & Astrophysic

Anatomy of rocky planets formed by rapid pebble accretion III. Partitioning of volatiles between planetary core, mantle, and atmosphere

Volatile molecules containing hydrogen, carbon, and nitrogen are key components of planetary atmospheres. In the pebble accretion model for rocky planet formation, these volatile species are accreted during the main planetary formation phase. For this study, we modelled the partitioning of volatiles within a growing planet and the outgassing to the surface. The core stores more than 90\% of the hydrogen and carbon budgets of Earth for realistic values of the partition coefficients of H and C between metal and silicate melts. The magma oceans of Earth and Venus are sufficiently deep to undergo oxidation of ferrous Fe$^{2+}$ to ferric Fe$^{3+}$. This increased oxidation state leads to the outgassing of primarily CO$_2$ and H$_2$O from the magma ocean of Earth. In contrast, the oxidation state of Mars' mantle remains low and the main outgassed hydrogen carrier is H$_2$. This hydrogen easily escapes the atmosphere due to the irradiation from the young Sun in XUV wavelengths, dragging with it the majority of the CO, CO$_2$, H$_2$O, and N$_2$ contents of the atmosphere. A small amount of surface water is maintained on Mars, in agreement with proposed ancient ocean shorelines, for moderately low values of the mantle oxidation. Nitrogen partitions relatively evenly between the core and the atmosphere due to its extremely low solubility in magma; the burial of large reservoirs of nitrogen in the core is thus not possible. The overall low N contents of Earth disagree with the high abundance of N in all chondrite classes and favours a volatile delivery by pebble snow. Our model of rapid rocky planet formation by pebble accretion displays broad consistency with the volatile contents of the Sun's terrestrial planets. The diversity of the terrestrial planets can therefore be used as benchmark cases to calibrate models of extrasolar rocky planets and their atmospheres.Comment: Version accepted for Astronomy & Astrophysic