Shock-darkening in ordinary chondrites : Mesoscale modelling of the shock process and comparison with shock-recovery experiments

Ordinary chondrites are primitive materials of the solar system; they were subject to thermal and shock metamorphism during asteroid accretion and collision history. Shock-darkening is a shock metamorphic process which occurs in ordinary chondrites where iron sulphides and metals form a network of tiny melt veins, optically darkening the lithology. Together space weathering and shock-darkening can be a major factor in alteration of reflectance spectra, suppressing the 1 and 2 micron silicate absorption bands. S-complex asteroids, hosting ordinary chondrites, display silicate absorption bands. C/X-complex asteroids are either devoid of 1 and 2 micron silicate absorption bands or presenting a weak silicate absorption band at 1 micron. If shock-darkening can alter the spectra of S-complex asteroids, they can appear like C/X-complex asteroids and induce a mismatch in the asteroid distribution. This thesis provides an in-depth study of shock-darkening in order to determine the pressure-temperature conditions for shock melting of both iron sulphides and metals, in ordinary chondrites. In order to perform this study the following actions were required: I. observing shock wave interactions in heterogeneous mediums composed of silicates, metals, and iron sulphides, the principal components of ordinary chondrites II. quantifying post-shock heating and melting of the individual phases III. comparing my results with observations of shock metamorphism in ordinary chondrites IV. investigating on the best conditions to reproduce shock-darkening in shock-recovery experiments. In contrast to shock-recovery experiments, I adopted a numerical modelling method which calculated the post-shock heating and melting of individual phases and provided observation of shock wave interactions in heterogeneous mediums. The shock physics code iSALE was used on a mesoscale to study shock compression of ordinary chondrites. Using complex models, the numerical study lead to the following results: A) 40−60 GPa is the likely range for shock-darkening, dominated by melting of iron sulphides. B) Heterogeneous distribution of peak shock pressures and post-shock heating is caused by strong impedance contrasts between phases (with strong pressure increases through reflections from high density phases to lower density phases, e.g. metals to silicates). C) Special conditions, such as eutectic melting, hotspots from convergence of shock waves, or pore crushing, are necessary to melt metals. D) Porosity and pre-heating are important boundary conditions affecting shock metamorphism. E) Results from the mesoscale models are compatible to observations of shock metamorphism in ordinary chondrites. Finally, simulations of shock-recovery experiments showed that the reverberation technique may prevent shock-darkening from occurring. Compared to a single pressure load, the reverberation technique reduces the rise in entropy from super-imposing pressures, thus, if sufficient pressure for shock-darkening (40–60 GPa) is achieved, melting of iron sulphides or metals may not occur. Alternatively, I showed that spherical shock-recovery experiments, which use spherically induced shock waves to shock spherical samples, are ideal to study shock-darkening because the rise in entropy is directly related to the peak-shock pressure in the sample. With my results, a more in depth quantitative study of the volume of shock-induced darkened materials during asteroid collisions is now possible.Les chondrites ordinaires sont des roches primitives du système solaire sujettes au métamorphisme thermique et de choc survenant sur les astéroïdes. Le noircissement par choc dans les chondrites ordinaires est un processus exclusif au métamorphisme de choc. Par fusion, les sulfures de fer et de métaux forment un réseau de minuscules veines qui noircissent la lithologie. Avec la météorologie spatiale, le noircissement par choc est un facteur majeur dans l’altération des spectres de réflexion puisqu’il élimine les bandes d’absorption à 1 et 2 microns des composés silicatés. Les spectres des astéroïdes du groupe S (intégrant les chondrites ordinaires) possèdent ces bandes d’absorption, là où ceux des astéroïdes du groupe C/X en sont dénués ou possèdent une faible absorption à 1 micron. Si le noircissement par choc altère les spectres des astéroïdes du groupe S, ceux-ci peuvent ressembler à ceux du groupe C/X et occasionner une incohérence dans la distribution des astéroïdes. Dans ma thèse, j’ai étudié le noircissement par choc en déterminant les conditions de pression et température nécessaires à la fusion des sulfures de fer et métaux dans les chondrites ordinaires. Plus précisément, j’ai eu besoin de : I. observer les interactions d’ondes de choc dans des milieux hétérogènes composés de silicates, métaux et sulfures de fer, principales phases minérales des chondrites ordinaires II. quantifier les températures post-choc et la fusion de ces phases minérales III. comparer mes résultats avec la littérature sur le métamorphisme de choc dans les chondrites ordinaires IV. explorer les conditions idéales pour reproduire le noircissement par choc dans des expériences de récupération (de choc). Au détriment des expériences de récupération, j’ai opté pour la modélisation numérique afin de quantifier les températures post-choc et la fusion des différentes phases minérales, et observer in situ les interactions d’onde de choc dans un milieu hétérogène. Par l’usage d’un code de physique des chocs (iSALE), j’ai mené cette étude sur les chondrites ordinaires. Profitant de modèles complexes, les résultats obtenus sont les suivants : A) Le noircissement par choc, dominé par la fusion des sulfures de fer, se produirait aux pressions de 40–60 GPa. B) Les forts contrastes d’impédance entre phases (e.g. entre métaux et silicates) provoquent une distribution hétérogène des pics de pression et de température post-choc causés par des réflexions de choc. C) La fusion eutectique et les zones de hautes températures (provoquées par la convergence d’ondes de choc ou la fermeture de pores) sont des conditions nécessaires pour la fonte des métaux. D) La porosité initiale et le métamorphisme thermique sont d’autres conditions ayant un effet sur le métamorphisme de choc. E) Les résultats inhérents aux modèles numériques sont en accord avec la littérature sur le métamorphisme de choc dans les chondrites ordinaires. Finalement, des modèles numériques sur les expériences de récupération montrent que la technique de réverbération est limitée pour le noircissement par choc. Contrairement à une hausse instantanée de pression, la réverbération réduit l’entropie en accumulant les pressions. Ainsi, si la pression requise pour le noircissement par choc est atteinte (40–60 GPa), la fusion des sulfures de fer ou des métaux peut ne pas se produire. Cependant, j’ai démontré que les expériences de récupération qui profitent d’ondes de choc sphériques sont idéales pour produire le noircissement par choc. En effet, dans ces expériences, l’entropie correspond à la pression atteinte dans l’échantillon. Avec mes résultats, une étude sur la quantification du volume de matériaux soumis au noircissement par choc pendant les collisions entre astéroïdes est désormais envisageable

