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AND IMPLICATIONS FOR THE IMPACT SCENARIO
LuigiFOLCO1,MassimoD’ORAZIO1, AgneseFAZIO1, Carole CORDIER2,3,AntonioZEOLI4, Matthias VAN GINNEKEN5,andAhmedEL-BARKOOKY6
1Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy 2Université de Grenoble Alpes, ISTerre, BP 53, 38041 Grenoble CEDEX 9, France
3CNRS, ISTerre, BP 53, 38041 Grenoble CEDEX 9, France
4Museo Nazionale dell’Antartide, Università di Siena, Via Laterina 8, 53100 Siena, Italy
5Department of Earth Science and Engineering, Imperial Collage, Prince Consort Road, London SW 2BP, United Kingdom 6Department of Geology, Faculty of Science, Cairo University, Giza, Egypt
In Meteoritics & Planetary Science, Accepted on 12th December 2014
Abstract – 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, 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 (<290 kg) derived from Ni-concentrations in the soil is a small fraction of the total impactor mass (~10 t) in the form of macroscopic shrapnel. That Kamil Crater was generated by a relatively small impactor is consistent with literature estimates of its pre-atmospheric mass (>20 t, likely 50-60 t).
INTRODUCTION
The characterization of microscopic projectile debris occurring at small-scale meteorite craters (≤1500 m in diameter) can improve our understanding of the physical- chemical interaction between projectile and target, including impactor fragmentation and dispersion, impact melt (and vapor) production, ejecta and plume evolution in small impact events (e.g., Hörz et al. 2002; Artemieva and Pierazzo 2009; 2011; Ebert et al. 2013; Hamann et al. 2013; Kenkmann et al. 2013; Shuvalov and Dypvik 2013).
132 Kamil Crater (Egypt; Fig. 1) is a pristine (or nearly so) type structure for very small impacts on Earth (Folco et al. 2010; 2011) and it thus offers the possibility to document the characteristics of the microscopic impactor debris, and its expansion and distribution in very small-scale meteorite impacts.
Kamil Crater is a 45-m-diameter simple crater, produced by the hypervelocity impact of the Ni-rich (~20 wt%; D'Orazio et al. 2011) Gebel Kamil iron meteorite on layered sandstones with subhorizontal bedding, interbedded with minor siltstones and wacke (Urbini et al. 2012; Fazio et al. 2014). Due to its very young age (probably <5,000 yr; Folco et al. 2011) and the dry climatic conditions in the Sahara (e.g., Kuper and Kröpelin 2006), it is exceptionally well-preserved with an unaltered crater structure, rayed pattern of bright ejecta, various types of shock-metamorphosed and impact melt rocks (from cm-sized bombs to microscopic impact spherules), and a nearly intact assemblage of fragments of the projectile that fragmented into thousands of shrapnel (i.e., unmolten projectile fragments; O’Keefe and Ahrens 1985) upon impact and were deposited in and around the crater (Folco et al. 2011; D'Orazio et al. 2011; Urbini et al. 2012; Fazio et al. 2014). The concentration of large shrapnel fragments (from ~10 g to ~30 kg) due SE of the crater indicates an oblique impact from the NW (D'Orazio et al. 2011). The mass of shrapnel found through visual search (D'Orazio et al. 2011) and geomagnetic survey (Urbini et al. 2012) indicates that the mass of the projectile was at least ~5,000 kg. Both noble gases and radionuclides in Gebel Kamil point to a pre-atmospheric mass >20 t, with a preferred estimate of 50-60 t (Ott et al. 2014). Shock metamorphic and melting features in the target indicate peak pressure between 30 and 60 GPa and impact velocities of 3.5 to 5.5 km s-1 for vertical impact or 5.0 to 7.5 km s-1 for an impact angle of 45° (Fazio et al. 2014).
