Shock Metamorphism and Impact Melting at Kamil Crater, Egypt

Shock effects in small terrestrial impact craters (diameter < 300 m) have been poorly studied because small craters are rare and often deeply eroded. Kamil is a young (< 5000 yr), small (45-m-in-diameter), and well preserved impact structure caused by the hypervelocity impact of the iron meteorite Gebel Kamil on sedimentary rocks in southwestern Egypt. Its pristine state of preservation makes Kamil a natural laboratory for the study of the cratering process of small impactors (about 1-m-in-diameter) on Earth, their consequences, and their impact on the terrestrial environment for hazard assessment. This PhD Thesis deals with the definition of the shock metamorphism and impact melting in small terrestrial impact craters through a comprehensive mineralogical, petrographic, and geochemical study of shock-metamorphosed rocks and impact melts from Kamil. This study also allows us to constrain the impact cratering process related to the impact of meter-sized iron meteorites on Earth. The results of this PhD Thesis highlight for the first time that a meter size iron body impacting on a sedimentary target can produce a wide range of shock features. These divide into two categories as a function of their abundance at the thin section scale: i) pervasive shock features (the most abundant), including fracturing, planar deformation features, and impact melt lapilli and bombs, and ii) localized shock features including high-pressure phases and localized impact melting in the form of intergranular melt, melt veins, and melt films in shatter cones. Pervasive shock features indicate the shock pressure suffered by rocks. The most shocked samples (impact melt lapilli and bombs) indicate that the shock pressure at the contact point between the projectile and the target was between 30 and 60 GPa. Based on the planar impact approximation model, this implies that the impact velocity of Gebel Kamil was at least 5 km s-1, for an impact angle of 45°. Localized shock features formed from the local enhancement of shock pressure and temperature at pores and/or at the heterogeneities of the target rocks. Thus, it is possible to find high-pressure phases and intergranular melting in sample that suffered low or moderate shock pressures. In small meteorite impacts, the projectile may survive the impact through fragmentation. In addition, it may melt and interact with both shocked and melted target rocks. The interaction between target and projectile liquids is a process yet to be completely understood. Impact melt lapilli and bombs from Kamil are very fresh and their study can help constrain the target-projectile interaction. Two types of glasses constitute the impact melt lapilli and bombs: a white glass and a dark glass. The white glass is inclusion-free, mostly SiO2, and has negligible amounts of Ni and Co, suggesting derivation from the target rocks with negligible interaction with the projectile liquid (<0.1 wt% of projectile contamination). The dark glass is made of a silicate glass with variable amounts of Al, Fe, and Ni. It also includes variously shocked and melted fragments from the target and projectile (Ni-Fe metal blebs). All this indicates an extensive interaction with the projectile liquid. The dark glass is thus a mixture of target and projectile (estimated projectile contamination 11-12 wt%) liquids. Based on the recently proposed models for the target-projectile interaction and for impact glass formation, we propose a model for the glass formation at Kamil. Between the contact and compression stage and the excavation stage, projectile and target liquids can chemically interact in a restricted zone. The projectile contamination affected only a shallow portion of the impacted target rocks. White glass formed out of this zone, escaping interaction with the projectile. During the excavation stage, due to a brief and chaotic time sequence and the high temperature, dark glass engulfed and coated white glass and target fragments and stuck on iron meteorite shrapnel fragments. The microscopic impactor debris, systematically collected from the soil around Kamil, includes vesicular masses, spherules, and coatings of dark impact melt glass that is a mixture of impactor and target materials (Si, Fe, Al-rich glass), and Fe-Ni oxide spherules and mini shrapnel fragments. As a consequence of an oblique impact, this material formed a downrange ejecta curtain of microscopic impactor debris due SE-SW of the crater (extension ~300,000 m2, up to ~400 m from the crater), consistent with previous determination of the impactor trajectory. The Ni contents of the soil provided an estimate of the mass of the microscopic debris of the Gebel Kamil meteorite dispersed in the soil. This mass (20 t, likely 50-60 t)

Studio chimico-petrografico della meteorite metallica anomala Northwest Africa 6583: modello genetico e relazioni con le condriti enstatitiche e le aubriti

