Hydrated phases encountered in meteorites are considered as clues of the earliest interactions between their primary components and water in the solar system. We ran hydrothermal experiments associated with thermochemical modeling to constrain the alteration processes, especially the reaction pathways toward hydrated phases on the parent body(ies) from which the primitive meteorites (chondrites) come from. These parent bodies first accreted rocks and ices. We focused on the early stage after ice melting at moderate temperature and low oxygen fugacity to mimic the alteration conditions present on the CM parent body. Synthetic chondritic mixtures made of olivine (Fo 90), GEMS-like material, pyrite, and alpha-iron were reacted at T = 80 degrees C for a time period of 128 days at a water-rock mass ratio of 10. Three kinds of solutions were used in order to investigate the interactions between mineral phases and H2O, NH3, or CO2 ices, respectively. According to scanning electron microscopy (SEM) observations, Raman, X-ray photoelectron spectroscopy (XPS), and X-ray characterizations, secondary phases formed were, respectively, spinel iron oxide, magnetite, magnesium silicate hydroxide, and iron sulfide in the case of saline solution and goethite, sulfur, and ferrotochilinite +/- dolomite in the case of ammonia or carbonate solution. Thermochemical calculations, validated by experiments, simulated this complex natural history. These findings help to unravel the pathways of the alteration processes in CM chondrites.

The petrologic and geochemical diversity of meteorites is a function of the bulk composition of their parent bodies, but also the result of how and when internal differentiation took place. Here we focus on this second aspect considering the two principal parameters involved: size and accretion time of the body. We discuss the interplay of the various time scales related to heating, cooling and drainage of silicate liquids. Based on two phase flow modeling in 1-D spherical geometry, we show that drainage time is proportional to two independent parameters: μm/R2, the ratio of the matrix viscosity to the square of the body radius and μf/a2, the ratio of the liquid viscosity to the square of the matrix grain size. We review the dependence of these properties on temperature, thermal history and degree of melting, demonstrating that they vary by several orders of magnitude during thermal evolution. These variations call into question the results of two phase flow modeling of small body differentiation that assume constant properties. For example, the idea that liquid migration was efficient enough to remove 26Al heat sources from the interior of bodies and dampen their melting (e.g. Moskovitz and Gaidos, 2011; Neumann et al., 2012) relies on percolation rates of silicate liquids overestimated by six to eight orders of magnitude. In bodies accreted during the first few million years of solar-system history, we conclude that drainage cannot prevent the occurrence of a global magma ocean. These conditions seem ideal to explain the generation of the parent-bodies of iron meteorites. A map of the different evolutionary scenarios of small bodies as a function of size and accretion time is proposed.

The petrologic and geochemical diversity of meteorites is a function of the bulk composition of their parent bodies, but also the result of how and when internal differentiation took place. Here we focus on this second aspect considering the two principal parameters involved: size and accretion time of the body. We discuss the interplay of the various time scales related to heating, cooling and drainage of silicate liquids. Based on two phase flow modelling in 1-D spherical geometry, we show that drainage time is proportional to two independent parameters: $\mu_m/R^2$, the ratio of the matrix viscosity to the square of the body radius and $\mu_f/a^2$, the ratio of the liquid viscosity to the square of the matrix grain size. We review the dependence of these properties on temperature, thermal history and degree of melting, demonstrating that they vary by several orders of magnitude during thermal evolution. These variations call into question the results of two phase flow modelling of small body differentiation that assume constant properties.For example, the idea that liquid migration was efficient enough to remove $^{26}$Al heat sources from the interior of bodies and dampen their melting (e.g. Moskovitz and Gaidos, 2011; Neumann et al., 2012) relies on percolation rates of silicate liquids overestimated by six to eight orders of magnitude. In bodies accreted during the first few million years of solar-system history, we conclude that drainage cannot prevent the occurrence of a global magma ocean. These conditions seem ideal to explain the generation of the parent-bodies of iron meteorites. A map of the different evolutionary scenarios of small bodies as a function of size and accretion time is proposed.

