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Results Project 8

On the fate of the projectile in impact cratering experiments

The project

The major goal of this sub-project is to reach an improved knowledge of the parameters that affect the dissemination of projectile material into e.g., ejecta or crater basement. The physicochemical conditions during projectile emplacement, modes of target-projectile mixing, and associated chemical fractionation processes are not well constrained. In addition, it is of great importance for our knowledge of impact cratering to quantify to which extent impact energy, water-saturation and porosity of the target control the fate of the projetile.

Our study addresses these problems under defined experimental conditions: Target compositions (Seeberger sandstone, quartzite and tuff) and projectile compositions (Campo del Cielo Fe-meteorite, steel and aluminium) were chosen such that strong chemical differences will enable (1) unambiguous identification of small projectile particles, and (2) constraining relative contributions of target/projectile to mixtures.

Characterization and selection of projectile and target materials

The application of well-characterized target and projectile material is mandatory in this MEMIN project. The Campo del Cielo meteorite was chosen as a projectile material that represents natural “meteorite” impacts. The steel D290-1 has rather high concentrations of several siderohpile and some more lithophile tracer elements like Co, Cr, V, Mo, and W – elements that are (nearly) absent in the target allowing the tracing of projectile matter in detail.

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Fig. 1 (Top) Sandstone target: photograph and backscatter electron image (BSE) with main phases. (Bottom) Campo del Cielo meteorite projectile: photograph and backscatter electron image (BSE) with main phases.

Sampling and preparation of experimental products

The experimental setup of the MEMIN hypervelocity impact experiments provides that a large part of the ejecta could be collected with a new designed ejecta catcher; consisting inter alia of phenolic foam plates (Fig. 2).

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Fig. 2 (Left) Quarter part of the ejecta catcher system; consisting of phenolic foam plates. The red-lined radial cutout marked the foam slices (right image), which were used for ejecta separation.

A first characterization of different ejecta types were performed with optical and scanning electron microscopy (SEM), followed by sorting of different ejecta types according to the ejection angle. Three ejecta types are distinguished, (1) weakly deformed, (2) highly deformed, and (3) highly shocked and projectile-rich material.

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Fig. 3 Three different ejecta types separated from the phenolic foam slices (Fig. 2 left).

The ejecta catcher system was used to collect material of the evolving ejecta curtain at different ejection angles. Ejecta type 3 (right box in Fig. 3) which contains approximately 10-30 % projectile material (in each fragment) is mainly distributed between angles of 71° and 81° (Fig. 4). Consequently, during the impact cratering process projectile-rich ejecta fragments will be ejected in a steep angle with regard to the impact angle. The rest of the projectile is represented as mm-sized, irregular formed fragments or as bowl shaped fragments of almost pure projectile material (cp. Fig. 5).

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Fig. 4 Ejecta mass for different ejecta types as function of the ejecta angle (Sandstone target and iron meteorite projectile).

 

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Fig. 5 Bowl shaped projectile fragments (left: front view; middle: back view) at lower impact velocities; Projectile fragments (right) at higher impact velocities.

Tracing of projectile material mixed into ejecta and target rocks

Tracing of projectile material in highly shocked target material was mainly done by high-resolution electron microprobe (at MfN, Fig. 6) because of the small dimension of projectile spheres and target melt sections.

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Fig. 6 Microprobe measurements at the Museum of Natural History Berlin (PhD student Matthias Ebert).

Impact-induced features and modes of projectile-target mixing

Our study is focused on target-projectile interaction observed in highly shocked fragments from the recovered ejecta material. The highly shocked ejecta shows shock-metamorphic features such as planar deformation features (PDF) in quartz, formation of diaplectic quartz glass, partial melting of the sandstone, and partially molten projectile, mixed mechanically and chemically with target melt (Fig. 7).

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Fig 7. (A) Shock-metamorphic features of the sandstone and projectile droplets; BSE-image (Ebert et al. 2012).

First TEM measurements provided more details of the heterogeneity within the shocked target, like nm-scale projectile droplets and small scale liquid immiscibilty between Fe- and Si-rich target melts (Fig. 8). One section covers the quartz with PDF, silica glass margin, and projectile-bearing sandstone melt (see xy profile in Fig. 4A).

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Fig. 8 (A) BSE image of a highly shocked ejecta fragment. The yellow dashed line marks the position of the TEM foil. (B) HAADF image of the TEM foil (line x-y in 8A) shows typical textures, components and high shock features. (C) TEM bright field image shows the magnification of Fig. 8B. The sandstone melt is an emulsion of at least three almost immiscible melts: two silicate melts (Si-rich and Fe-rich) and the metallic projectile droplets. The projectile droplets reach diameters down to a few nanometers (Fig. 8C).

Inter-element fractionation during target-projectile mixing

Campo del Cielo meteorite and sandstone target

During the highly dynamic and turbulent impact process projectile melt was injected into the sandstone melt and disseminated as small metallic droplets. Thus, two coexisting but largely immiscible melts exist in the highly shocked ejecta fragments. In these projectile droplets Ni and Co are enriched over Fe, yielding a Fe/Ni that is generally higher than Fe/Ni in Campo del Cielo meteorite (Fig. 9).

