Publications by authors named "Julien Siebert"

12 Publications

  • Page 1 of 1

Early oxidation of the martian crust triggered by impacts.

Sci Adv 2020 Oct 30;6(44). Epub 2020 Oct 30.

Université de Paris, Institut de physique du globe de Paris, CNRS, 75005 Paris, France.

Despite the abundant geomorphological evidence for surface liquid water on Mars during the Noachian epoch (>3.7 billion years ago), attaining a warm climate to sustain liquid water on Mars at the period of the faint young Sun is a long-standing question. Here, we show that melts of ancient mafic clasts from a martian regolith meteorite, NWA 7533, experienced substantial Fe-Ti oxide fractionation. This implies early, impact-induced, oxidation events that increased by five to six orders of magnitude the oxygen fugacity of impact melts from remelting of the crust. Oxygen isotopic compositions of sequentially crystallized phases from the clasts show that progressive oxidation was due to interaction with an O-rich water reservoir. Such an early oxidation of the crust by impacts in the presence of water may have supplied greenhouse gas H that caused an increase in surface temperature in a CO-thick atmosphere.
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http://dx.doi.org/10.1126/sciadv.abc4941DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7608801PMC
October 2020

The niobium and tantalum concentration in the mantle constrains the composition of Earth's primordial magma ocean.

Proc Natl Acad Sci U S A 2020 11 26;117(45):27893-27898. Epub 2020 Oct 26.

Université de Paris, Institut de physique du globe de Paris, CNRS, F-75005 Paris, France.

The bulk silicate Earth (BSE), and all its sampleable reservoirs, have a subchondritic niobium-to-tantalum ratio (Nb/Ta). Because both elements are refractory, and Nb/Ta is fairly constant across chondrite groups, this can only be explained by a preferential sequestration of Nb relative to Ta in a hidden (unsampled) reservoir. Experiments have shown that Nb becomes more siderophile than Ta under very reducing conditions, leading the way for the accepted hypothesis that Earth's core could have stripped sufficient amounts of Nb during its formation to account for the subchondritic signature of the BSE. Consequently, this suggestion has been used as an argument that Earth accreted and differentiated, for most of its history, under very reducing conditions. Here, we present a series of metal-silicate partitioning experiments of Nb and Ta in a laser-heated diamond anvil cell, at pressure and temperature conditions directly comparable to those of core formation; we find that Nb is more siderophile than Ta under any conditions relevant to a deep magma ocean, confirming that BSE's missing Nb is in the core. However, multistage core formation modeling only allows for moderately reducing or oxidizing accretionary conditions, ruling out the need for very reducing conditions, which lead to an overdepletion of Nb from the mantle (and a low Nb/Ta ratio) that is incompatible with geochemical observations. Earth's primordial magma ocean cannot have contained less than 2% or more than 18% FeO since the onset of core formation.
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http://dx.doi.org/10.1073/pnas.2007982117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7668002PMC
November 2020

Iron isotope evidence for very rapid accretion and differentiation of the proto-Earth.

Sci Adv 2020 Feb 12;6(7):eaay7604. Epub 2020 Feb 12.

Institut de Physique du Globe de Paris, Université Sorbonne Paris Cité, 75005 Paris, France.

Nucleosynthetic isotope variability among solar system objects provides insights into the accretion history of terrestrial planets. We report on the nucleosynthetic Fe isotope composition (μFe) of various meteorites and show that the only material matching the terrestrial composition is CI (Ivuna-type) carbonaceous chondrites, which represent the bulk solar system composition. All other meteorites, including carbonaceous, ordinary, and enstatite chondrites, record excesses in μFe. This observation is inconsistent with protracted growth of Earth by stochastic collisional accretion, which predicts a μFe value reflecting a mixture of the various meteorite parent bodies. Instead, our results suggest a rapid accretion and differentiation of Earth during the ~5-million year disk lifetime, when the volatile-rich CI-like material is accreted to the proto-Sun via the inner disk.
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http://dx.doi.org/10.1126/sciadv.aay7604DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7015677PMC
February 2020

Examining the homeostatic distribution of metals and Zn isotopes in Göttingen minipigs.

