Discovery of a 25-cm asteroid clast in the giant Morokweng impact crater, South Africa

Abstract-Here we present the results of a geochemical study of the projectile component in impactmelt rocks from the Lappajärvi impact structure, Finland. Main-and trace-element analyses, including platinum group elements (PGEs), were carried out on twenty impact-melt rock samples from different locations and on two shocked granite fragments. The results clearly illustrate that all the impact melt rocks are contaminated with an extraterrestrial component. An identification of the projectile type was performed by determining the projectile elemental ratios and comparing the corresponding element ratios in chondrites. The projectile elemental ratios suggest an H chondrite as the most likely projectile type for the Lappajärvi impact structure. The PGE composition of the highly diluted projectile component (~0.05 and 0.7 wt% in the impact-melt rocks) is similar to the recent meteorite population of H chondrites reaching Earth. The relative abundance of ordinary chondrites, including H, L, and LL chondrites, as projectiles at terrestrial impact structures is most likely related to the position of their parent bodies relative to the main resonance positions. This relative abundance of ordinary chondrites suggests a strong bias of the impactor population toward inner Main Belt objects.

Section: Introductionmentioning

confidence: 56%

Traces of an H chondrite in the impact‐melt rocks from the Lappajärvi impact structure, Finland

Tagle

Öhman

Schmitt

et al. 2007

“…The fact that the projectile element ratios of these impact craters (work on the identification of several more impact crater projectiles is currently in progress) overlap with certain chondrite types supports the assumption that chondrite element ratios obtained here are not only representative for the composition of meteorite fragments, but also for the composition of larger bodies. Moreover, the recent discovery of a 25 cm projectile fragment, found almost at the bottom of the impact melt sheet of the Morokweng impact structure (Maier et al 2006) and its classification as an ordinary chondrite conclusively supports the suggested projectile type ) and the method presented herein.…”

Section: Projectile Identification In Terrestrial and Lunar Impact Meltsmentioning

confidence: 79%

A database of chondrite analyses including platinum group elements, Ni, Co, Au, and Cr: Implications for the identification of chondritic projectiles

Tagle

Berlin

2008

Abstract-Siderophile elements have been used to constrain projectile compositions in terrestrial and lunar impact melt rocks. To obtain a better knowledge of compositional differences between potential chondritic projectile types, meteorite analyses of the elements Ru, Rh, Pd, Os, Ir, Pt, Cr, Co, Ni, and Au were gathered into a database. The presented compilation comprises 806 analyses of 278 chondrites including new ICP-MS analyses of Allende and two ordinary chondrites. Each data set was evaluated by comparing element ratios of meteorites from the same chondrite group. Characteristic element abundances and ratios were determined for each group. Features observed in the element abundance patterns can be linked directly to the presence of certain components, such as the abundance of refractory elements Os, Ir, and Ru correlating with the occurrence of refractory inclusions in CV, CO, CK, and CM chondrites. The refined characteristic element ratios appear to be representative not only for meteorites, but also for related asteroidal bodies. Chondrite element ratios were compared to previously published values from impact melt rocks of the Popigai and Morokweng impact structures confirming that an identification of the specific type of projectile (L and LL chondrite, respectively) is possible. The assessment for Morokweng is supported by the recent discovery of an LL chondrite fragment in the impact melt rocks. Ultimately, the database provides valuable information for understanding processes in the solar nebula as they are recorded in chondrites. A new type of complementarity between element patterns of CK and EH chondrites is suggested to be the result of condensation, redox, and transportation processes in the solar nebula.

“…It may be expected, therefore, that a significant amount of material may also survive a planetary-scale impact into water. Moreover, while material interpreted as fragments of the parent meteorite have been recovered from large oceanic impacts such as Eltanin (Kyte 2002), a recent discovery has shown that fragments can also survive impact into a continental basement, as found in the Morokweng crater, South Africa (Maier et al 2006). For our dry impacts, projectile survivability could not be estimated due to the surviving projectile fragments being fused into sandstone grains, preventing uncontaminated projectile extraction.…”

Section: Projectile Survivabilitymentioning

confidence: 99%

Laboratory impacts into dry and wet sandstone with and without an overlying water layer: Implications for scaling laws and projectile survivability

Baldwin

Milner

Burchell

et al. 2007

Abstract-Scaling laws describing crater dimensions are defined in terms of projectile velocity and mass, densities of the materials involved, strength of the target, and the local gravity. Here, the additional importance of target porosity and saturation, and an overlying water layer, are considered through 15 laboratory impacts of 1 mm diameter stainless steel projectiles at 5 km s −1 into a) an initially uncharacterized sandstone (porosity ~17%) and b) Coconino Sandstone (porosity ~23%). The higher-porosity dry sandstone allows a crater to form with a larger diameter but smaller depth than in the lower-porosity dry sandstone. Furthermore, for both porosities, a greater volume of material is excavated from a wet target than a dry target (by 27-30%). Comparison of our results with Pi-scaling (dimensionless ratios of key parameters characterizing cratering data over a range of scales) suggests that porosity is important for scaling laws given that the new data lie significantly beneath the current fit for ice and rock targets on a π v versus π 3 plot (π v gives cratering efficiency and π 3 the influence of target strength). An overlying water layer results in a reduction of crater dimensions, with larger craters produced in the saturated targets compared to unsaturated targets. A water depth of approximately 12 times the projectile diameter is required before craters are no longer observed in the targets. Previous experimental studies have shown that this ratio varies between 10 and 20 (Gault and Sonett 1982). In our experiments ~25% of the original projectile mass survives the impact.