T ARGET P ROJECTILE I NTERACTION DURING I MPACT M ELTING

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AgneseFAZIO1*,MassimoD’ORAZIO1, Carole CORDIER2,3,and LuigiFOLCO1

1_{Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy} 2_{Université de Grenoble Alpes, ISTerre, BP 53, 38041 Grenoble CEDEX 9, France}

3_{CNRS, ISTerre, BP 53, 38041 Grenoble CEDEX 9, France}

Abstract – In small meteorite impacts, the projectile may survive the impact through

fragmentation; in addition it may melt and chemically and physically interact with both shocked and melted target rocks. However, the mixing between projectile and target melts is a process that is 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 very 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 inclusion-free, it is mostly made of SiO2, and it has

negligible amount of Ni and Co, suggesting derivation from the target rocks with negligible interaction with the projectile melt (<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 iron meteorite Gebel Kamil. 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. 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.

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INTRODUCTION

Impact melting is a common process occurring during meteorite impacts on solid bodies of the Solar System. On Earth, evidences of this process have been found as impact melt rocks (kilometer- to centimeter-scale) forming layered bodies in the crater or in the ejecta blanket, as dykes, veins, and vein networks (kilometer to centimeter-scale) in the crater basement, or as centimeter- to micrometer-sized masses and spherules in distal strewn fields called tektites and microtektites (e.g., Stöffler and Grieve, 2007; Osinski et al. 2013). Most impact melt rocks have been produced by large meteoritic impacts of projectiles more than tens of meters in diameter or larger. Impact melt rocks reflect the chemical composition of the target rocks; indeed the contribution of the projectile is typically lower than 1 vol%, because the projectile is entirely vaporized during large meteoritic impacts. Thus, the nature of the projectile can be detected only through the analysis of few geochemical fingerprints such as the distribution of siderophile elements or the isotope composition of osmium and chromium (e.g., Koeberl et al. 2012; Goderis et al. 2013). During small impact events (final crater diameter < 1.5 km) impact melt rocks are formed by the melting of both the target and the projectile; the projectile survives to the impact and its partially melted and fragments can be found in the proximity of the crater (e.g., Wabar (largest crater 116 m in diameter), Kamil (45 m in diameter), Henbury (157 m in diameter), Aouelloul (390 m), Barringer (1.2 km in diameter)). Nineteen out of 27 small impact craters were generated by the impact of iron or stony-iron projectiles (Earth Impact Database, accessed on December 2014). Target rocks and iron projectiles have extremely different chemical compositions, thereby facilitating our understanding of the mixing-mingling processes between projectile and target. In recent years, new steps forward in this field have been done thanks to new experiments carried out in the framework of the MEMIN (Multidisciplinary Experimental and Modeling Impact Research Network) program (Ebert et al. 2013 and 2014) and thanks to new evidence from natural impactites (e.g., Wabar crater; Hamann et al. 2013).

In this paper we present the results of the petrographic and geochemical study of impact melt lapilli and bombs from Kamil Crater (southwester Egypt; Folco et al. 2010; 2011). Impact melt lapilli and bombs are made by two types of glasses. These glasses have different petrographic features (glass color, type and number of inclusions) and show a bimodal distribution of the projectile contamination: white glass (white in color, almost inclusion free, and < 0.1 wt% of projectile contamination) and dark glass (grayish to greenish in color, inclusion rich, and 11-12 wt% of projectile contamination). These results will be

used to constrain the mechanism of formation of impact melt rocks in small terrestrial impact craters.

BACKGROUND

Kamil is a small impact structure (45 m in diameter) in southwestern Egypt (22°01’06’’N, 26°05’16’’E). It was discovered in 2008 (Folco et al. 2010 and 2011). In 2010, an Italian-Egyptian geophysical campaign was organized with the aim to conduct a geological and geophysical survey of the crater, a systematic collection of macroscopic meteorite specimens, a systematic collection of microscopic impactor debris and an explorative sampling of macroscopic ejecta, including impact melt lapilli and bombs and target blocks. The results of the geological and geophysical survey are reported in Urbini et al. (2012). The petrography and the geochemistry of the iron meteorite Gebel Kamil are discussed in D’Orazio et al. (2011). The main petrographic and geochemical features of shocked and melted ejecta are presented in Fazio et al. (2014). The study of the microscopic impactor debris is reported in Folco et al. (2015). In the following paragraphs we summarize the main features of the target rocks, of the iron projectile, and of the ejecta.