Metallomics 2018 09;10(9):1264-1281

Institut de Physique du Globe de Paris, Université Paris Diderot, Sorbonne Paris Cité, CNRS UMR 7154, 1 rue Jussieu, 75238 Paris Cedex 05, France.

The role of metals in biologic systems is manifold, and understanding their behaviour in bodily processes, especially those relating to neurodegenerative diseases, is at the forefront of medical science. The function(s) of metals - such as the transition metals - and their utility in both the diagnosis and treatment of diseases in human beings, is often examined via the characterization of their distribution in animal models, with porcine models considered exceptional proxies for human physiology. To this end, we have investigated the homeostatic distribution of numerous metals (Mg, K, Ca, Mn, Fe, Cu, Zn, Rb and Mo), the non-metal P, and Zn isotopes in the organs and blood (red blood cells, plasma) of Göttingen minipigs. These results represent the first set of data outlining the homeostatic distribution of metals and Zn isotopes in Göttingen minipigs, and indicate a relatively homogeneous distribution of alkali/alkaline earth metals and P among the organs, with generally lower levels in the blood, while indicating more heterogeneous and systematic abundance patterns for transition metals. In general, the distribution of all elements analysed is similar to that found in humans. Our elemental abundance data, together with data reported for humans in the literature, suggest that element-to-element ratios, e.g. Cu/Mg, show potential as simple diagnostics for diseases such as Alzheimer's. Isotopic data indicate a heterogeneous distribution of Zn isotopes among the organs and blood, with the liver, heart and brain being the most depleted in heavy Zn isotopes, and the blood the most enriched, consistent with observations in other animal models and humans. The Zn isotopic composition of Göttingen minipigs displays a systematic offset towards lighter δ66Zn values relative to mice and sheep models, suggesting physiology that is more closely aligned with that of humans. Cumulatively, these observations strongly suggest that Göttingen minipigs are an excellent animal model for translational research involving metals, and these data provide a strong foundation for future research.
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http://dx.doi.org/10.1039/c8mt00179kDOI Listing
September 2018

Volatile element evolution of chondrules through time.

Proc Natl Acad Sci U S A 2018 08 6;115(34):8547-8552. Epub 2018 Aug 6.

Institut de Physique du Globe de Paris, Université Paris Diderot, Sorbonne Paris Cité, CNRS UMR 7154, 75238 Paris Cedex 05, France.

Chondrites and their main components, chondrules, are our guides into the evolution of the Solar System. Investigating the history of chondrules, including their volatile element history and the prevailing conditions of their formation, has implications not only for the understanding of chondrule formation and evolution but for that of larger bodies such as the terrestrial planets. Here we have determined the bulk chemical composition-rare earth, refractory, main group, and volatile element contents-of a suite of chondrules previously dated using the Pb-Pb system. The volatile element contents of chondrules increase with time from ∼1 My after Solar System formation, likely the result of mixing with a volatile-enriched component during chondrule recycling. Variations in the Mn/Na ratios signify changes in redox conditions over time, suggestive of decoupled oxygen and volatile element fugacities, and indicating a decrease in oxygen fugacity and a relative increase in the fugacities of in-fluxing volatiles with time. Within the context of terrestrial planet formation via pebble accretion, these observations corroborate the early formation of Mars under relatively oxidizing conditions and the protracted growth of Earth under more reducing conditions, and further suggest that water and volatile elements in the inner Solar System may not have arrived pairwise.
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http://dx.doi.org/10.1073/pnas.1807263115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6112700PMC
August 2018

An early geodynamo driven by exsolution of mantle components from Earth's core.