The textures of solid and molten metal in the presence of varying fractions of silicate melt at high temperature have been investigated to shed light on differentiation processes occurring in magma oceans formed on rocky bodies of the early solar system. Analogue experiments have been performed in a three-phase system (composed of coexisting metal, forsterite and silicate melt) in both static (1 GPa, 1723 K) and dynamic (i.e. agitated, at 1 bar, 1713 K and 1743 K) conditions. Micro-textures were analyzed with SEM and EBSD techniques, while mesotextures of the metallic phase were analyzed using ex-situ 3D microtomography. Although all samples exhibit the same micro-scale organization consistent with the minimization of local interfacial energies, their meso-scale textures differ significantly. Static conditions produce metal grains that have shapes close to spherical, corresponding to the state predicted by the grain-scale minimization of interfacial energies. In contrast, under dynamic conditions and in the presence of high silicate melt fractions (>= 50 vol%), molten metal coalesces to form pools with sizes that are several orders of magnitude larger than those predicted by grain growth mechanisms. Furthermore, in agreement with expectations based upon an interfacial energy budget, images show that nickel grains, whether solid or molten, do not occur surrounded entirely by silicate melt, but rather in contact with both forsterite crystals and silicate melt, leading to the formation of composite aggregates. Assuming that a magma ocean has less than 50 vol% of crystals (the upper limit that permits convective motion), thermodynamic calculations indicate that at the necessary temperatures, the metallic subsystem (Fe-NiS) of the planetesimal is entirely molten and the silicate residue is only composed of olivine. Convective motions in such a body will drive agitation, promoting the formation of composite aggregates of olivine and molten ironsulfide, their initial coalescence and subsequent fragmentation. In detail, these composite aggregates have a reduced density contrast with the surrounding silicate melt that reduces their settling velocities compared to pure metal. They also entrain olivine during the downward migration of iron-sulfide pools. Olivine grains concentrate at the surface of the metallic pools, hindering coalescence between pools or with a pre-existing core. An alternative differentiation scenario for core formation is explored in which the simple compaction of partially molten mixtures in the basal non-convecting layer of the magma ocean expels the interstitial silicate melt upward, such that the local fraction of iron-sulfide increases by mass-balance, reaching its percolation threshold and allowing core formation. This process is not only limited to early accreted planetesimals but may also occur in terrestrial bodies.

Acapulcoites and lodranites both represent partially differentiated meteorites, intermediate between pristine chondrites on one hand and fully differentiated iron meteorites / achondrites on the other hand. In primitive achondrites, metal differentiation started but did not reach completion due to their low peak temperature and limited degree of partial melting (<5 vol% for acapulcoites and up to 20 vol% for lodranites [1-2]). Both groups display similar oxygen isotope compositions, indicating a close relationship [3]. However, the petrogenetic link between them is still unclear, as well as the physical processes responsible for the aborted differentiation and the precise timing of these events. Here, we conducted a detailed mineralogical and chemical study of seven acapulcoites and lodranites using optical microscopy, SEM-EDS, EBSD and EPMA. Based on the olivine-chromite and pyroxene-pyroxene thermometers, the equilibrium temperatures could be estimated and the oxygen fugacity at which the meteorites formed has been inferred. The bulk composition of the samples suggests that the precursor was similar to reduced H chondrite material. Considering all data together, we propose a global physical model of partial differentiation that can explain the suite of acapulcoite-lodranite meteorites. The next step is to unravel the exact timing of

The fractionation of Ni isotopes during Ni coprecipitation with calcite was measured at pH = 6.2 and pCO(2) = 1 atm as a function of calcite growth rate. Light Ni isotopes are preferentially incorporated into calcite during its coprecipitation, which is likely due to a longer NiAO bond length in calcite compared to that of the Ni aquo complex. The extent of Ni isotope fractionation between Ni in the solid and the aqueous fluid phase increases from -0.3 to -0.9 parts per thousand as the calcite growth rate slows from 10(-7.3) to 10(-8.3) mol m(-2) s(-1). This behaviour can be attributed to the strong hydration of the Ni2+ aqueous ion. As mineral growth rates depend strongly on the degree of supersaturation of the fluid relative to the mineral, the results of this study suggest that the Ni isotopic composition of natural calcite can potentially provide insight into the saturation state of seawater with respect to calcite at the time that this mineral formed. In addition, calculations based on our results suggest that the incorporation of Ni into calcite could be a significant sink of light Ni in the ocean. (C) 2021 Elsevier Ltd. All rights reserved.

