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 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

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

Metal-silicate silicon isotopic fractionation and the composition of the bulk Earth

F. M. acknowledges funding from the European Research Council under the H2020 framework program/ERC grant agreement (#637503-Pristine).PostprintPeer reviewe

Titanium isotopes as a tracer for the plume or island arc affinity of felsic rocks

F.M. acknowledges funding from the European Research Council (ERC) under Horizon 2020 Framework Programme/ERC Grant Agreement 637503 (Pristine). F.M. and M.C. acknowledge the financial support of the UnivEarthS Labex Program at Sorbonne Paris Cité (Grants ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). Parts of this work were supported by IPGP Plateau d’Analyse haute Résolution (PARI) and by Region Île-de-France Sesame Grant 12015908.Indirect evidence for the presence of a felsic continental crust, such as the elevated 49Ti/47Ti ratios in Archean shales, has been used to argue for ongoing subduction at that time and therefore plate tectonics. However, rocks of intermediate to felsic compositions can be produced in both plume and island arc settings. The fact that Ti behaves differently during magma differentiation in these two geological settings might result in contrasting isotopic signatures. Here, we demonstrate that, at a given SiO2 content, evolved plume rocks (tholeiitic) are more isotopically fractionated in Ti than differentiated island arc rocks (mainly calc-alkaline). We also show that the erosion of crustal rocks from whether plumes (mafic in average) or island arcs (intermediate in average) can all produce sediments having quite constant 49Ti/47Ti ratios being 0.1–0.3 per mille heavier than that of the mantle. This suggests that Ti isotopes are not a direct tracer for the SiO2 contents of crustal rocks. Ti isotopes in crustal sediments are still a potential proxy to identify the geodynamical settings for the formation of the crust but only if combined with additional SiO2 information.PostprintPeer reviewe

Earth’s evolving geodynamic regime recorded by titanium isotopes

Earth’s mantle has a two-layered structure, with the upper and lower mantle domains separated by a seismic discontinuity at about 660 km (refs.). The extent of mass transfer between these mantle domains throughout Earth’s history is, however, poorly understood. Continental crust extraction results in Ti-stable isotopic fractionation, producing isotopically light melting residues. Mantle recycling of these components can impart Ti isotope variability that is trackable in deep time. We report ultrahigh-precision ⁴⁹Ti/⁴⁷Ti ratios for chondrites, ancient terrestrial mantle-derived lavas ranging from 3.8 to 2.0 billion years ago (Ga) and modern ocean island basalts (OIBs). Our new Ti bulk silicate Earth (BSE) estimate based on chondrites is 0.052 ± 0.006‰ heavier than the modern upper mantle sampled by normal mid-ocean ridge basalts (N-MORBs). The ⁴⁹Ti/⁴⁷Ti ratio of Earth’s upper mantle was chondritic before 3.5 Ga and evolved to a N-MORB-like composition between approximately 3.5 and 2.7 Ga, establishing that more continental crust was extracted during this epoch. The +0.052 ± 0.006‰ offset between BSE and N-MORBs requires that <30% of Earth’s mantle equilibrated with recycled crustal material, implying limited mass exchange between the upper and lower mantle and, therefore, preservation of a primordial lower-mantle reservoir for most of Earth’s geologic history. Modern OIBs record variable ⁴⁹Ti/⁴⁷Ti ratios ranging from chondritic to N-MORBs compositions, indicating continuing disruption of Earth’s primordial mantle. Thus, modern-style plate tectonics with high mass transfer between the upper and lower mantle only represents a recent feature of Earth’s history.ISSN:0028-0836ISSN:1476-468

Impact Induced Oxidation and Its Implications for Early Mars Climate

Abstract H2 in a CO2 atmosphere may serve as a potential solution to the early Mars climate paradox, but its unknown sources cast doubts on the proposed mechanism. Impact cratering is an energetic process that may modify the surface redox budget. Here, we investigate the potential influence of impact‐related melt oxidation and serpentinization on global climate conditions. We show that impact melt and the projectile's significant oxidizing potential during basin‐forming impacts (Basin size ≥1,250 km) result in sufficient H2 to raise the global mean temperature to above 273K, which lasts for up to 105 − 106 yr considering rate‐limited regime. Impact‐induced serpentinization has limited consequences on the global climate in comparison. Episodic warming after large impacts may have enabled the presence of liquid water for up to several million years in the Noachian, resulting in the chemical evolution of the planet's surface co‐evolving with the planetary atmosphere in an episodic manner

Hydrophobic Agglomeration of Fine Pyrite Particles Induced by Flotation Reagents

Flotation reagents can change the surface properties of minerals, leading to differences in the interaction between mineral particles and affecting the mutual aggregation or dispersion of particles. In this work, we studied the role of activator copper sulfate, collector butyl xanthate and frother terpineol in adjusting the potential energy of pyrite particles from the perspective of the interfacial interaction. We evaluated the surface characteristics using contact angle analysis and zeta potential measurements under different reagents. A microscope was used to observe aggregation state of particles. The hydrophobic agglomeration kinetics of pyrite was studied through the turbidity meter measurement, and the interaction energy between pyrite particles was calculated using the extended-Derjaguin-Landau-Verwey-Overbeek (extended-DLVO) theory. The results showed that the repulsive potential energy is dominant among pyrite particles in aqueous suspensions and that the particles are easy to disperse. Flotation reagents can effectively reduce the repulsive energy between pyrite particles and increase the attraction energy between particles, which is conducive to the hydrophobic agglomeration of fine pyrite. Reagent molecules can greatly reduce the electrostatic repulsion potential energy of the pyrite particles&rsquo; interface, increase the hydrophobic attraction potential energy between the particle interfaces, and its size is 2 orders of magnitude larger than the van der Waals attraction potential energy, which is the main reason for induced the agglomeration of fine pyrite and is conducive to the flotation recovery of fine pyrite. Generally, the order in which the reduction of pyrite agglomeration was affected by the additions of flotation reagents was butyl xanthate &gt; terpineol &gt; copper sulfate