^1,2Agata M. Krzesińska, ¹Natasha V. Almeida
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13190]
¹Department of Earth Sciences, Natural History Museum, London, UK
²Institute of Geological Sciences, Polish Academy of Sciences, Wrocław, Poland
Published by arrangement with John Wiley & Sons

Baszkówka is an equilibrated, apparently low‐shock, unusually porous chondrite. Some earlier studies were undertaken to understand whether the porosity in Baszkówka, and similar porous chondrites, is a relic of a primordial feature or rather the effect of atypical reprocessing on the parent body. Neither of the studies reconstructed the accurate thermal and deformational evolution of chondrites, however, while it is known that shock‐induced compaction is the main means to affect chondritic porosity. Here we use a combination of 3‐D and 2‐D petrographic examination to understand how the evolution of pores correlates with thermal and shock history recorded in the Baszkówka chondrite. The grain framework silicates in Baszkówka contain healed shock fractures—a clear recorder of significant shock process and postshock annealing. Simultaneously, metal grains do not exhibit any preferred orientation or fabric, which would be expected to develop in response to the deformation as recorded by silicates. We interpret this as evidence for re‐agglomeration and annealing of shocked material. Pore spaces in Baszkówka are connected and decorated by fine‐grained plagioclase‐dominated mass and bulky euhedral olivine crystals, which exhibit growth steps on crystal surfaces. The euhedral olivine must have formed owing to the condensation of a vapor, while plagioclase most likely crystallized from melted chondritic matrix. During the shock event, fine‐grained matrix in Baszkówka was melted and vaporized. Vapor expansion added to ballistic velocity led to ejection and opening of the pore spaces. After re‐agglomeration in a hot ejecta blanket the rock was annealed, melted material circulated in created pore spaces and vapor condensed.

¹Edwin Gnos, ¹Beda A. Hofmann, ²Khalid Al‐Wagdani,²Ayman Mahjub, ²Abdulaziz Abdullah Al‐Solami, ²Siddiq N. Habibullah, ³Albert Matter, ²Mohammed A. Halawani
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13187]
¹Natural History Museum Geneva, CP 6434, Geneva 6, Switzerland
²Saudi Geological Survey, Jiddah, Saudi Arabia
³Institute of Geological Sciences, University of Bern, Bern, Switzerland
Published by arrangement with John Wiley & Sons

The ~5.5 km sized Jabal Rayah ring structure located at 28°39′N/37°12′E in Saudi Arabia has been classified as a possible complex impact structure located in flat‐lying Paleozoic clastic sediments. Previous, detailed mapping showed that erosional processes led to a relief inversion, with displaced, folded, and faulted blocks of Silurian to Early Devonian strata, interpreted to form a ring syncline, now forming a topographically outstanding, 150 m high ring crest. The drainage toward the center of the structure seems controlled by a set of radial faults. This central part is eroded to the level of the surrounding plateau and partially covered with gravel. Analysis of 28 Qusaiba Formation sandstones showed that at the present outcrop level, the sediments seem devoid of shock features. Measurement of fold axes in the central part of the structure shows radially outward plunging fold axes, becoming steeper toward the center, and also fold axes of other orientation, and folded folds. This fold axis pattern is interpreted as an upward‐pointing, kilometer‐sized sheath fold. Assuming an impact scenario and using the present size of the structure, the minimum central structural uplift is estimated at ~500 m, which is consistent with Qusaiba Formation occupying the center of the ring structure.