Growth of calcium-aluminum-rich inclusions by coagulation and fragmentation in a turbulent protoplanetary disk: observations and modelisation

Whereas it is generally accepted that calcium-aluminum-rich inclusions (CAIs) from chondritic meteorites formed in a hot environment in the solar protoplanetary disk, the conditions of their formation remain debated. Recent laboratory studies of CAIs have provided new kind of data: their size distributions. We show that size distributions of CAIs measured in laboratory from sections of carbonaceous chondrites have a power law size distribution with cumulative size exponent between -1.7 and -1.9, which translates into cumulative size exponent between -2.5 and -2.8 after correction for sectioning. To explain these observations, numerical simulations were run to explore the growth of CAIs from micrometer to centimeter sizes, in a hot and turbulent protoplanetary disk through the competition of coagulation and fragmentation. We show that the size distributions obtained in growth simulations are in agreement with CAIs size distributions in meteorites. We explain the CAI sharp cut-off of their size distribution at centimeter sizes as the direct result from the famous fragmentation barrier, provided that CAI fragment for impact velocities larger than 10 m/s. The growth/destruction timescales of millimeter- and centimeter-sized CAIs is inversely proportional to the local dust/gas ratio and is about 10 years at 1300 K and up to 104 years at 1670K. This implies that the most refractory CAIs are expected to be smaller in size owing to their long growth timescale compared to less refractory CAIs. Conversely, the least refractory CAIs could have been recycled many times during the CAI production era which may have profound consequences for their radiometric age.Comment: Accepted in Icaru

Physical, Chemical, and Petrological Characteristics of Chondritic Materials and Their Relationships to Small Solar System Bodies

Chondrite materials with varying abundances of volatile-bearing phases are expected at the destinations for the asteroid sample-return missions Hayabusa2 and OSIRIS-REx. The targets of the missions are 162173 (1999 JU3) Ryugu and 101955 (1999 RQ36) Bennu. Spectroscopic analyses of these asteroids suggest that their surface materials are related to types 1 and 2 carbonaceous chondrites. Some studies suggest that the parent bodies of these chondrites may have also experienced some thermal and/or shock metamorphism. The physical properties of boulders at asteroid surfaces and fine particles in asteroid regoliths are consequences of the diverse processes that fragmented them, mobilized them, and redeposited them in unique accumulations. Sample-return missions are likely to encounter a broad range of carbonaceous chondrite (CC)-like materials, to which aqueous alteration, thermal, and shock metamorphism imparted changes affecting their sub-micron- to meter-scale physical properties. Consequently, implementation of scale-dependent analytical techniques to the study of the chemical, physical, and geotechnical characteristics of these CC-like materials is fundamental to safe mission operations, sample selection, and return. However, most of the available knowledge for informing and formulating expectations about regolith processes, products, and properties on carbonaceous small bodies comes from missions that studied anhydrous (e.g., Itokawa studied by Hayabusa) and/or much larger asteroids (e.g., Vesta studied by Dawn). No previous mission is likely directly relevant to small ice-free carbonaceous NEOs 162173 Ryugu or 101955 Bennu, although the Rosetta Spaceraft performed a flyby of the large asteroid Lutetia which has variously been classified as M and C type (Ptzold et al., 2011). Carbonaceous chondrites carry the best record of the history, distribution, and activity of water in the early solar system. Ordinary and Enstatite chondrites carry only partial records, but these are still critical to understanding the full story. We will describe the records of water-rock interactions on asteroids, as recorded in these meteorites, with particular emphasis on the timing, nature, settings, and fluid compositions. An integral part of this story is the rare, but fortunate, preservation of actual early solar system water as aqueous fluid inclusions

Elephant Moraine 96029, a very mildly aqueously altered and heated CM carbonaceous chondrite: Implications for the drivers of parent body processing