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Figure 9. Fe/Ni and Fe/Co for different phases of the projectile before and after the impact. The curved line represents the mixing line between unshocked kamacite and rhabdite.

Obviously this Fe depletion in projectile droplets is associated with the enrichment of Fe and to a lesser degree of other siderophile elements in the sandstone melt and in shocked quartz (Fig. 10). The average Fe/Ni of quartz with PDF and of silica glasses resembles the Fe/Ni-ratio of the projectile (Ebert et al. 2012). In this case we suggest an alternative process of projectile-target interaction, maybe condensation from projectile vapour. Further studies are needed to check for chemical contamination of shocked quartz.

In the laser-experiments (see last chapter of this site) the sandstone melt with projectile contamination (green field Fig. 10) match the Fe-enrichment observed in the hypervelocity impact experiments. Comparing the contaminated sandstone melt with a melt of pure sandstone (yellow field) highlights the significant enrichment of meteoritic iron.

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Fig. 10 Fe vs. Si for various materials of the highly shocked ejecta fragments . The Fe depletion in projectile droplets is associated with the enrichment of Fe and to a lesser degree of other siderophile elements in the sandstone melt and in shocked quartz. The green and yellow field represent data from two laser-induced melting experiments with (green) and without projectile (yellow). D.L. = detection limit. LE= laser experiments.

D290-1 steel projectile and sandstone target

During interaction of projectile and target melts Cr, V and Fe of the steel projectile are preferentially partitioned into sandstone melt (cp. Fig. 11 and 12). This enrichment is even more significant in the laser-induced melting experiments (Fig. 11; green field), probably due to higher temperatures. The element mapping in Fig. 12 also shows that nearly the entire Cr and V of the projectile droplets partition into the sandstone melt. Due to the siderophile character of Co, Mo and W, these elements almost entirely remain in the projectile droplets (Fig. 12).

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Fig. 11 Fe vs. Cr for various materials of the highly shocked ejecta fragments (microprobe data).  The green field represents data from the laser-induced melting experiments; mixing steel and sandstone melts.

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Fig. 12 Microprobe element mapping for various elements of a highly shocked ejecta fragment with shocked Qtz (BSE - dark grey), sandstone melt (light grey) and projectile droplets (white spheres of D290-1 steel projectile); White arrow points to Al-rich sandstone melt that is significantly enriched in the meteoritic (lithophile) elements Cr and V. The siderophile elements (Co, W, Mo) remain in the projectile droplets.

P-T conditions during impact

Melting and vaporization of projectile requires temperatures higher than expected from the calculation of maximum shock pressures (~55 GPa). We suggest three mechanisms for enhanced thermal input: 1) friction and deformation (i.e., plastic work) of the projectile, 2) more effective transfer of kinetic energy to porous material (in case of the sandstone) including the local increase of shock pressure due to pore collapse, and 3) heat transfer from shock compressed air during projectile flight (projectile pre-heating). The process of pore collapse is well documented in the results of our co-researches (project 5, project 7)

Effects of experimental conditions on projectile relevant processes

A systematic relation between impact energy (velocity and/or projectile mass) and the relative proportion of projectile relicts could not be found. At higher velocities a stronger fragmentation of the projectile can be assumed (Kenkmann et al. 2012) but we did not observe any influence of the impact energy on inter-element fractionation processes. Projectile fragmentation seems to be enhanced in experiments with water-saturated targets (Kenkmann et al. 2012) but geochemical processes between projectile and target are obviously not affected by the degree of water-saturation.

Recent MEMIN hypervelocity cratering experiments were carried out with quartzite target with almost 0 % porosity. First SEM investigations of highly shocked ejecta material from these experiments suggest enhanced inter-element fractionation processes during the impact compared to the experiments with sandstone targets (~23 % porosity). Partitioning of more or less lithophile, projectile derived elements (Cr, V, Fe) into the silicate target is much more enhanced in experiments with quartzite target compared to those with sandstone; e.g. the quarzite melt may have Cr2O3 contents up to about 10 wt.%

Laser-induced melting experiments

In addition to the proposed research program some laser-induced melting experiments were done to have an additional control on the melting behavior and chemical mixing of projectile and target materials. This was especially important to constrain the compositional range of the heterogeneous target melts.

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Fig. 13 Laser welding facility at TU Berlin YAG-Laser Trumpf Haas HL 3006D

 

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Fig. 14 (A) Setup of the laser-induced melting experiments; (B) Top view of the setup after laser application which clearly shows the laser-induced melt lines; (C) Polished section of area C in Fig. 6B shows different melt degrees: 1=non-molten sandstone; 2=partly molten sandstone; 3=completely molten sandstone; 4=projectile melt.

In the laser-induced melting experiments we revealed melting textures very similar to those of the highly shocked targets: partly to completely fused quartz as well as partially to completely molten sandstone (Fig. 15A, B). The projectile matter mostly occurs within schlieren of the sandstone melt in contact to the projectile (Fig. 15B), suggesting heterogeneous chemical mixing between projectile and target melts.

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Fig. 15 (A) Melting texture produced in a laser-induced melting experiment [LE: pure sandstone]; (B) Mixing of projectile and target matter in a laser induced melting experiment [LE: sandstone with steel]

 

Matthias Ebert

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