The fractionation of Ni isotopes during Ni adsorption from aqueous fluids onto calcite surfaces was measured at 25 degrees C and as a function of pH from 7.7 to 8.9. Experiments showed that the percent Ni adsorbed and the degree of Ni isotope fractionation attained constant values in less than 30 hours after the calcite was exposed to the Ni bearing, calcite saturated aqueous solution. The percentage of Ni adsorbed from the fluid onto the calcite surfaces increased from 9 to 67% as the pH increased over this range. Calcite preferentially adsorbs light Ni isotopes during adsorption, resulting in a fractionation between adsorbed and aqueous, Delta Ni-60(calcite-fluid), of -0.52 +/- 0.16 parts per thousand. This value is pH independent, within uncertainty, over the experimental pH range. The preferential adsorption of light Ni isotopes into calcite likely results from the change in coordination environment between adsorbed and aqueous nickel; the Ni-O length in the Ni-CO3 bond formed at the calcite surface is greater than that in the Ni2+ aquo ion. (C) 2020 Elsevier Ltd. All rights reserved.

Context: Until recently, camera networks designed for monitoring fireballs worldwide were not fully automated, implying that in case of a meteorite fall, the recovery campaign was rarely immediate. This was an important limiting factor as the most fragile - hence precious - meteorites must be recovered rapidly to avoid their alteration. Aims: The Fireball Recovery and InterPlanetary Observation Network (FRIPON) scientific project was designed to overcome this limitation. This network comprises a fully automated camera and radio network deployed over a significant fraction of western Europe and a small fraction of Canada. As of today, it consists of 150 cameras and 25 European radio receivers and covers an area of about 1.5 million square kilometers.

Understanding metal-silicate differentiation in small rocky bodies that accreted early in solar system history requires quantification of the effects of variable amounts of silicate melt and molten metal on the connectivity of metal-rich liquids. To shed light on this question, the equilibrium geometry and textural ripening of metal grains in the vicinity of the metal interconnection threshold have been determined experimentally. High pressure and temperature experiments were performed in the three-phase system forsterite + silicate melt + nickel at conditions of 1 GPa and up to 2080 K using piston-cylinder and Paris-Edinburgh presses. Sample textures were analyzed by 3D X-Ray microtomography either in-situ at the PSICHE beamline of the SOLEIL synchrotron, or on quenched samples using a laboratory Computed Tomography scan. Although dihedral angles point to textural equilibrium at the scale of individual grains at the end of each experiment, the attainment of textural equilibrium at sample scale is not straightforward. Depending on the relative proportions of the phases, different states of textural maturation are revealed. A particularly important issue is that time-resolved in-situ microtomography data show that cold (subsolidus) compression at the beginning of the experiment leads to soft metal grains being squeezed between silicates, leading to a forced interconnectivity of nickel. High temperature experiments with metal contents <= 20 vol% resulted in disruption of these forced networks, while the networks persisted in time for metal contents >= 25 vol%. This is taken to indicate that the stable interconnection threshold of pure nickel in a partially molten silicate matrix lies between 20 and 25 vol%. Therefore, we conclude that care must be taken when defining the interconnection threshold: not only should there be existence of a network of touching grains, but this network must persist in time in the absence of external forces and at pressure-temperature conditions that permit grain-boundary movement (i.e. excluding kinetically arrested systems). Growth of silicate grains is identified as the process driving textural maturation, and may explain the variability of interconnection thresholds reported in the literature. In addition, these considerations shed light on the diversity of textures observed in natural meteoritic samples (e.g. carbonaceous and ordinary chondrites and primitive achondrites), providing textural arguments to constrain the processes that affected these meteorites.