Gli scopi della presente tesi sono stati quelli di classificare e proporre un modello per la genesi e l’evoluzione della meteorite metallica Northwest Africa 6583 (NWA 6583). Per raggiungere questi scopi sono stati condotti un dettagliato studio petrografico e chimico e numerosi confronti con altre meteoriti sia metalliche sia non metalliche. Lo studio petrografico è stato effettuato mediante osservazioni al microscopio ottico a luce riflessa e al microscopio elettronico a scansione. L’analisi chimica totale del metallo è stata realizzata tramite Inductively Coupled Plasma – Mass Spectrometry (ICP-MS), mentre le analisi chimiche delle diverse fasi sono state eseguite alla microsonda elettronica. Da queste osservazioni e analisi è emerso che NWA 6583 è una meteorite metallica ungrouped, ovvero non appartiene a nessuno dei gruppi fino ad oggi conosciuti. Il metallo si contraddistingue per essere ricco di nichel (~18 wt%) e di elementi incompatibili (ad esempio Ga, Ge, As e Cu), inoltre questo contiene quantità significative di silicio (~0.13 wt%). Il metallo della meteorite ha una tessitura policristallina a grana fine compresa tra i 100 μm e i 2 mm. I singoli grani hanno tessitura martensitica. L’assemblaggio dei minerali accessori è costituito da fasi silicatiche e da solfuri, fosfuri e elementi nativi. I silicati sono rappresentati da enstatite priva di ferro e in misura ridotta da diopside e forsterite anch’essi privi di ferro. Il solfuro più abbondante è la troilite (3.4 vol.%) che contiene elevate concentrazioni di cromo (~0.87 wt%) e titanio (~0.15 wt%); questa fase spesso include cristalli di solfuri di Mn-Mg-Fe appartenenti al gruppo della alabandite-keilite-niningerite, solfuri di Fe-Zn-Mn non ancora identificato, solfuri di Cu-Fe e solfuri di Fe-Ni. La schreibersite si trova sia come cristalli millimetrici sia come cristalli micrometrici diffusi nella matrice metallica. La grafite si trova come abbondanti rosette (fino a 5 mm di diametro) e come cristalli isolati. Il rame nativo è stato raramente trovato all’interfaccia tra il metallo Fe,Ni e la troilite (spessore di 10 μm). Le caratteristiche composizionali del metallo e della troilite (alti contenuti di Si e di Cr+Ti rispettivamente), la presenza di solfuri costituiti da elementi litofili, di grafite e di silicati privi di ferro sono tutti importanti indizi di una formazione in un ambiente caratterizzato da una bassissima fugacità di ossigeno. Simili condizioni di riduzione sono tipiche di un altro importante insieme di meteoriti: le meteoriti enstatitiche (comprendente due gruppi di condriti, EH e EL, un gruppo di acondriti, le aubriti, e numerose meteoriti anomale). Date le somiglianze mineralogiche, è probabile che NWA 6583 si sia formata a partire da uno di questi gruppi. Diversi modelli petrologici comunemente utilizzati in letteratura (e.g., Chabot, 2004, e Wasson, 1999) sono stati utilizzati in questo studio per descrivere la genesi di NWA 6583 attraverso la cristallizzazione di un liquido metallico. La tessitura del metallo e la composizione di NWA 6583 testimoniano un complesso processo di raffreddamento e cristallizzazione: la composizione altamente differenziata indica un lungo e lento raffreddamento; il metallo martensitico è invece indicativi di un raffreddamento molto rapido. È possibile quindi ipotizzare che la formazione di NWA 6583 sia avvenuta in più stadi. Il primo stadio è quello relativo alla formazione di un liquido metallico e alla sua cristallizzazione (tasso di raffreddamento >104 °C/Ma, Goldstein et al., 2009). Sulla base dei dati attuali non è possibile dare una spiegazione univoca a questa prima fase della storia di NWA 6583. I due possibili scenari limite sono legati o alla differenziazione di un asteroide di composizione simile a quella delle condriti enstatitiche con la conseguente formazione di un nucleo metallico, o a un violento impatto che ha riscaldato e fuso la superficie del corpo genitore. Il secondo scenario è il più probabile, considerata anche la presenza di silicati in NWA 6583. Il secondo