Target rocks

Kamil Crater is located in a rocky desert area in the East Uweinat district in southwestern Egypt. The impact of Gebel Kamil affected only the sedimentary rocks of the Gilf Kebir Formation, without involving the Precambrian crystalline basement, cropping out due northwest of the crater (> 150 m). In the crater area, the Gilf Kebir Formation consists of sandstone with subhorizontal bedding consisting of reddish brown rocks topped by a meter- thick level of pale rocks (Urbini et al. 2012).

Pale and reddish brown rocks are both made of up to 98 vol% quartz; Fe-Ti oxides, tourmaline and zircon are the most common accessory phases. Pale rocks are coarse quartzarenite to medium-fine-grained wacke with siltstone levels. The matrix is mainly composed by kaolinite (Table 1), and its abundance increases with decreasing grain-size, ranging from negligible contents in coarse quartzarenite up to 40 vol% in medium-fine- grained wacke. Porosity is usually lower than 4 vol%. In coarse quartzarenite rocks, pores are filled by an extensive overgrowth of syntaxial quartz cement. Reddish rocks are very coarse- to coarse-grained, gritty and ferruginous quartzarenite. Iron oxides and hydroxides are important components of the matrix together with kaolinite (Table 1). The porosity of reddish quartzarenites is generally higher than that of pale rocks (up to 24 vol%). Target rocks from Kamil area show values of loss on ignition (L.O.I.) ranging from 0.65 and 3.91. These

92   contents are mainly attributable to the presence of H2O from the kaolinite of the matrix. The

contribution of other volatile compounds is negligible due to the mineral composition of the target rocks.

Table 1. Chemical composition of target rocks at Kamil Crater. Major elements (wt%) were determined by X-ray fluorescence analyses, except where otherwise specified. Trace elements (ppm) were determined by

ICP-MS. Modified after Folco et al. (2015).

Pale quartzarenite, negligible matrix

contents

Pale wacke with siltstone levels Reddish quartzarenite, ~ 5 vol.% matrix Kaolinite-rich matrix (n=18)a SiO2 99.8 87.8 95.7 44.5 TiO2 0.07 1.15 0.09 0.55 Al2O3 0.25 10.1 1.98 34.8 FeO T 0.10 0.66 1.54 1.18 MnO <0.01 <0.02 0.06 <0.01 MgO 0.18 0.14 0.25 0.06 CaO 0.05 0.11 0.31 0.04 Na2O 0.05 0.13 0.08 0.04 K2O <0.01 0.04 <0.01 0.05 P2O5 <0.01 0.06 0.04 0.09 sum 100.5 100.2 100.1 81.3b L.O.I. 0.65 3.91 1.19 Trace elements Co 0.5 2.1 10 Ni 6.9 7.3 13 Cu 5.2 8.9 15 V 3.0 53 32 Cr 20 57 9.5 Ni/Fe 0.009 0.001 0.001 Ni/Co 13.2 3.51 1.19

a _{Average of EPMA analyses.}

b _{The low total of the average of the EPM analyses of kaolinite is assumed to be due to the water content of this}

mineral (~19 wt%). FeOT: total iron as FeO.

Abbreviation: L.O.I.: Loss On Ignition.

Projectile

Kamil was formed by the hypervelocity impact of the iron meteorite Gebel Kamil. Gebel Kamil is an ungrouped ataxite (Ni = 20.6 wt%) characterized by a very fine-grained duplex plessite metal matrix (D'Orazio et al. 2011). Accessory minerals are schreibersite, troilite, daubréelite and native copper, in order of decreasing abundance.

Thousands of meteorite fragments were found around the crater. They are all shrapnel in the <1 g to 34 kg mass range with the exception of a single regmaglypted individual of 83 kg.

The systematic visual search of specimens > 10 g (D'Orazio et al. 2011) and the geomagnetic survey (buried shrapnel > 100 g; Urbini et al. 2012) indicate that the minimum projectile mass is close to 5 t. However, the total mass of the Gebel Kamil meteorite is close to 10 t taking into account the estimate of the mass of shrapnel < 10 g, buried shrapnel < 100 g, and impactor debris fraction (Folco et al. 2015). This value is in agreement with estimates of its pre-atmospheric mass, namely >20 t, most likely 50-60 t (Ott et al. 2014). The absence of companion craters suggests that Gebel Kamil meteorite impacted the ground as a single mass or a very tight cluster of fragments, and that it underwent only minor fragmentation and separation during atmospheric flight (Folco et al. 2010 and 2011).