Nature 2016 08 18;536(7616):326-8. Epub 2016 Jul 18.

Recent palaeomagnetic observations report the existence of a magnetic field on Earth that is at least 3.45 billion years old. Compositional buoyancy caused by inner-core growth is the primary driver of Earth's present-day geodynamo, but the inner core is too young to explain the existence of a magnetic field before about one billion years ago. Theoretical models propose that the exsolution of magnesium oxide--the major constituent of Earth's mantle--from the core provided a major source of the energy required to drive an early dynamo, but experimental evidence for the incorporation of mantle components into the core has been lacking. Indeed, terrestrial core formation occurred in the early molten Earth by gravitational segregation of immiscible metal and silicate melts, transporting iron-loving (siderophile) elements from the silicate mantle to the metallic core and leaving rock-loving (lithophile) mantle components behind. Here we present experiments showing that magnesium oxide dissolves in core-forming iron melt at very high temperatures. Using core-formation models, we show that extreme events during Earth's accretion (such as the Moon-forming giant impact) could have contributed large amounts of magnesium to the early core. As the core subsequently cooled, exsolution of buoyant magnesium oxide would have taken place at the core–mantle boundary, generating a substantial amount of gravitational energy as a result of compositional buoyancy. This amount of energy is comparable to, if not more than, that produced by inner-core growth, resolving the conundrum posed by the existence of an ancient magnetic field prior to the formation of the inner core.
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http://dx.doi.org/10.1038/nature18594DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4998958PMC
August 2016

Core formation and core composition from coupled geochemical and geophysical constraints.

Proc Natl Acad Sci U S A 2015 Oct 21;112(40):12310-4. Epub 2015 Sep 21.

Institut de Physique du Globe de Paris, Sorbonne Paris Cité, UMR CNRS 7154, 75005 Paris, France; Lawrence Livermore National Laboratory, Livermore, CA 94550.

The formation of Earth's core left behind geophysical and geochemical signatures in both the core and mantle that remain to this day. Seismology requires that the core be lighter than pure iron and therefore must contain light elements, and the geochemistry of mantle-derived rocks reveals extensive siderophile element depletion and fractionation. Both features are inherited from metal-silicate differentiation in primitive Earth and depend upon the nature of physiochemical conditions that prevailed during core formation. To date, core formation models have only attempted to address the evolution of core and mantle compositional signatures separately, rather than seeking a joint solution. Here we combine experimental petrology, geochemistry, mineral physics and seismology to constrain a range of core formation conditions that satisfy both constraints. We find that core formation occurred in a hot (liquidus) yet moderately deep magma ocean not exceeding 1,800 km depth, under redox conditions more oxidized than present-day Earth. This new scenario, at odds with the current belief that core formation occurred under reducing conditions, proposes that Earth's magma ocean started oxidized and has become reduced through time, by oxygen incorporation into the core. This core formation model produces a core that contains 2.7-5% oxygen along with 2-3.6% silicon, with densities and velocities in accord with radial seismic models, and leaves behind a silicate mantle that matches the observed mantle abundances of nickel, cobalt, chromium, and vanadium.
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http://dx.doi.org/10.1073/pnas.1505672112DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4603515PMC
October 2015

Nonlinearity of local dynamics promotes multi-chimeras.

Chaos 2015 Aug;25(8):083104

Institut für Theoretische Physik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany.

Chimera states are complex spatio-temporal patterns in which domains of synchronous and asynchronous dynamics coexist in coupled systems of oscillators. We examine how the character of the individual elements influences chimera states by studying networks of nonlocally coupled Van der Pol oscillators. Varying the bifurcation parameter of the Van der Pol system, we can interpolate between regular sinusoidal and strongly nonlinear relaxation oscillations and demonstrate that more pronounced nonlinearity induces multi-chimera states with multiple incoherent domains. We show that the stability regimes for multi-chimera states and the mean phase velocity profiles of the oscillators change significantly as the nonlinearity becomes stronger. Furthermore, we reveal the influence of time delay on chimera patterns.
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http://dx.doi.org/10.1063/1.4927829DOI Listing
August 2015

Dynamics of reaction-diffusion patterns controlled by asymmetric nonlocal coupling as a limiting case of differential advection.