stadio è sicuramente attribuibile ad un impatto che ha riscaldato improvvisamente e liberato il solido metallico γ-Fe,Ni che si è trasformato senza diffusione in α2, martensite, alla temperatura di circa 270°C. Lo stadio finale della storia di NWA 6583 è il riscaldamento (T > 1000°C, Axon, 1963) durante l’attraversamento ablativo dell’atmosfera terrestre, questo è responsabile della fusione dei cristalli di troilite e schreibersite entro 2 mm dalla superficie esterna della meteorite. The aims of this thesis are the full petrographic and geochemical characterization of the Northwest Africa 6583 (NWA 6583, 1.8 kg) iron meteorites and the interpretation of its petrogenesis and evolution. The petrographic study was carried out using the optical (reflected light) and scanning electron microscope (SEM-EDS), while chemical analyses were performed by Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) (bulk metal) and Electron Microprobe Analysis (EPMA) plus SEM-EDS (mineral phases). The results show that NWA 6583 is a chemically “ungrouped” iron, characterized by high contents of Ni (~ 18 wt%) and incompatible siderophile and chalcophile elements (e.g. Ga, Ge, As, Cu, Sn, Sb). In particular the metal of NWA 6583 shows an anomalously high content of Si (~0.13 wt%). NWA 6583 has a polycrystalline texture (grain size 0.1-2.0 mm) and the metal shows a martensitic texture. The accessory mineral assemblage includes silicates and sulphides, phosphides and native elements. Silicates are represented by iron-free enstatite and subordinate iron-free diopside and forsterite. The most abundant sulfide is troilite (~ 3.4 vol.%) characterized by high contents of Cr (~0.87 wt%) and Ti (~0.15 wt%); this phase commonly hosts crystals of Mn-Mg-Fe sulfides belonging to the alabandite-keilite-niningerite group, an unidentified Fe-Zn-Mn sulfide, chalcopyrite and Fe-Ni sulfides. Schreibersite occurs as large crystals and as tiny crystals scattered in the metal matrix. Graphite occurs as abundant subspherical rosettes (up to 5 mm in diameter) and as isolated flakes. Native copper has been rarely found at the interface between Fe-Ni metal and troilite (~10 μm thickness). The compositional features of metal and troilite (high Si and high Cr+Ti, respectively), the occurrence of sulfides of lithophile elements and graphite, and the iron-free composition of the silicates are all important clues to the formation of this meteorite in an extremely reducing environment. Similar redox conditions are typical of three important groups of stony meteorites: the EH and EL chondrites, and the aubrites. Indeed the meteorites belonging to these groups are characterized by mineral assemblages and compositions similar to NWA 6583. Several petrologic models currently used in the literature (e.g. Wasson and Richardson 2001; Chabot 2004) have been used to describe the genesis of NWA 6583 by crystallization of a liquid metal. The metal texture and composition of NWA 6583 testify to a complex process of cooling and crystallization: the highly differentiated composition indicates protracted, slow cooling; in turn, the martensite metal is indicative of very fast cooling. We therefore suggest a three step origin of NWA 6583. The first, high temperature step is represented by the crystallization of a liquid metal generated by a melting episode of a chondritic precursor, likely of E chondrite parentage. It isn’t possible provide a unique explanation for this first phase of NWA 6583 history. The two solutions are a magmatic origin or an impact-melt origin. The latter is the more likely, based on the presence of silicate inclusions. The second step is related to an impact. It disrupted the parent body inducing an increase of the subsolidus cooling rate through the martensitic transition (~270°C). The final step in the history of NWA 6583 is the reheating during its ablative flight though the Earth atmosphere which was responsible of the melting of troilite and schreibersite crystals close to the external surface of the meteorite (T > 1000°C, Axon, 1963).

