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Tesi etd-02162015-160141

Thesis type
Tesi di dottorato di ricerca
Shock Metamorphism and Impact Melting at Kamil Crater, Egypt
Settore scientifico disciplinare
Corso di studi
tutor Prof. D'Orazio, Massimo
tutor Dott. Folco, Luigi
Parole chiave
  • coesite
  • impact melting
  • shock metamorphism
  • meteoritic impact crater
  • Kamil Crater
  • stishovite
  • diamond
  • impact glass
  • impactites
Data inizio appello
Riassunto analitico
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 (<290 kg) is a small fraction of the total impactor mass (~10 t) in the form of macroscopic shrapnel. Kamil Crater was generated by a relative small impactor that is consistent with literature estimates of its pre-atmospheric mass (>20 t, likely 50-60 t).