Elephant Moraine (EET) 96029 is a CMcarbonaceous chondrite regolith breccia with evidence for unusually mild aqueous alteration, a later phase of heating and terrestrial weathering. The presence of phyllosilicates and carbonates within chondrules and the fine-grained matrix indicates that this meteorite was aqueously altered in its parent body. Features showing that water-mediated processing was arrested at a very early stage include a matrix with a low magnesium/iron ratio, chondrules whose mesostasis contains glass and/or quench crystallites, and a gehlenite-bearing calcium- and aluminium-rich inclusion. EET 96029 is also rich in Fe,Ni metal relative to other CM chondrites, and more was present prior to its partial replacement by goethite during Antarctic weathering. In combination, these properties indicate that EET 96029 is one of the least aqueously altered CMs yet described (CM2.7) and so provides new insights into the original composition of its parent body. Following aqueous alteration, and whilst still in the parent body regolith, the meteorite was heated to ~400–600 °C by impacts or solar radiation. Heating led to the amorphisation and dehydroxylation of serpentine, replacement of tochilinite by magnetite, loss of sulphur from the matrix, and modification to the structure of organic matter that includes organic nanoglobules. Significant differences between samples in oxygen isotope compositions, and water/hydroxyl contents, suggests that the meteorite contains lithologies that have undergone different intensities of heating. EET 96029 may be more representative of the true nature of parent body regoliths than many other CM meteorites, and as such can help interpret results from the forthcoming missions to study and return samples from C-complex asteroids

Modal mineralogy of CI and CI-like chondrites by X-ray diffraction

The CI chondrites are some of the most hydrated meteorites available to study, making them ideal samples with which to investigate aqueous processes in the early Solar System. Here, we have used position-sensitive-detector X-ray diffraction (PSD-XRD) to quantify the abundance of minerals in bulk samples of the CI chondrite falls Alais, Orgueil and Ivuna, and the Antarctic CI-like chondrites Y-82162 and Y-980115. We find that Alais, Orgueil and Ivuna are dominated by a mixed serpentine/saponite phyllosilicate (81–84 vol%), plus minor magnetite (6–10%), sulphides (4–7%) and carbonates (<3%). This reflects an extended period of aqueous alteration and the near-complete transformation of anhydrous phases into a secondary mineral assemblage. The similarity in total abundance of phyllosilicate suggests that the CI chondrites all experienced the same degree of aqueous alteration on the parent body. In contrast, Y-82162 contains a highly disordered serpentine/saponite phyllosilicate (68 vol%), sulphide (19%), olivine (11%) and magnetite (2%). This mineralogy is distinct from that of the CI chondrites, attesting to both a different starting mineralogy and alteration history. The structure and relatively low abundance of the phyllosilicate, and the high abundance of olivine, are consistent with previous observations that Y-82162 represents CI-like material that following aqueous alteration suffered thermal metamorphism at temperatures >500 °C. Similarly, Y-980115 contains disordered serpentine/saponite (71 vol%), sulphide (19%), olivine (8%) and magnetite (2%), confirming that it too is a thermally metamorphosed CI-like chondrite. We suggest that the CI-like chondrites are derived from a different parent body than the CI chondrites, which underwent short-lived thermal metamorphism due to impacts and/or solar radiation

The most primitive CM chondrites, Asuka 12085, 12169, and 12236, of subtypes 3.0–2.8: Their characteristic features and classification

CM chondrites (CMs) are the most abundant group of carbonaceous chondrites. CMs experienced varying degrees of secondary aqueous alteration and heating that modified or destroyed their primitive features. We have studied three chondrites, Asuka (A) 12085, A 12169, and A 12236. Their modal compositions, chondrule size distributions, and bulk composition indicate that they are CMs. However, the common occurrence of melilite in CAIs and glass in chondrules, abundant Fe–Ni metal, the absence of tochilinite-cronstedtite intergrowths, and almost no phyllosilicates, all suggest that these chondrites, especially A 12169, experienced only minimal aqueous alteration. The textures and compositions of metal and sulfides, the lack of ferroan rims on AOA olivines, the compositional distribution of ferroan olivine, and the Raman spectra of their matrices, indicate that these chondrites experienced neither significant heating nor dehydration. These chondrites, especially A 12169, are the most primitive CMs so far reported. The degree of the alteration increases from A 12169, through A 12236, to A 12085. We propose the criteria for subtypes of 3.0–2.8 for CMs. A 12169, A 12236, and A 12085 are classified as subtype 3.0, 2.9, and 2.8, respectively. The oxygen isotopic composition of the Asuka CMs is consistent with these samples having experienced only a limited degree of aqueous alteration. The CM and CO groups are probably not derived from a single heterogeneous parent body. These chondrites are also of particular significance in view of the imminent return of sample material from the asteroids Ryugu and Bennu