In this paper, we investigate the microscopic impactor debris of Gebel Kamil. Soil samples around Kamil Crater were collected during our 2010 geophysical expedition (Folco et al. 2010). Following similar studies, mainly conducted at Barringer Crater (e.g., Niniger 1956; Rinehart 1958), this work provides an inventory of the various types of microscopic impactor material that can be produced in small-scale impact craters and constrains the trajectory of the Gebel Kamil iron meteorite, providing an estimate of its total mass at the time of impact.
SAMPLES AND ANALYTICAL METHODS
In order to study the microscopic impactor debris at Kamil Crater, we collected 44 soil samples around the crater (Fig. 1; Table 1). Samples were collected at incremental distances
from the crater rim along eight radial traverses (45° apart, starting from the north) extending up to 1.6 km from the crater rim (i.e., at about the distance from the crater rim where the most distant macroscopic shrapnel was found (D'Orazio et al. 2011). Each sample consisted of a 30 x 30 x ~5 cm soil volume, i.e. we assumed that this volume contained all the microscopic impactor debris deposited on the considered surface. This assumption is based on the fact that the fine grained ejecta deposit is typically less than some centimeters in thickness anywhere around the crater; even within the ejecta blanket, i.e., within ~50 m from the crater rim (Urbini et al. 2012), the bulk and the thickness of the deposit is given by boulders and blocks with thin fine-grained ejecta layers in between. Samples were collected using shovel, paint brush, blade and measuring tape (Fig. 2a). Samples were dry sieved in the field to obtain the <5 mm size fraction. Exploiting the contrasting magnetic properties of projectile and target rocks, the magnetic particles were subsequently extracted using a field extractor (Fig. 2b) and weighed. To complete magnetic separation, we used the Extractor-SE in-line magnetic separator, with a stainless steel housing and three Nd magnet rods arranged in a triangular matrix producing a magnetic force of 0.6 T (S+S Separation and Sorting Technology GmbH, Schönberg, Germany). A separation efficiency >80% was derived through a number of random field tests at various locations.
Fig. 1. Kamil Crater, southern Egypt, and location map of the collected soil samples. Base map: enhanced true color QuickBird satellite image (22 October 2005; courtesy of Telespazio S.p.A.).
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Table 1. Magnetic extract <5 mm of soil samples from Kamil Crater: location and mass. Samples are listed according to the bearing of the profiles starting from the north, and to their increasing distance from crater rim.
Lable Location Profile/ Distance from crater rim (m) Latitude S Longitude E a.s.l. (m) Altitude magnetic Mass of extract (g) S12 N 15 26° 05' 18" 22° 01' 06" 599 4.5 S13 N 100 26° 05' 19" 22° 01' 05" 602 6.6 S14 N 200 26° 05' 23" 22° 01' 06" 602 7.8 S15 N 400 26° 05' 29" 22° 01' 07" 609 8.8 S16 N 800 26° 05' 42" 22° 01' 06" 615 13.3 S35 NE 15 26° 05' 59" 22° 01' 03" 597 2.8 S36 NE 100 26° 05' 14" 22° 01' 07" 596 10.5 S37 NE 200 26° 05' 11" 22° 01' 06" 608 18.4 S38 NE 400 26° 05' 07" 22° 01' 05" 604 11.0 S39 NE 800 26° 05' 00" 22° 01' 03" 605 9.0 S01 E 15 26° 04' 47" 22° 01' 07" 603 15.2 S02 E 100 26° 05' 15" 22° 01' 07" 601 11.7 S03 E 200 26° 05' 15" 22° 01' 10" 605 13.3 S04 E 400 26° 05' 16" 22° 01' 13" 604 4.0 S05 E 800 26° 05' 14" 22° 01' 19" 615 4.0 S06 E 1200 26° 05' 13" 22° 01' 33" 606 23.1 S23 SE 15 26° 05' 15" 22° 01' 04" 597 12.1 S24 SE 100 26° 05' 15" 