Possible shock-induced crystallization of skeletal quartz from supercritical SiO2-H2O fluid: A case study of impact melt from Kamil impact crater, Egypt

Since its discovery, the Kamil crater (Egypt) has been considered a natural laboratory for studying small-scale impact cratering. We report on a previously unknown shock-related phenomenon observed in impact melt masses from Kamil; that is, the shock-triggered formation of skeletal quartz aggregates from silica-rich fluids. These aggregates are unshocked and characterized by crystallographically oriented lamellar voids and rounded vesicles. The distribution of the aggregates can be correlated with former H2O- and impurity-rich heterogeneities in precursor quartz; i.e., fluid inclusions. The heterogeneities acted as hot spots for local melting. Due to the presence of H2O and the high impact pressure and temperature, the formation of a localized supercritical fluid is plausible. Below the upper critical end point of the SiO2–H2O system (temperature &lt;1100 °C and pressure &lt;1 GPa), SiO2 melt and H2O fluid become immiscible, leading to the rapid and complete crystallization of skeletal quartz

Target-projectile interaction during impact melting at Kamil Crater, Egypt

In small meteorite impacts, the projectile may survive through fragmentation; in addition, it may melt, and chemically and physically interact with both shocked and melted target rocks. However, the mixing/mingling between projectile and target melts is a process still not completely understood. Kamil Crater (45 m in diameter; Egypt), generated by the hypervelocity impact of the Gebel Kamil Ni-rich ataxite on sandstone target, allows to study the target-projectile interaction in a simple and fresh geological setting. We conducted a petrographic and geochemical study of macroscopic impact melt lapilli and bombs ejected from the crater, which were collected during our geophysical campaign in February 2010. Two types of glasses constitute the impact melt lapilli and bombs: a white glass and a dark glass. The white glass is mostly made of SiO2 and it is devoid of inclusions. Its negligible Ni and Co contents suggest derivation from the target rocks without interaction with the projectile (<0.1 wt% of projectile contamination). The dark glass is a silicate melt with variable contents of Al2O3 (0.84-18.7 wt%), FeOT (1.83-61.5 wt%), and NiO (<0.01-10.2 wt%). The dark glass typically includes fragments (from few μm to several mm in size) of shocked sandstone, diaplectic glass, lechatelierite, and Ni-Fe metal blebs. The metal blebs are enriched in Ni compared to the Gebel Kamil meteorite. The dark glass is thus a mixture of target and projectile melts (11-12 wt% of projectile contamination). Based on recently proposed models for target-projectile interaction and for impact glass formation, we suggest a scenario for the glass formation at Kamil. During the transition from the contact and compression stage and the excavation stage, projectile and target liquids formed at their interface and chemically interact in a restricted zone. Projectile contamination affected only a shallow portion of the target rocks. The SiO2 melt that eventually solidified as white glass behaved as an immiscible liquid and did not interact with the projectile. During the excavation stage dark glass melt engulfed and coated the white glass melt, target fragments, and got stuck to iron meteorite shrapnel fragments. This model could also explain the common formation of white and dark glasses in small impact craters generated by iron bodies (e.g., Wabar)

Microscopic impactor debris in the soil around Kamil crater (Egypt): Inventory, distribution, total mass, and implications for the impact scenario

We report on the microscopic impactor debris around Kamil crater (45 m in diameter, Egypt) collected during our 2010 geophysical expedition. The hypervelocity impact of Gebel Kamil (Ni‐rich ataxite) on a sandstone target produced a downrange ejecta curtain of microscopic impactor debris due SE–SW of the crater (extending ~300,000 m2, up to ~400 m from the crater), in agreement with previous determination of the impactor trajectory. The microscopic impactor debris include vesicular masses, spherules, and coatings of dark impact melt glass which is a mixture of impactor and target materials (Si‐, Fe‐, and Al‐rich glass), plus Fe‐Ni oxide spherules and mini shrapnel, documenting that these products can be found in craters as small as few tens of meters in diameter. The estimated mass of the microscopic impactor debris (20 t, likely 50–60 t)

Impact on NK cell functions of acute versus chronic exposure to extracellular vesicle-associated MICA. Dual role in cancer immunosurveillance

Natural killer (NK) cells are innate cytotoxic lymphocytes that play a key role in cancer immunosurveillance thanks to their ability to recognize and kill cancer cells. NKG2D is an activating receptor that binds to MIC and ULBP molecules typically induced on damaged, transformed or infected cells. The release of NKG2D ligands (NKG2DLs) in the extracellular milieu through protease-mediated cleavage or by extracellular vesicle (EV) secretion allows cancer cells to evade NKG2D-mediated immunosurveillance. In this work, we investigated the immunomodulatory properties of the NKG2D ligand MICA*008 associated to distinct populations of EVs (i.e., small extracellular vesicles [sEVs] and medium size extracellular vesicles [mEVs]). By using as model a human MICA*008-transfected multiple myeloma (MM) cell line, we found that this ligand is present on both vesicle populations. Interestingly, our findings reveal that NKG2D is specifically involved in the uptake of vesicles expressing its cognate ligand. We provide evidence that MICA*008-expressing sEVs and mEVs are able on one hand to activate NK cells but, following prolonged stimulation induce a sustained NKG2D downmodulation leading to impaired NKG2D-mediated functions. Moreover, our findings show that MICA*008 can be transferred by vesicles to NK cells causing fratricide. Focusing on MM as a clinically and biologically relevant-model of tumour-NK cell interactions, we found enrichment of EVs expressing MICA in the bone marrow of a cohort of patients. All together our results suggest that the accumulation of NKG2D ligands associated to vesicles in the tumour microenvironment could favour the suppression of NK cell activity either by NKG2D downmodulation or by fratricide of NK cell dressed with EV-derived NKG2D ligands