Characterising the CI and CI-like carbonaceous chondrites using thermogravimetric analysis and infrared spectroscopy

The CI and CI-like chondrites provide a record of aqueous alteration in the early solar system. However, the CI-like chondrites differ in having also experienced a late stage period of thermal metamorphism. In order to constrain the nature and extent of the aqueous and thermal alteration, we have investigated the bulk mineralogy and abundance of H2O in the CI and CI-like chondrites using thermogravimetric analysis and infrared spectroscopy. The CI chondrites Ivuna and Orgueil show significant mass loss (28.5–31.8 wt.%) upon heating to 1000 °C due to dehydration and dehydroxylation of abundant phyllosilicates and Fe-(oxy)hydroxides and the decomposition of Fe-sulphides, carbonates and organics. Infrared spectra for Ivuna and Orgueil have a prominent 3-μm feature due to bound −OH/H2O in phyllosilicates and Fe-(oxy)hydroxides and only a minor 11-μm feature from anhydrous silicates. These characteristics are consistent with previous studies indicating that the CI chondrites underwent near-complete aqueous alteration. Similarities in the total abundance of H2O and 3 μm/11 μm ratio suggest that there is no difference in the relative degree of hydration experienced by Ivuna and Orgueil. In contrast, the CI-like chondrites Y-82162 and Y-980115 show lower mass loss (13.8–18.8 wt.%) and contain >50 % less H2O than the CI chondrites. The 3-μm feature is almost absent from spectra of Y-82162 and Y-980115 but the 11-μm feature is intense. The CI-like chondrites experienced thermal metamorphism at temperatures >500 °C that initially caused dehydration and dehydroxylation of phyllosilicates before partial recrystallization back into anhydrous silicates. The surfaces of many C-type asteroids were probably heated through impact metamorphism and/or solar radiation, so thermally altered carbonaceous chondrites are likely good analogues for samples that will be returned by the Hayabusa-2 and OSIRIS-REx missions

Signatures of the post-hydration heating of highly aqueously altered CM carbonaceous chondrites and implications for interpreting asteroid sample returns

The CM carbonaceous chondrites have all been aqueously altered, and some of them were subsequently heated in a parent body environment. Here we have sought to understand the impact of short duration heating on a highly aqueously altered CM through laboratory experiments on Allan Hills (ALH) 83100. Unheated ALH 83100 contains 83 volume per cent serpentine within the fine-grained matrix and altered chondrules. The matrix also hosts grains of calcite and dolomite, which are often intergrown with tochilinite, Fe(Ni) sulphides (pyrrhotite, pentlandite), magnetite and organic matter. Some of the magnetite formed by replacement of Fe(Ni) sulphides that were accreted from the nebula. Laboratory heating to 400 °C has caused partial dehydroxylation of serpentine and loss of isotopically light oxygen leading to an increase in bulk δ18O and fall in Δ17O. Tochilinite has decomposed to magnetite, whereas carbonates have remained unaltered. With regards to infrared spectroscopy (4000–400 cm-1; 2.5–25 µm), heating to 400 °C has resulted in decreased emissivity (increased reflectance), a sharper and more symmetric OH band at 3684 cm-1 (2.71 µm), a broadening of the Si—O stretching band together with movement of its minimum to longer wavenumbers, and a decreasing depth of the Mg—OH band (625 cm-1; 16 µm). The Si—O bending band is unmodified by mild heating. With heating to 800 °C the serpentine has fully dehydroxylated and recrystallized to ∼Fo60/70 olivine. Bulk δ18O has further increased and Δ17O decreased. Troilite and pyrrhotite have formed, and recrystallization of pentlandite has produced Fe,Ni metal. Calcite and dolomite were calcined at ∼700 °C and in their place is an un-named Ca-Fe oxysulphide. Heating changes the structural order of organic matter so that Raman spectroscopy of carbon in the 800 °C sample shows an increased (D1 + D4) proportional area parameter. The infrared spectrum of the 800 °C sample confirms the abundance of Fe-bearing olivine and is very similar to the spectrum of naturally heated stage IV CM Pecora Escarpment 02010. The temperature-related mineralogical, chemical, isotopic and spectroscopic signatures defined in ALH 83100 will help to track the post-hydration thermal histories of carbonaceous chondrite meteorites, and samples returned from the primitive asteroids Ryugu and Bennu

