Chemical modification of projectile residues and target material in a MEMIN cratering experiment

Abstract-The very young Wabar craters formed by impact of an iron meteorite and are known to the scientific community since 1933. We describe field observations made during a visit to the Wabar impact site, provide analytical data on the material collected, and combine these data with poorly known information discovered during the recovery of the largest meteorites. During our visit in March 2008, only two craters (Philby-B and 11 m) were visible; Philby-A was completely covered by sand. Mapping of the ejecta field showed that the outcrops are strongly changing over time. Combining information from different visitors with our own and satellite images, we estimate that the large seif dunes over the impact site migrate by approximately 1.0-2.0 m yr À1 southward. Shock lithification took place even at the smallest, 11 m crater, but planar fractures (PFs) and undecorated planar deformation features (PDFs), as well as coesite and stishovite, have only been found in shock-lithified material from the two larger craters. Shock-lithified dune sand material shows perfectly preserved sedimentary structures including cross-bedding and animal burrows as well as postimpact structures such as open fractures perpendicular to the bedding, slickensides, and radiating striation resembling shatter cones. The composition of all impact melt glasses can be explained as mixtures of aeolian sand and iron meteorite. We observed a partial decoupling of Fe and Ni in the black impact glass, probably due to partitioning of Ni into unoxidized metal droplets. The absence of a Ca-enriched component demonstrates that the craters did not penetrate the bedrock below the sand sheet, which has an estimated thickness of 20-30 m.

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“…The latter is comparable to experimental work by Ebert et al. () and earlier studies on Wabar material (Mittlefehldt et al. ).…”

Section: The Wabar Impact Sitesupporting

confidence: 92%

The Wabar impact craters, Saudi Arabia, revisited

Gnos

Hofmann

Halawani

et al. 2013

Meteorit & Planetary Scien

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“…We observe this brightening typically also for other crushed or shocked ejecta in comparison to the less damaged host rock and ejecta (Ebert et al. ). Some of the fragments were also coated with crushed, powdered host‐rock material.…”

Section: Resultssupporting

confidence: 78%

“…For this reason we also see a variety of shock features from crushing, over PF and PDF formation, up to melting and vaporization, as well documented in Ebert et al. (). Nevertheless, with the given fast shock wave attenuation rates, the volume of higher shocked particles is very limited.…”

Section: Discussionsupporting

confidence: 78%

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Formation of shatter cones in MEMIN impact experiments

Wilk

Kenkmann

2016

Meteorit & Planetary Scien

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Shatter cones are the only macroscopic feature considered as evidence for shock metamorphism. Their presence is diagnostic for the discovery and verification of impact structures. The occurrence of shatter cones is heterogeneous throughout the crater record and their geometry can diverge from the typical cone shape. The precise formation mechanism of shatter cones is still not resolved. In this study, we aim at better constraining the boundary conditions of shatter cone formation in impact experiments and test a novel approach to qualitatively and quantitatively describe shatter cone geometries by white light interferometry. We recovered several ejected fragments from MEMIN cratering experiments that show slightly curved, striated surfaces and conical geometries with apices of 36°–52°. These fragments fulfilling the morphological criteria of shatter cones were found in experiments with 20–80 cm sized target cubes of sandstone, quartzite and limestone, but not in highly porous tuff. Targets were impacted by aluminum, steel, and iron meteorite projectiles at velocities of 4.6–7.8 km s−1. The projectile sizes ranged from 2.5–12 mm in diameter and produced experimental peak pressures of up to 86 GPa. In experiments with lower impact velocities shatter cones could not be found. A thorough morphometric analysis of the experimentally generated shatter cones was made with 3D white light interferometry scans at micrometer accuracy. SEM analysis of the surfaces of recovered fragments showed vesicular melt films alternating with smoothly polished surfaces. We hypothesize that the vesicular melt films predominantly form at strain releasing steps and suggest that shatter cones are probably mixed mode fractures.

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“…A diameter of approximately 0.1 mm corresponds to spherules in the first size range, with a voxel number n ≤25. This elemental variation, called fractionation, was attributed to partitioning and diffusion of Fe from the spherules into the surrounding impact glass and to selective oxidation of Fe [15,48], causing the formation of a Ni-rich rim in the spherule surrounded by a Fe-rich halo (Fe >30%) in the silicate melt [30]. Although the available resolution was not sufficient to detect such detailed features, it was possible to distinguish such spherules by the different attenuation values depending on their size.…”

Section: Spherules Analysis and Quantificationmentioning

confidence: 99%

Investigation of a Monturaqui Impactite by Means of Bi-Modal X-ray and Neutron Tomography

et al. 2018

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X-ray and neutron tomography are applied as a bi-modal approach for the 3D characterisation of a Monturaqui impactite formed by shock metamorphism during the impact of an iron meteorite with the target rocks in the Monturaqui crater (Chile). The particular impactite exhibits structural heterogeneities on many length scales: its composition is dominated by silicate-based glassy and crystalline materials with voids and Fe/Ni-metal and oxihydroxides particles generally smaller than 1 mm in diameter. The non-destructive investigation allowed us to apply a novel bi-modal imaging approach that provides a more detailed and quantitative understanding of the structural and chemical composition compared to standard single mode imaging methods, as X-ray and neutron interaction with matter results in different attenuation coefficients with a non-linear relation. The X-ray and neutron data sets have been registered, and used for material segmentation, porosity and metallic content characterization. The bimodal data enabled the segmentation of a large number of different materials, their morphology as well as distribution in the specimen including the quantification of volume fractions. The 3D data revealed an evaporite type of material in the impactite not noticed in previous studies. The present study is exemplary in demonstrating the potential for non-destructive characterisation of key features of complex multi-phase objects such as impactites.

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Chemical modification of projectile residues and target material in a MEMIN cratering experiment

Cited by 45 publications

References 46 publications

The Wabar impact craters, Saudi Arabia, revisited

The Wabar impact craters, Saudi Arabia, revisited

Formation of shatter cones in MEMIN impact experiments

Investigation of a Monturaqui Impactite by Means of Bi-Modal X-ray and Neutron Tomography

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