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Shock Metamorphism and Impact Melting at Kamil Crater, Egypt

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Shock Metamorphism and Impact Melting at Kamil Crater, Egypt Università di Pisa Dipartimento di Scienze della Terra Scuola di Dottorato in Scienze di Base “Galileo Galilei” Programma in Scienze della Terra XXVII Ciclo SSD GEO/07 SHOCK METAMORPHISM AND IMPACT MELTING AT KAMIL CRATER, EGYPT PhD Student Advisor Prof. Massimo D’Orazio Agnese Fazio Co-advisor Dott. Luigi Folco Anno Accademico 2013-2014 Ricorda: “Quando stai per rinunciare, quando senti che la vita è stata troppo dura con te, ricordati chi sei. Ricorda il tuo sogno”. (Il Delfino - S. Bambarén) TABLE OF CONTENTS ABSTRACT 7 RIASSUNTO 9 PREFACE 11 1. INTRODUCTION 13 1.1. IMPACT CRATERING AS A TERRESTRIAL GEOLOGICAL PROCESS 13 1.2. IMPACT CRATERING STAGES 17 1.3. SHOCK METAMORPHISM 21 1.3.1. Quartz 24 1.3.2. Deformation in other minerals 28 1.3.3. Selective and localized melting 29 1.4. IMPACT MELTING 31 1.5. SHOCK EFFECTS IN QUARTZ-BEARING ROCKS: CRYSTALLINE VS. SEDIMENTARY TARGETS 34 1.6. REFERENCES 37 2. SHOCK METAMORPHISM AND IMPACT MELTING IN SMALL IMPACT CRATERS ON EARTH: EVIDENCE FROM KAMIL CRATER, EGYPT 41 3. TARGET-PROJECTILE INTERACTION DURING IMPACT MELTING AT KAMIL CRATER, EGYPT 89 4. MICROSCOPIC IMPACTOR DEBRIS IN THE SOIL AROUND KAMIL CRATER (EGYPT): INVENTORY, DISTRIBUTION, TOTAL MASS AND IMPLICATIONS FOR THE IMPACT SCENARIO 131 5. CONCLUSIONS 161 6. FUTURE WORK 165 6.1. COMBINED MICRO-RAMAN AND TEM STUDY OF HIGH-PRESSURE PHASES FROM KAMIL CRATER (EGYPT): IMPLICATIONS FOR THEIR FORMATION IN SMALL IMPACT CRATERS ON EARTH 165 6.2. LIQUID IMMISCIBILITY FEATURES IN IMPACT MELTS 165 6.3. REFERENCES 166 APPENDIX I. USE OF THE UNIVERSAL STAGE (U-STAGE) FOR INDEXING PLANAR DEFORMATION FEATURES IN QUARTZ 169 APPENDIX II. THE EXTREMELY REDUCED SILICATE-BEARING IRON METEORITE NORTHWEST AFRICA 6583: IMPLICATIONS ON THE VARIETY OF THE IMPACT MELT ROCKS OF THE IAB-COMPLEX PARENT BODY 175 APPENDIX III. OTHER ACTIVITIES 207 ACKNOWLEDGMENTS 209 ABSTRACT 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 7 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). 8 RIASSUNTO Gli effetti di shock registrati in piccoli crateri di impatto terrestri (diametro < 300 m) sono stati poco studiati perché i piccoli crateri sono rari e spesso profondamente erosi. Kamil è una struttura di impatto giovane (<5000 anni), di piccole dimensioni (45 m di diametro) e ben preservata. È stata prodotta dall’impatto iperveloce della meteorite metallica Gebel Kamil su rocce sedimentarie dell’Egitto sudoccidentale. Il suo ottimo stato di preservazione permette di considerare Kamil un laboratorio naturale per studiare il processo di craterizzazione di piccoli impattori (circa 1 m di diametro) sulla Terra, le loro conseguenze e il loro impatto sull’ambiente terrestre per la valutazione del rischio. La presente Tesi di Dottorato contribuisce alla definizione del metamorfismo da shock e della fusione da impatto in piccoli crateri terrestri attraverso uno studio comprensivo di tipo mineralogico, petrografico e geochimico delle impattiti e dei vetri da impatto di Kamil. Questo studio permette inoltre di ampliare le conoscenze sul processo di craterizzazione legato all’impatto di meteoriti metalliche di dimensioni metriche sulla Terra. I risultati di questa Tesi di Dottorato evidenziano per la prima volta che un corpo metallico di dimensioni metriche può produrre una vasta gamma di strutture e associazioni mineralogiche da shock impattando su un target sedimentario. Queste sono state divise in due categorie, in funzione della loro abbondanza a scala della sezione sottile: i) effetti di shock pervasivi (più abbondanti), comprendenti fratturazione irregolare, piani di materiale amorfo orientati parallelamente agli indici cristallografici del quarzo o Planar Deformation Features (PDFs), lapilli e bombe di vetro da impatto; ii) effetti di shock localizzati (meno abbondanti) comprendenti fasi di alta pressione e fusione localizzata in forma di vetro intergranulare, vene e film di vetro su shatter cones (strutture coniche caratterizzate da strie disposte a coda di cavallo). Gli effetti di shock pervasivi indicano la pressione subita dalla roccia. I campioni più shockati (lapilli e bombe di vetro) indicano che la pressione al punto di contatto tra la meteorite e il proiettile era tra i 30 e 60 GPa. Sulla base del modello dell’approssimazione planare di impatto, la velocità minima di impatto della meteorite Gebel Kamil era ~5 km s-1, assumendo un angolo di impatto di 45°. Gli effetti di shock localizzati si sono formati come conseguenza di un aumento della pressione e temperatura di shock in corrispondenza dei pori e/o di eterogeneità delle rocce del target. Per questo è possibile trovare fasi di alta pressione e vetro intergranulare in campioni che hanno subito basse o moderate pressioni di shock. 9 In piccoli impatti di meteoriti, il proiettile può sopravvivere all’impatto, frammentandosi e fondendo. Il suo fuso potrà interagire con rocce del target shockate e fuse. L’interazione tra i fusi del proiettile e del target è un processo ancora non pienamente compreso. I lapilli e le bombe di vetro da impatto di Kamil sono molto freschi e il loro studio può aiutare a vincolare il processo di interazione tra target e proiettile.
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