22° 01' 02" 595 21.7 S25 SE 200 26° 05' 15" 22° 00' 58" 600 18.2 S26 SE 400 26° 05' 14" 22° 00' 58" 596 17.1 S27 SE 800 26° 05' 15" 22° 00' 36" 592 9.8 S28 SE 1220 26° 05' 22" 22° 00' 24" 588 10.1 S17 S 15 26° 05' 16" 22° 01' 04" 600 10.3 S18 S 100 26° 05' 17" 22° 01' 02" 594 25.3 S19 S 200 26° 05' 20" 22° 01' 00" 596 13.7 S20 S 400 26° 05' 25" 22° 00' 56" 595 15.6 S21 S 800 26° 05' 35" 22° 00' 47" 591 7.6 S22 S 1280 26° 05' 44" 22° 00' 36" 592 7.5 S29 SW 15 26° 05' 14" 22° 01' 05" 599 11.7 S30 SW 100 26° 05' 12" 22° 01' 03" 597 6.4 S31 SW 200 26° 05' 10" 22° 01' 01" 602 12.7 S32 SW 400 26° 05' 04" 22° 01' 01" 596 5.3 S33 SW 800 26° 04' 58" 22° 00' 46" 596 6.1 S34 SW 1160 26° 04' 46" 22° 00' 46" 592 13.7 S07 W 15 26° 05' 17" 22° 01' 07" 597 2.9 S08 W 100 26° 05' 18" 22° 01' 09" 593 5.9 S09 W 200 26° 05' 21" 22° 01' 11" 592 5.7 S10 W 400 26° 05' 24" 22° 01' 16" 596 7.9 S11 W 800 26° 05' 33" 22° 01' 25" 603 8.6 S41 NW 100 26° 05' 12" 22° 01' 09" 596 3.0 S43 NW 400 26° 05' 06" 22° 01' 11" 600 5.0 S44 NW 800 26° 04' 53" 22° 01' 27" 596 7.6
Fig. 2. Field photos of steps in the collection of soil samples around Kamil Crater (A, B) and stereomicrographs of representative samples of their magnetic components (C, D, E). A) A surface soil volume measuring 30 x 30 x ~5 cm (arrowed) was collected at each site and sieved (<5 mm). Inset: a detail of the sampling surface. B) The sieved sample was then let through a powerful field magnetic separator (arrowed) to obtain the magnetic extract
studied in this work. The picture features one of the preliminary extraction test. C) The magnetic extract of sample S23 (<5 mm; 15 m due SE of crater rim) showing abundant dark impact melt glass particles mainly in the shape of vesicular clastic masses, mini-to-microscopic individual or compound spheroids, and coatings of pale target rock particles (mainly shocked sandstone fragments and or lechatelierite, as revealed by electron microscopy and microanalyses). Subordinate mini-to-microscopic iron meteorite shrapnel and microscopic Fe-
Ni-oxide spherules also occur. D) The magnetic extract of sample S44 (<5 mm; 800 m due NW of crater rim) dominated by Precambrian crystalline basement rock fragments (mainly amphibolite gneiss). E) Close up view of the magnetic extract of sample S27 (<400 µm; 800 m due SE crater rim) showing abundant Fe-Ti oxide plus garnet grains of terrestrial origin. Abbreviations: CS: cosmic spherule; DG: individual masses of dark impact melt glass; DGc: dark glass coatings; DG sph: dark glass spheroids; Fe-Ti ox: Fe-Ti oxides; GK: Gebel Kamil
iron meteorite shrapnel; Grt: garnet; Fe-Ni sph: Fe-Ni spherule; Qtz: quartz.
The magnetic extracts from each soil sample were first observed under the stereomicroscope in order to identify the various types of magnetic particles, including impactor debris and terrestrial particles, and their overall distribution around the crater.
A selection of the magnetic particles from a representative number of samples was then studied under the Scanning Electron Microscope (SEM). Particles were first observed as whole specimens to characterize their morphology and structure. They were then sectioned in order to define their texture and overall composition. Analyses were performed using a Philips XL30 SEM, operating at 20 kV coupled with an energy-dispersive X-ray fluorescence spectrometer (EDX), available at Pisa University’s Dipartimento di Scienze della Terra, and
136 Field Emission Scanning Electron Microscope (FE-SEM) Jeol JSM 6500F (upgraded to 7000 series), available at the Istituto di Geofisica e Vulcanologia (INGV) of Rome.