Metal-silicate fractionation of nickel isotopes has been experimentally quantified at 1623 K, with oxygen fugacities varying from 10(-8.2) to 10(-9.9) atm and for run durations from 0.5 to 1 h. Both kinetic and equilibrium fractionations have been studied. A wire loop set-up was used in which the metal reservoir is a pure nickel wire holding a silicate melt droplet of anorthite-diopside eutectic composition. During the course of the experiment, diffusion of nickel from the wire to the silicate occurred. The timescale to reach chemical equilibrium was fO(2) dependent and decreased from 17 to 1 hour, as conditions became more reducing. The isotopic composition of each reservoir was determined by Multicollector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICPMS) after Ni purification. The isotopic composition was found to be constant in the metallic wire, which therefore behaved as an infinite reservoir. On the contrary, strong kinetic fractionation was observed in the silicate melt (delta Ni down to -0.98 parts per thousand.amu(-1) relative to the standard). Isotopic equilibrium was typically reached after 24 hours. For equilibrated samples at 1623 K, no metal-silicate fractionation was observed within uncertainty (2SD), with Delta NiMetal-Silicate = 0.02 +/- 0.04 parts per thousand. amu(-1). Theoretical calculations of metal-silicate isotope fractionation at equilibrium were also performed on different metal-silicate systems. These calculations confirm (1) the absence of fractionation at high temperature and (2) a weak temperature dependence for Ni isotopic fractionation for the metal-olivine and metal-pyroxene pairs with the metal being slightly lighter isotopically. Our experimental data were finally compared with natural samples. Some mesosiderites (stony-iron meteorites) show a Delta NiMetal-Silicate close to experimental values at equilibrium, whereas others exhibit positive metal-silicate fractionation that could reflect kinetic processes. Conversely, pallasites display a strong negative metal-silicate fractionation. This most likely results from kinetic processes with Ni diffusion from the silicate to the metal phase due to a change of Ni partition coefficient during cooling. In this respect we note that in these pallasites, iron isotopes show metal-silicate fractionation that is opposite direction to Ni, supporting the idea of kinetic isotope fractionation, associated with Fe-Ni interdiffusion. (C) 2019 Elsevier Ltd. All rights reserved.

High temperature experiments have been performed to constrain interfacial energies in a three-phase system (metal–forsterite–silicate melt) representative of partially differentiated planetesimals accreted early in the solar system history, with the aim of providing new insights into the factors affecting the interconnection threshold of metal-rich phases. Experiments were run under controlled oxygen fugacity (ΔNi-NiO=−3) at 1440°C, typically for 24 h. Quantification of the true dihedral angles requires a resolution of at least 30 nm per pixel in order to reveal small-angle wedges of silicate melt at crystal interfaces. At this level of resolution, dihedral angle distributions of silicate melt and olivine appear asymmetric, an observation interpreted in terms of anisotropy of olivine crystals. Based upon the theoretical relation between dihedral angles and interfacial energies in a three-phase system, the relative magnitudes of interfacial energies have been determined to be: γMelt-Ol<γMelt-Ni<γOl-Ni. This order differs from that obtained with experiments using an iron sulfide liquid close to the Fe–FeS eutectic for which γMelt-Sulfide<γMelt-Ol<γOl-Sulfide, implying a lower interconnection threshold for sulfur-rich melts than for pure metallic phases. This dependence of the interconnection threshold on the sulfur content will affect the drainage of metallic phases during melting of small bodies. Assuming a continuous extraction of silicate melt, evolution of the metal volume fraction has been modeled. Several sulfur-rich melts extraction events are possible over a range of temperatures relevant with thermometric data on primitive achondrites (1200–1400°C and 25% of silicate melt extracted). These successive events provide novel insight into the variability of sulfur content in primitive achondrites, which are either representative of a region that experienced sulfide extraction or from a region that accumulated sulfide melt from overlying parts of the parent body.