Etude de la résistance du melon à un virus émergent, le CVYV

7 annexes 14 p. Diplôme : Master Professionne

Petrological and experimental study of CV-CK chondrites and conditions of metamorphism in carbonaceous asteroids

Les chondrites carbonées (CCs) sont des objets primitifs accrétés lors de la formation du Système Solaire. Composées en grande partie de chondres, de matrice et d’inclusions réfractaires, elles ont enregistré les hétérogénéités chimiques, isotopiques et minéralogiques de la nébuleuse solaire. Contrairement aux autres classes de chondrites, la grande majorité des CCs sont primitives (types pétrologiques 1 à 3). Elles n’ont donc pas subi de métamorphisme important sur leur corps parent. Toutefois, un groupe de CCs, les CKs, montre un métamorphisme thermique intense (types pétrologiques 4 à 6). Ces chondrites sont caractérisées par des matrices recristallisées, des olivines équilibrées à ∼Fa31, un degré d’oxydation important (olivines riches en NiO, rapport métal/magnétite proche de zéro), des teneurs en éléments réfractaires lithophiles intermédiaires aux CVs et aux COs, ou encore des compositions isotopiques en oxygène se situant dans le champ défini par les CVs et les COs. Les CKs ont été peu étudiées jusqu’au début des années 90, car peu nombreuses (seulement 210 classifiées au 6 décembre 2011) et de petite taille (masse médiane ∼33,5g). Leurs compositions isotopiques et chimiques laissent supposer l’existence d’un lien génétique avec les CV3. Les découvertes récentes de nouvelles CKs depuis 1990, et notamment de CK3 par le biais de collectes systématiques au Sahara et en Antarctique, permettent l’étude détaillée de l’évolution métamorphique des CKs, notamment à la transition 3–4. Ce travail a pour but de caractériser les conditions dans lesquelles s’est déroulé cet épisode métamorphique, et grâce à l’observation de plusieurs CK3–4, d’étudier la relation CV-CK. La caractérisation détaillée de l’évolution métamorphique de 19 CKs dont 5 CK3 a permis de confirmer que les différences observées entre les divers composants chondritiques (abondance, minéralogie, texture) des CVs et des CKs peuvent être expliquées par un épisode thermique secondaire de HT-BP (∼300–650°C) en conditions oxydantes (∼NNO). De plus, l’analyse de profils de diffusions dans les chondres des CKs indique des durées de métamorphisme intermédiaires à celles communément invoquées pour du choc (de quelques secondes à quelques jours) et pour la désintégration d’éléments à courte durée de vie (plusieurs millions d’années). Une série d’expériences réalisées en four 1 atmosphère avec contrôle de la fugacité d’oxygène nous a permis de reproduire les textures caractéristiques des CKs et d’obtenir une teneur en fer d’équilibre des olivines des CVs, valeur proche de celle mesurée dans les CKs. Cela semble donc confirmer que les CKs sont des CVs rééquilibrées. Par conséquent, la classification actuelle de ces chondrites en deux groupes distincts devrait être modifiée afin de rendre compte de l’existence de cette série métamorphique CV-CK continue. Nous proposons de considérer le chauffage radiatif comme cause possible du métamorphisme des CKs. Un modèle numérique nous a permis de confirmer que des météoroïdes carbonés avec des périhélies situés entre 0,07 et 0,15 UA peuvent être chauffés à des températures pouvant aller jusqu’à 780°C. Les tailles pré-atmosphériques estimées pour les CV-CK (de quelques centimètres à 2,5 mètres) sont compatibles avec ce type de processus. La fragmentation d’un corps parent homogène de type CV (possiblement l’astéroïde à l’origine de la famille d’Eos) pourrait former des météoroïdes qui, sous l’effet de phénomènes