The mineral chemistry of the various phases in a selection of the above particles was obtained by electron probe micro-analyses (EPMA) using a Cameca SX50 electron microprobe at IGG-CNR, Padua (Table 2). Running conditions were 15 kV accelerating voltage, 15 nA beam current, and 1 µm nominal beam spot. The manufacturer-supplied PAP procedure was employed for raw data reduction. Standards used for instrumental calibration were natural minerals (quartz, diopside, apatite and sphalerite), and pure elements (Fe, Ni, and Co).
Table 2. Average major element bulk composition by EPMA of dark glass impact melt (in the form of vesicular particles, spherules and coatings), Fe-Ni oxide blebs in dark glass, Fe-oxide spherules, lechatelierite in dar
glass, and Fe-Ni metal blebs in dark glass (distinguished in high-Ni and very high-Ni compositional types). Limits of detection (LOD) for the analyses of silicates and oxides are: Mg = 0.03 wt%, Al = 0.04 wt%, Si = 0.04
wt%, P = 0.05 wt%, S = 0.04 wt%, K = 0.04 wt%, Ca = 0.04 wt%, Ti = 0.04 wt%, Cr = 0.06 wt%, Mn = 0.07 wt%, Fe = 0.04 wt%, Co = 0.08 wt%, Ni = 0.04 wt%. LOD for the analyses of the metal phases are: Si = 0.03
wt%, P = 0.04 wt%, S = 0.04 wt%, Cr = 0.05 wt%, Fe = 0.04 wt%, Co = 0.06 wt%, Ni = 0.04 wt%. Glass - vesicular Glass - spherules Glass - coatings Fe-oxide blebs Fe-Ni-oxide spherules L
a Fe-Ni metal blebs
H- Nid VH-Nie n. p.b 1 6 3 2 11 9 2 n. an.c 9 9 48 5 29 28 2 ox wt% σ ox wt% σ ox wt% σ ox wt% σ ox wt% σ el wt% σ el wt% MgO 0.18 0.07 0.23 0.04 0.30 0.04 0.14 0.01 0.04 0. 01 bdl Si 0.23 0.62 0.17 Al2O3 10.4 2.8 12.3 1.9 16.1 1.4 4.79 2.48 0.06 0.05 0.08 P 0.67 0.57 bdl SiO2 57.0 10.1 49.2 4.0 47.3 3.2 4.81 1.65 0.77 1.05 99.5 S 2.19 1.71 2.38 P2O5 bdl 0.09 0.05 0.13 0.03 0.57 0.62 0.10 0.11 bdl Cr bdl bdl SO3 0.20 0.05 0.12 0.07 0.08 0.10 0.02 0.02 bdl 0.09 Fe 69.1 4.7 4.45 K2O 0.13 0.03 0.23 0.07 0.38 0.06 na na bdl Co 0.91 0.18 0.10 CaO 0.57 0.14 0.49 0.09 0.50 0.28 0.27 0.03 bdl 0.00 bdl Ni 26.3 4.5 94.2 TiO2 1.06 0.31 0.76 0.05 0.81 0.05 0.44 0.15 bdl bdl tot 99.4 101.3 Cr2O3 0.10 0.05 bdl bdl 0.07 0.05 bdl bdl MnO bdl 0.79 0.30 1.16 0.09 0.08 0.04 bdl 0.00 bdl FeO* 27.6 7.5 30.9 3.7 29.2 3.0 79.6 4.32 71.6 1.92 0.23 CoO bdl 0.10 0.05 bdl bdl 0.82 0.09 bdl NiO 0.16 0.10 2.71 1.08 1.54 0.63 0.08 0.02 20.9 1.90 bdl tot 97.4 97.9 97.5 90.9 94.3 99.9 a Lechatelierite.
b Number of analysed particles. c Number of total analyses in the average. d High-Ni blebs.
e Very high-Ni blebs.
*All Fe as FeO.
Abbreviations: na = not analyzed; bdl = below detection limit.