Phys Rev E Stat Nonlin Soft Matter Phys 2014 May 14;89(5):052909. Epub 2014 May 14.

Technische Universität, Institut für Theoretische Physik, Hardenberstrasse 36, 10623 Berlin, Germany.

A one-component bistable reaction-diffusion system with asymmetric nonlocal coupling is derived as a limiting case of a two-component activator-inhibitor reaction-diffusion model with differential advection. The effects of asymmetric nonlocal couplings in such a bistable reaction-diffusion system are then compared to the previously studied case of a system with symmetric nonlocal coupling. We carry out a linear stability analysis of the spatially homogeneous steady states of the model and numerical simulations of the model to show how the asymmetric nonlocal coupling controls and alters the steady states and the front dynamics in the system. In a second step, a third fast reaction-diffusion equation is included which induces the formation of more complex patterns. A linear stability analysis predicts traveling waves for asymmetric nonlocal coupling, in contrast to a stationary Turing patterns for a system with symmetric nonlocal coupling. These findings are verified by direct numerical integration of the full equations with nonlocal coupling.
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http://dx.doi.org/10.1103/PhysRevE.89.052909DOI Listing
May 2014

Terrestrial accretion under oxidizing conditions.

Science 2013 Mar 10;339(6124):1194-7. Epub 2013 Jan 10.

Institut de Minéralogie et de Physique des Milieux Condensés, Université Pierre et Marie Curie, UMR CNRS 7590, Institut de Physique du Globe de Paris, 75005 Paris, France.

The abundance of siderophile elements in the mantle preserves the signature of core formation. On the basis of partitioning experiments at high pressure (35 to 74 gigapascals) and high temperature (3100 to 4400 kelvin), we demonstrate that depletions of slightly siderophile elements (vanadium and chromium), as well as moderately siderophile elements (nickel and cobalt), can be produced by core formation under more oxidizing conditions than previously proposed. Enhanced solubility of oxygen in the metal perturbs the metal-silicate partitioning of vanadium and chromium, precluding extrapolation of previous results. We propose that Earth accreted from materials as oxidized as ordinary or carbonaceous chondrites. Transfer of oxygen from the mantle to the core provides a mechanism to reduce the initial magma ocean redox state to that of the present-day mantle, reconciling the observed mantle vanadium and chromium concentrations with geophysical constraints on light elements in the core.
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http://dx.doi.org/10.1126/science.1227923DOI Listing
March 2013

Spin crossover in ferropericlase at high pressure: a seismologically transparent transition?

Science 2011 Jan;331(6013):64-7

Institut de Minéralogie et de Physique des Milieux Condensés, UMR CNRS 7590, Institut de Physique du Globe de Paris, Université Pierre et Marie Curie, Université Paris Diderot, 75005 Paris, France.

Seismic discontinuities in Earth typically arise from structural, chemical, or temperature variations with increasing depth. The pressure-induced iron spin state transition in the lower mantle may influence seismic wave velocities by changing the elasticity of iron-bearing minerals, but no seismological evidence of an anomaly exists. Inelastic x-ray scattering measurements on (Mg(0.83)Fe(0.17))O-ferropericlase at pressures across the spin transition show effects limited to the only shear moduli of the elastic tensor. This explains the absence of deviation in the aggregate seismic velocities and, thus, the lack of a one-dimensional seismic signature of the spin crossover. The spin state transition does, however, influence shear anisotropy of ferropericlase and should contribute to the seismic shear wave anisotropy of the lower mantle.
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http://dx.doi.org/10.1126/science.1198429DOI Listing
January 2011