Melting efficiency of troilite-iron assemblages in shock-darkening : insight from numerical modeling

We studied shock-darkening in ordinary chondrites by observing the propagation of shock waves and melting through mixtures of metals and iron sulfides. We used the shock physics code iSALE at the mesoscale to simulate shock compression of modeled ordinary chondrites (using olivine, iron and troilite). We introduced FeS-FeNi eutectic properties and partial melting in a series of chosen configurations of iron and troilite grains mixtures in a sample plate. We observed, at a nominal pressure of 45 GPa, partial melting of troilite in all models. Only few of the models showed partial melting of iron (a phase difficult to melt in shock heating) due to the eutectic properties of the mixtures. Iron melting only occurred in models presenting either strong shock wave concentration effects or effects of heating by pore crushing, for which we provided more details. Further effects are discussed such as the frictional heating between iron and troilite and the heat diffusion in scenarios with strongly heated troilite. We also characterized troilite melting in the 32-60 GPa nominal pressure range. We concluded that specific dispositions of the iron and troilite grains in mixtures exist that lead to melting of iron and explain why it is possible to find a mix of metals and iron sulfides in shock-darkened ordinary chondrites.Peer reviewe

Shock physics mesoscale modeling of shock stage 5 and 6 in ordinary and enstatite chondrites