Whole-rock major elements of unshocked target material of Kamil area (namely, sandstones ranging from very coarse quartzarenite to coarse siltstone with intercalated levels of very fine wacke; Table 3) were determined for geochemical comparison with impact glass.
Major elements were determined on glass beads by X-Ray Fluorescence (XRF, ARL 9400 XP spectrometer) at Pisa University’s Dipartimento di Scienze della Terra (Italy), using the procedure described by Tamponi et al. (2003). Nickel and Co were determined by Inductively Coupled Plasma Mass Spectrometry (ICPMS; Perkin Elmer NexION 300x spectrometer) available at the Dipartimento di Scienze della Terra of the University of Pisa.
Table 3. Chemical composition of Kamil Crater target rocks (X-ray fluorescence analyses, except where otherwise specified). Samples were collected from the crater walls (Urbini et al. 2012; Fazio et al. 2014). Target
rocks consist of layered sandstones with subhorizontal bedding belonging to the Gilf Gebir Formation. A 70 cm thick, pale quartzarenite layer (sample M26) is at the top of the impacted sedimentary sequence. The underlying rocks consists of reddish, ferruginous quartzarenite layers up to some tens of cm in thickness (sample M27). The pale quartzarenite sporadically contains pale wake with siltstone levels. A kaolinite-rich matrix occurs in the
reddish quartzarenite.
Pale
quartzarenite
Pale wacke with silstone levels Reddish quartzarenite, ~ 5 vol.% matrix Kaolinite-rich matrix (n=18)a sample M26 M25 M27 M27 oxide wt% SiO2 99.8 87.8 95.7 43.9 TiO2 0.07 1.15 0.09 0.55 Al2O3 0.25 10.1 1.98 34.7 Fe2O3T 0.11 0.74 1.71 1.88 MnO <0.01 0.02 0.06 b.d.l. MgO 0.18 0.14 0.25 0.22 CaO 0.05 0.11 0.31 0.05 Na2O 0.05 0.13 0.08 0.12 K2O <0.01 0.04 <0.01 0.04 P2O5 <0.01 0.06 0.04 0.09 sum 100.5 100.3 100.2 81.5 L.O.I. 0.65 3.91 1.19 element µg g-1 Nib 7 15 Cob <1 12
a Average of EPMA analyses. b Analyses by ICP-MS. Fe2O3 T: total iron as Fe2O3.
L.O.I.: Loss On Ignition.
In order to quantify the amount of the iron impactor debris and to define its distribution around the crater, we determined the concentration of a number of elements in aliquots of the magnetic extracts (obtained using a small stainless steel splitter), including the expected tracers of the iron impactor Fe, Ni, Co, P (Table 4). Elemental concentrations were determined through the total dissolution method (HF + HNO3 + HClO4 + HCl) followed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) analyses at ACTALABS. Due to the high concentration of Ni (>10000 µg g-1) in samples S23 and S24, they were re-run for siderophile elements at an appropriate dilution using ICPS-MS, at the Dipartimento di Scienze della Terra of the University of Pisa. Note that prior to grinding and
138 powdering of the magnetic extract for geochemical analyses, we separated, counted and weighed Gebel Kamil shrapnel down to 1 mm in size from each magnetic sample under the stereo microscope (the two fractions are denominated "shrapnel fraction" and "soil fraction" in Table 4). This procedure was adopted in order to avoid problems in powdering and dissolving relatively large metal particles, and discuss the distribution and mass of the impactor debris in the soil with and without mini shrapnel.