Differentiation of small bodies (diameter < 100 km) is likely to have occurred within 3 Ma of CAI formation (e.g. [1-2]). Meteorites record various stages of this process: from undifferentiated (chondrites) to fully differentiated (iron meteorites), while the intermediate step (incipient differentiation) is typically illustrated by primitive achondrites (e.g. Acapulco & Lodran). Most studies up to now have used a two-phase system with a metallic / sulfide melt segregating in a crystalline silicate matrix. However, this simplistic model is unable to account for the meteoritic record (e.g. Acapulco and Lodran-like meteorites), as percolation velocities are too low [3] and primitive achondrites have produced up to 20 vol.% of silicate melt [4]. The addition of a third phase (silicate melt) allows the experimental charges to be more realistic; it also leads to segregation timescales in agreement with partially or fully differentiated meteorites. One way to quantify the mobility of a melt is to measure its dihedral angle, the angle at the junction of a melt with two grains. In a two-phase system the interconnection threshold (volume fraction above which a phase forms an interconnected network) can be predicted from the dihedral angles [5]. In a three-phase system, the interconnection threshold cannot be predicted, it has to be determined experimentally. In order to better understand the formation of partially differentiated meteorites and their characteristics (melt fraction extracted, presence or absence of sulfide or feldspar), the present study aims at determining equilibrium geometries (dihedral angles and interfacial energy) in a 3-phase system: a metal (molten or not), a solid silicate matrix and a silicate melt. Experiments were performed in a reduced environment, below and above the melting point of nickel (log(fO2) = -8.5, T = 1440 and 1470 ◦C) to simulate conditions before and after differentiation is triggered.

V. Debaille, S. McKibbin, S. Goderis, L. Pittarello, N. Shirai, G. Hublet, G. Quitté, T. Iizuka, R. C. Greenwood, P. Claeys, The University of Tokyo, Tokyo, Japan, National Institute of Polar Research (NIPR), Tokyo, Japan, Université Libre de Bruxelles, Brussels, Belgium, Vrije Universiteit Brussel, Brussels, Belgium, University of Vienna, Vienna, Austria, Tokyo Metropolitan University, Tokyo, Japan, IRAP, Observatoire Midi-Pyrénées, Toulouse, France, Open University, Milton Keynes, UK, E-mail: mikouchi@eps.s.u-tokyo.ac.jp.

Introduction: The isotope compositions of the three major elements iron, magnesium and silicon have been widely used to trace processes occurring during planetary accretion and differentiation. Nickel is also an element of interest as it is expected to behave similarly to iron, showing comparable condensation temperatures, yet being much less sensitive to redox processes. Thus, a combined set of Fe and Ni isotope data obtained on a suite of planetary materials will permit to disentangle volatility-related processes and redox reactions including planetary core formation. Here we present new Ni isotope data for a variety of samples (meteorites and lunar samples); most of them have been previously studied for Si isotopes [1].

Tafassasset is a primitive meteorite, the origin of which is still debated. Its possible relationship to either the CR chondrites – considered among the most primitive meteorites – or the brachinites – complex primitive achondrites – makes it an interesting sample for studying the initial stages of planetary accretion and differentiation in the early solar system. Here, we report tungsten (W) isotope data for bulk rock samples as well as for mineral fractions from Tafassasset, along with micro-computed tomography of a piece of the meteorite. Silicates show mass-independent W isotope anomalies, while the metal phase does not. These nucleosynthetic anomalies are interpreted as reflecting the presence of SiC presolar grains in the matrix of the meteorite, carrying s-process 184W. After correction of the nucleosynthetic anomalies, a correlation is observed between the 182W/184W isotope compositions and the Hf/W ratios of the different fractions. A 182Hf–182W age of ca. 2.9 Ma after CAIs is inferred from the 182Hf–182W chronometer, slightly older than other estimates based on the 53Mn–53Cr, 26Al–26Mg, and Pb/Pb chronometers, but consistent with the difference in closure temperatures of the different isotopic systems. Numerical modeling of the thermal evolution of Tafassasset indicates accretion of a parent-body less than ∼50 km in diameter, ≤1 Ma after the formation of CAIs, at a time when short-lived radio-nuclides induced metal–silicate separation and partial melting of the silicates with extraction of a basaltic component. According to our new data, Tafassasset may represent an inner part of a CR-like parent body, with a differentiation history similar to, but less severe than, that of brachinites.