de résonances, seraient redirigés vers l’intérieur du Système Solaire et pourraient ainsi être métamorphisés par chauffage radiatif. Ce type de processus thermique secondaire n’étant efficace que pour de petits fragments d’astéroïdes, il ne doit pas être considéré comme un processus corps-parent stricto sensu.Carbonaceous chondrites (CCs) are primitive objects accreted during the earliest stage of the Solar System formation. Mainly composed of chondrules, matrix and refractory inclusions, CCs recorded chemical, isotopic and mineralogical heterogeneities of the solar nebula. Unlike other chondrite classes, most CCs are primitive (petrologic types 1 to 3), i.e., they have not been affected by thermal parent-body processes. However, CK chondrites suffered an intense metamorphism (petrologic types 4 to 6). The CK group is characterized by recrystallized matrices, equilibrated olivines (∼Fa31), a high level of oxidation (Ni-rich olivines, metal/magnetite ratio close to zero), low contents of refractory inclusions, refractory lithophile abundances intermediate between CV and CO groups, and oxygen isotope compositions overlapping the CV and CO groups. CKs have been poorly studied until the 1990’s, in part due to the small number of classified samples (210 as of December 6th, 2011), and their small masses (median mass∼33.5g). Isotopic and major element compositions support a genetic link with CV3s. Since1990, recent discoveries of CKs, in particular of CK3s recovered by systematic Antarctic and Saharan collects, allow a detailed study of the CK metamorphic evolution, especially at the 3–4 transition. The objective of this study is the characterization of the conditions of metamorphism of CKs, and through analyses of several CK3–4 samples, the study of the CV-CK relationship. The detailed characterization of the metamorphic evolution of 19 CKs, including 5 CK3, confirms that observed differences between chondritic components in CVs and CKs (abundance, mineralogy, texture) can be explained by a secondary HT-BP thermal process (∼300–650°C) under oxidizing conditions (∼NNO). Moreover, durations of metamorphism obtained by the analysis of diffusion profiles in CK chondrules are intermediate between those commonly admitted for shock (few seconds to several days) and for short-lived radionuclides decay (several million years). An experimental study, using a 1-atmosphere furnace with controlled oxygen fugacity, provides additional arguments for the CV-CK relationship. We reproduced characteristic CK textures and obtained olivine iron contents of equilibrated CVs close to those measured in CKs. These experiments confirm that CKs can be considered as reequilibrated CVs. Thus, the current classification of CVs and CKs in two distinct groups should be modified in order to account for the existence of the CV-CK continuous metamorphic series from type 3 to 6. We propose to consider radiative heating as a possible cause of metamorphism for CKs. Numerical thermal modeling indicates that carbonaceous meteoroids with low perihelia (between 0.07 and 0.15 AU) can be heated at temperatures up to 780°C. Pre-atmospheric sizes estimated for CVs and CKs (from a few centimeters to 2.5 meters) support this thermal process. Fragmentation of an homogeneous CV-type parent body (possibly the parent asteroid at the origin of the Eos family) could be the source of meteoroids which, due to resonances, move toward the Sun and thus be metamorphosed by radiative heating. This secondary thermal process, affecting only small asteroid fragments, should not be considered as a parent-body process in the sense that it did not occur on the asteroid before its disruption