Shock-darkening, the melting of metals and iron sulfides into a network of veins within silicate grains, altering reflectance spectra of meteorites, was previously studied using shock physics mesoscale modeling. Melting of iron sulfides embedded in olivine was observed at pressures of 40-50 GPa. This pressure range is at the transition between shock stage 5 (C-S5) and 6 (C-S6) of the shock metamorphism classification in ordinary and enstatite chondrites. To better characterize C-S5 and C-S6 with a mesoscale modeling approach and assess post-shock heating and melting, we used multi-phase (i.e. olivine/enstatite, troilite, iron, pores, and plagioclase) meshes with realistic configurations of grains. We carried out a systematic study of shock compression in ordinary and enstatite chondrites at pressures between 30 and 70 GPa. To setup mesoscale sample meshes with realistic silicate, metal, iron sulfide, and open pore shapes, we converted backscattered electron microscope images of three chondrites. The resolved macroporosity in meshes was 3-6%. Transition from shock C-S5 to C-S6 was observed through (1) the melting of troilite above 40 GPa with melt fractions of similar to 0.7-0.9 at 70 GPa, (2) the melting of olivine and iron above 50 GPa with melt fraction of similar to 0.001 and 0.012, respectively, at 70 GPa, and (3) the melting of plagioclase above 30 GPa (melt fraction of 1, at 55 GPa). Post-shock temperatures varied from similar to 540 K at 30 GPa to similar to 1300 K at 70 GPa. We also constructed models with increased porosity up to 15% porosity, producing higher post-shock temperatures (similar to 800 K increase) and melt fractions (similar to 0.12 increase) in olivine. Additionally we constructed a pre-heated model to observe post-shock heating and melting during thermal metamorphism. This model presented similar results (melting) at pressures 10-15 GPa lower compared to the room temperature models. Finally, we demonstrated dependence of post-shock heating and melting on the orientation of open cracks relative to the shock wave front. In conclusion, the modeled melting and post-shock heating of individual phases were mostly consistent with the current shock classification scheme (Stoffler et al., 1991, 2018).Peer reviewe

Insight into the Distribution of High-pressure Shock Metamorphism in Rubble-pile Asteroids

Funding Information: This work was supported by the Academy of Finland, project Nos. 293975 and 335595, the European Regional Development Fund, the Mobilitas Pluss programme (grant No. MOBJD639), and the NASA Solar System Exploration Research Virtual Institute Center for Lunar and Asteroid Surface Science, and it was conducted within institutional support RVO 67985831 of the Institute of Geology of the Czech Academy of Sciences. R.L. appreciates funding from the European Union’s Horizon 2020 research and innovation program, NEO-MAPP, grant agreement No. 870377. Publisher Copyright: © 2022. The Author(s). Published by the American Astronomical Society.Shock metamorphism in ordinary chondrites allows for reconstructing impact events between asteroids in the main asteroid belt. Shock-darkening of ordinary chondrites occurs at the onset of complete shock melting of the rock (>70 GPa) or injection of sulfide and metal melt into the cracks within solid silicates (∼50 GPa). Darkening of ordinary chondrites masks diagnostic silicate features observed in the reflectance spectrum of S-complex asteroids so they appear similar to C/X-complex asteroids. In this work, we investigate the shock pressure and associated metamorphism pattern in rubble-pile asteroids at impact velocities of 4–10 km s−1. We use the iSALE shock physics code and implement two-dimensional models with simplified properties in order to quantify the influence of the following parameters on shock-darkening efficiency: impact velocity, porosity within the asteroid, impactor size, and ejection efficiency. We observe that, in rubble-pile asteroids, the velocity and size of the impactor are the constraining parameters in recording high-grade shock metamorphism. Yet, the recorded fraction of higher shock stages remains low (<0.2). Varying the porosity of the boulders from 10% to 30% does not significantly affect the distribution of pressure and fraction of shock-darkened material. The pressure distribution in rubble-pile asteroids is very similar to that of monolithic asteroids with the same porosity. Thus, producing significant volumes of high-degree shocked ordinary chondrites requires strong collision events (impact velocities above 8 km s−1 and/or large sizes of impactors). A large amount of asteroid material escapes during an impact event (up to 90%); however, only a small portion of the escaping material is shock-darkened (6%).Peer reviewe