RESULTS
Petrography of magnetic particles
A first survey of the magnetic extracts under the stereomicroscope and the microanalytical SEM-EDS revealed a systematic difference in the distribution around the crater of the various types of magnetic particles. The magnetic extracts from samples collected in the E to SW sector are dominated by dark Fe-Ni glass in a variety of structures and occurrences, plus minor Fe-Ni oxide spherules and Gebel Kamil shrapnel (Fig. 2c). Magnetic extracts from the NW sector are dominated by amphibolite gneiss particles (Fig. 2d) belonging to the Precambrian Basement that crops out 100-150 m due NW of the crater rim (Urbini et al. 2012). In the other sectors, the magnetic extracts are dominated by crystals or crystal fragments of terrestrial Fe- and Fe-Ti oxides, garnet and minor cosmic spherules mainly belonging to the barred olivine - type (Fig. 2e). Although aeolian quartz grains and sandstone particles are ubiquitous, they are more abundant in the samples collected due W of the crater. Although we cannot rule out that these particles could result from inefficient magnetic extraction, note that they often contain tiny inclusions of ferro-magnetic minerals or they are coated by ferruginous deposits. Furthermore, we shall see that the external surfaces of the aeolian quartz grains and sandstones particles from the E-to-the-S sectors are speckled with tiny impactor splashes of dark Fe-Ni dark impact melt.
In the following two sections, we will focus on Fe-Ni particles, i.e. dark Fe-Ni impact melt glass and Fe-Ni oxide spherules. Due to their Ni-rich composition, we shall see that they are the only microscopic tracers of the Ni-rich impactor debris. For details on Gebel Kamil shrapnel the reader should refer to D'Orazio et al. (2011).
Dark Fe-Ni impact glass
Dark Fe-Ni glass occurs in the form of highly vesicular particles (Fig. 3), spherules (Fig. 4) and coatings (Fig. 5).
Dark glass vesicular particles are irregular masses characterized by delicate reticulite to pumiceous microstructures (Figs. 2 and 3). Under the stereomicroscope they are brown to dark green. The composition of the glass (Table 2) is slightly variable and dominated by SiO2
(57.0 ± 10.1 wt%), FeO (27.6 ± 7.5 wt%) and Al2O3 (10.4 ± 2.8 wt%), with subordinate TiO2
(1.06 ± 0.31 wt%) and NiO (0.16 ± 0.10 wt%). Abundant oxide blebs up to tens of micrometers in size float within the glass (Fig. 3). They consist mainly of micrometer-sized Fe-oxides and are here referred to as "Fe-oxide blebs". Their bulk composition (Table 2) is dominated by FeO (79.6 ± 4.32 wt%), SiO2 (4.81 ± 1.65 wt%) and Al2O3 (4.79 ± 2.48 wt%)
with traces of NiO (0.08 ± 0.02 wt%) and P2O5 (0.57 ± 0.62 wt%). Fe-Ni metal blebs, here
referred to as "Fe-Ni metal blebs", are sometimes also observed in the dark glass. They contain abundant Ni (26.3 - 94.2 wt%; Table 2), 4.45 - 69.1 wt% Fe, minor S (~2 wt%), and traces of P (~0.6 wt%), Co (<1.0 wt%) and Si (<0.2 wt%), suggesting that there was some mixing with the target material. Both types of blebs show nanometer-sized cellular intergrowths of - as yet - unidentified phases indicative of eutectic melts (Fig. 3a, inset). Submillimeter-sized, bedrock clasts (mainly sandstones) may also be found embedded in the dark glass (Fig. 3a).
Fig. 3. Back scattered electron images of dark impact melt glass in the form of the vesicular masses from sample S23. A) A highly vesicular particle consisting of a tenuous reticular network of glass. Oxide blebs up to tens of
microns in size enriched in Fe, Ni, ± P, ± S float within the host glass (inset). They consist mainly of micron- sized oxides and are here referred as "Fe-oxide blebs". Less frequently, Ni-rich metal blebs occur and are here
referred as "Fe-Ni-metal blebs". A target rock clast (shocked sandstone) embedded in glass is arrowed. B) A detail of another pumiceous particle showing abundant Fe-oxide and Fe-Ni metal blebs. Abbreviations: CL:
target rock clast; DG: dark glass, Fe-Ni bl: Fe-oxide and Fe-Ni metal blebs.