Ejecta

A radial pattern of ejecta characterizes the surrounding of Kamil Crater (Urbini et al. 2012). It is formed by meteorite shrapnel fragments, shocked and unshocked sandstone blocks, and impact melt lapilli and bombs. The bulk of the ejecta material is preferentially concentrated between the two main ejecta rays trending to the north and southwest, respectively, indicating that the projectile arrived from NW. Within ~ 50 m from the crater rim unshocked sandstone clasts and impact melt lapilli and bombs are more common; further away, the shocked sandstone clasts dominate.

Shocked sandstone blocks show an almost complete set of shock metamorphic features including fracturing, planar deformation features (PDFs) in quartz, high-pressure SiO2 polymorphs, diamond, melt veins and melt in shatter cones (Fazio et al. 2014). Some of

these features have never been reported before from impact craters of comparable size.

Impact glass lapilli and bombs are constituted by two types of glasses: the white glass (Figs. 1a and 1b) and the dark glass (Figs. 1c and 1d; Table 2). Dark glass is dark-gray to brown in color, magnetic, and contains several target fragments, whereas white glass is white to light gray in color, non-magnetic, and nearly clast free. White glass can be completely or partially enveloped by dark glass (Fig. 1b), whereas the reverse has never been observed. Similar relations are common also among the micrometric impact melt particles sampled in the soils around the crater (Folco et al. 2015). Dark glass fragments were also found stuck onto the meteorite shrapnel (Fig. 1d).

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Fig. 1. Impact glasses. a) Cut surface of the white impact melt bomb L04. b) Cut surface of the white impact melt bomb L03 coated by dark glass. c) Cut surface of the dark impact melt bomb L06. The dark glass contains abundant fragments of sandstone and lechatelierite, and rare fragments of meteorite. Abbreviations: MF = meteorite fragment; SC = shocked sandstone clasts; L = lechatelierite. d) Dark glass stuck onto a meteorite shrapnel fragment (see white arrows). The side

Table 2. Location and description of impact melt lapilli and bombs from Kamil Crater (Table modified after Fazio et al., 2014).

Sample Latitude N Longitude E Location Description

Ejecta: impact melt lapilli and bombs

L15 22°01'6.05" 26°05'15.77" Inside the crater Lapillus made of dark glass with inclusions of sandstone clasts, diaplectic glass and lechatelierite. Metallic blebs are scattered in the dark glass.

M23 22°01'5.69" 26°05'16.27" Inside the crater Bomb made of white lechatelierite. Vesicles are rounded and generally micrometric. L08 22°01'4.05" 26°05'16.53" 52 m due ENE of CR Bomb made of white lechatelierite partially coated by < 2 mm thick dark glass _envelope.

L09 22°01'4.01" 26°05'16.57" 52 m due ENE of CR

Lapillus made of white lechatelierite stained by reddish-brown material (iron-oxi- hydroxides). Relicts of highly shocked quartz grains (melted PDFs, amorphization) are common in this sample.

L05 22°01'5.29" 26°05'18.17" 57 m due E of CR Lapillus made of dark glass with inclusions of sandstone clasts, diaplectic glass and lechatelierite. Metallic blebs are scattered in the dark glass.

L06 22°01'5.27" 26°05'18.14" 57 m due E of CR

Bomb made of dark glass with inclusions of sandstone clasts, diaplectic glass and lechatelierite. Metallic blebs are scattered in the dark glass. Rare occurrence of fragments of the iron projectile Gebel Kamil.

E30m.s. 22°01'3.39" 26°05'16.44" 57 m due ESE of CR

Dark glass found stuck to the surface of a shrapnel of the iron meteorite Gebel Kamil. Inclusions of sandstone clasts, diaplectic glass and lechatelierite are common. Metallic blebs are scattered in the dark glass.

MNA07m.s. 22°01'4.51" 26°05'18.11" 61 m due SSE of CR

Dark glass found stuck to the surface of a shrapnel of the iron meteorite Gebel Kamil. Inclusions of sandstone clasts, diaplectic glass and lechatelierite are common. Metallic blebs are scattered in the dark glass.

L03 22°01'7.91" 26°05'19.88" 120 m due ENE of CR Bomb made of white lechatelierite completely coated by < 2 mm thick dark glass _envelope. L04 22°01'7.93" 26°05'19.92" 120 m due ENE of CR Bomb made of white lechatelierite. Vesicles are elongated and can be up to 3 mm in

size.

Abbreviations: CR: crater rim; m.s.: meteorite shrapnel.