Bulk synthesis of stoichiometric/meteoritic troilite (FeS) by high-temperature pyrite decomposition and pyrrhotite melting

Stoichiometric troilite (FeS) is a common phase in differentiated and undifferentiated meteorites. It is the endmember of the iron sulfide system. Troilite is important for investigating shock metamorphism in meteorites and studying spectral properties and space weathering of planetary bodies. Thus, obtaining coarse-grained meteoritic troilite in quantities is beneficial for these fields. The previous synthesis of troilite was achieved by pyrite or pyrrhotite heating treatments or chemical syntheses. However, most of these works lacked a visual characterization of the step by step process and the final product, the production of large quantities, and they were not readily advertised to planetary scientists or the meteoritical research community. Here, we illustrate a two-step heat treatment of pyrite to synthesize troilite. Pyrite powder was decomposed to pyrrhotite at 1023-1073 K for 4-6 h in Ar; the run product was then retrieved and reheated for 1 h at 1498-1598 K in N-2 (gas). The minerals were analyzed with a scanning electron microscope, X-ray diffraction (XRD) at room temperature, and in situ high-temperature XRD. The primary observation of synthesis from pyrrhotite to troilite is the shift of a major diffraction peak from similar to 43.2 degrees 2 theta to similar to 43.8 degrees 2 theta. Troilite spectra matched an XRD analysis of natural meteoritic troilite. Slight contamination of Fe was observed during cooling to troilite, and alumina crucibles locally reacted with troilite. The habitus and size of troilite crystals allowed us to store it as large grains rather than powder; 27 g of pyrite yielded 17 g of stochiometric troilite.Peer reviewe

A shock recovery experiment and its implications for Mercury's surface : The effect of high pressure on porous olivine powder as a regolith analog

We conducted classic dynamic high - pressure experiments on porous San Carlos (SC) olivine powder to examine if and how different shock stages modify corresponding reflectance mid – infrared (MIR) spectra. Microscopic investigation of the thin sections produced of our shocked samples indicates local peak pressures of >60 GPa along with all lower grade shock stages. Spectral analyses of optically identified shock areas were documented and compared in terms of Christiansen Feature (CF) and the position of olivine – diagnostic Reststrahlenbands (RBs). We found that one RB (fundamental vibrations of the orthosilicate - ion) of olivine occurring at 980 cm−1 (corresponding to ≈ 10.2 μm) shows the least energetic shift in the investigated MIR spectra and could therefore serve as a proxy for the presence of olivine in remote sensing application. Furthermore, a peak located at ≈ 1060 cm−1 (≈ 9.4 μm) shows a significant intensity change probably related to the degree of shock exposure or grain orientation effects, as we observe a decline in intensity of this band from our averaged reference olivine spectra of our IRIS database (diffuse reflectance measurement) down to spectra of grains showing mosaicism and recrystallized areas. We also report the presence of a weak band in some of the olivine spectra located at ≈ 1100 cm−1 (9.1 μm) that has an influence on the position of the CF when spectral data of olivine are averaged.Peer reviewe

Heat diffusion in numerically shocked ordinary chondrites and its contribution to shock melting