The dark glass spherules (Fig. 4) are shiny and black under the stereomicroscope and show a number of splash form morphologies similar to those observed at Barringer Crater (e.g., Hörz et al. 2002): from ovoids and irregular dumbbell to nearly perfect spherules, in order of decreasing abundance. Some are delicately attached to each other and form
140 compound spherules (Fig. 4b). Others are partially covered by splashes of other spherules (Fig. 4c). The vesicularity of the dark glass varies from moderate to negligible (compare Fig. 4a and Fig. 4c). The external surfaces of the most vesicular particles are characterized by the occurrence of boiling and degassing microstructures including quenched bubbles and glass filaments (Fig. 4d). The filaments originate from the vesicles and are hollow. The external surfaces of the spherules are also finely decorated by dendrites of unidentified Fe-rich oxides <1 µm in size embedded in glass, often arranged in schlieren (Figs. 4a, 4d, 4e). Sectioned spherules (Fig. 4f) show the characteristic vesicularity of the dark glass, its compositional heterogeneity and the occurrence of several mineral and lithic inclusions. Inclusions mainly consist of bedrock fragments (quartzarenite), high temperature (T >1,500 °C) silica glass or lechatelierite and oxide and metal blebs (as described above). The heterogeneity of the glass is highlighted by schlieren and haloes with variable Si/Fe ratios around the inclusions. The composition of the glass (Table 2) is slightly variable, dominated by SiO2 (49.2 ± 4.0 wt%),
FeO (30.9 ± 3.7 wt%), and Al2O3 (12.3 ± 1.9 wt%), with subordinate NiO (2.71 ± 1.08 wt%)
and TiO2 (0.76 ± 0.01 wt%).
Fig. 4. Back scattered electron images of dark glass spherules from sample S23. A) An ovoid spheroid showing abundant vesicles and a large glass bubble. B) Three spheroids stuck together to form a compound particle. C) A Fe-rich liquid splashed onto a glass spherule. D) A detail of the typical glass spheroid surface showing, trails (schlieren) of Fe-rich oxides, glass bubbles and glass filaments. The insets show that the filaments originate from the vesicles and are hollow. E) A sectioned spherule showing the characteristic abundant vesicularity of
the dark glass, its compositional heterogeneity and the occurrence of several mineral and lithic inclusions. Inclusions mainly consists of fragments quartzite, lechatelierite (vesicular) and Fe-oxide and Fe-Ni-metal blebs.
The heterogeneity of the glass is highlighted by schlieren and haloes around the inclusions. Abbreviations: B: glass bubble; F: hollow glass filament; Fe-Ni B: Fe-oxide and Fe-Ni metal blebs; L: lechatelierite; Ox: oxides;
Qtz: quartz, Sch: schlieren.
Dark glass coatings (Fig. 5) consist of mantles of shiny black glass continuously (or nearly so) enveloping small target lithic fragments (mainly quartzarenite) and lechatelierite.
They are up to some 100 µm in thickness. The morphological, textural, microstructural and compositional characteristics of their external surfaces are similar to those observed in dark glass spherules (see above). Sectioned particles show that the glass coating is vesicular and envelops lithic fragments made by variably shocked and melted target quartzarenite, i.e. lechatelierite (Fig. 5d). The composition of the glass (Table 2) is slightly variable, dominated by SiO2 (47.3 ± 3.2 wt%), FeO (29.2 ± 3.0 wt%), and Al2O3 (16.1 ± 1.4 wt%), with
subordinate NiO (1.54 ± 0.63 wt%) and TiO2 (0.81 ± 0.05 wt%). Similar glass coatings have been reported from Wabar (e.g., Mittlefehldt et al. 1992) and lunar samples (e.g., Apollo 16 lunar samples; Grieve and Plant 1973) and, at a cm-scale, in impact melt lapilli and bombs at Barringer Crater (e.g., Niniger 1954) and Kamil Crater (Fazio et al. 2014).
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Fig. 5. Stereomicrographs (A, B) and back scattered electron images (C, D) of dark impact melt coatings from sample S23. A) A whole particle coated with a shiny dark bubbly glass. B) A broken particle showing their
typical structure: a dark glass coating few tens of micron in thickness continuously envelops a target rock