High pressure shock metamorphism in ordinary chondrites involves heating and melting of individual phases from shock entropy, pore collapse, frictional heating, and heat transfer. Numerical models using shock physics codes have recently been used to comprehend the mechanism of shock heating and melting in multiphase mesoscale models. Such models suggest that the formation of sulfide and metal melt veins in ordinary chondrites (shock-darkening) can be explained by preferential heating and melting of iron and iron sulfides during shock. However, those models usually dismissed heat transfer between heterogeneously shock heated phases. This leads to an underestimation of the degree of melting in phases that experienced low degrees of shock heating (e.g. iron metal) but are in direct contact with strongly shock heated phases (e.g. iron sulfides). In our study, we implemented a finite difference 2-D heat diffusion code to model heat diffusion among neighboring grains in shock heated multiphase meshes that represent typical textural relations of silicate, sulfide and metal grains in ordinary chondrites. Post-shock temperature maps for each textural model were calculated using the iSALE shock physics code and used as input for the diffusion code. We find that heat diffusion, not initial shock heating, is the principal cause for heating and melting of metals in eutectic contact with iron sulfides at similar to 50 GPa of pressure. In addition we study the effects of iron and troilite grain sizes, shock pressures and pre-shock porosities of the silicate matrix, and discuss the preservation of melt allowing melt migration in shock-darkened meteorites and the observation of metal-silicate intermixed melting. With our work, we demonstrate that the consideration of heat diffusion during and after shock is crucial for a better understanding of melting features in both experimentally and naturally shocked ordinary chondrites.Peer reviewe

Shawn McNeeley mows the grass with a manual push mower, location unknown, 1973

Herral Long was an award-winning photographer who infused his photojournalism with a touch of artistry. Mr. Long was hired by the Toledo Blade in 1949 and became their photographer in 1950. Mr. Long loved his job and his profession. He photographed still life foods for the Blade with meticulous precision. He enthusiastically covered news stories as well as everyday human interest stories. Herral Long retired from the Blade in December of 2008. Before he died Mr. Long donated his private collection to the Toledo Lucas County Public Library. The collection consists of news and personal photos. He passed away June 14, 2014

Insight into the Distribution of High-pressure Shock Metamorphism in Rubble-pile Asteroids

Funding Information: This work was supported by the Academy of Finland, project Nos. 293975 and 335595, the European Regional Development Fund, the Mobilitas Pluss programme (grant No. MOBJD639), and the NASA Solar System Exploration Research Virtual Institute Center for Lunar and Asteroid Surface Science, and it was conducted within institutional support RVO 67985831 of the Institute of Geology of the Czech Academy of Sciences. R.L. appreciates funding from the European Union’s Horizon 2020 research and innovation program, NEO-MAPP, grant agreement No. 870377. Publisher Copyright: © 2022. The Author(s). Published by the American Astronomical Society.Shock metamorphism in ordinary chondrites allows for reconstructing impact events between asteroids in the main asteroid belt. Shock-darkening of ordinary chondrites occurs at the onset of complete shock melting of the rock (>70 GPa) or injection of sulfide and metal melt into the cracks within solid silicates (∼50 GPa). Darkening of ordinary chondrites masks diagnostic silicate features observed in the reflectance spectrum of S-complex asteroids so they appear similar to C/X-complex asteroids. In this work, we investigate the shock pressure and associated metamorphism pattern in rubble-pile asteroids at impact velocities of 4–10 km s−1. We use the iSALE shock physics code and implement two-dimensional models with simplified properties in order to quantify the influence of the following parameters on shock-darkening efficiency: impact velocity, porosity within the asteroid, impactor size, and ejection efficiency. We observe that, in rubble-pile asteroids, the velocity and size of the impactor are the constraining parameters in recording high-grade shock metamorphism. Yet, the recorded fraction of higher shock stages remains low (<0.2). Varying the porosity of the boulders from 10% to 30% does not significantly affect the distribution of pressure and fraction of shock-darkened material. The pressure distribution in rubble-pile asteroids is very similar to that of monolithic asteroids with the same porosity. Thus, producing significant volumes of high-degree shocked ordinary chondrites requires strong collision events (impact velocities above 8 km s−1 and/or large sizes of impactors). A large amount of asteroid material escapes during an impact event (up to 90%); however, only a small portion of the escaping material is shock-darkened (6%).Peer reviewe