^1,2Lukas Manske,^1,2Kai Wünnemann,³Kosuke Kurosawa
Journal of Geophysical Research (Planets) (in Press) Open Access Link to Article [https://doi.org/10.1029/2022JE007426]
¹Museum für Naturkunde Berlin, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
²Department of Earth Sciences, Freie Universität Berlin, Berlin, Germany
³Planetary Exploration Research Center, Chiba Institute of Technology, Narashino, Japan
Published by arrangement with John Wiley & Sons

Melting and vaporization of rocks in impact cratering is mostly attributed to be a consequence of shock compression. However, other mechanism such as plastic work and decompression by structural uplift also contribute to melt production. In this study we expand the commonly used method to determine shock-induced melting in numerical models from the peak shock pressure by a new approach to account for additional heating due plastic work and internal friction. We compare our new approach with the straight-forward method to simply quantify melting from the temperature relative to the solidus temperature at any arbitrary point in time in the course of crater formation. This much simpler method does account for plastic work but suffers from reduced accuracy due to numerical diffusion inherent to ongoing advection in impact crater formation models. We demonstrate that our new approach is more accurate than previous methods in particular for quantitative determination of impact melt distribution in final crater structures. In addition, we assess the contribution of plastic work to the overall melt volume and find, that melting is dominated by plastic work for impacts at velocities smaller than 7.5–12.5 km/s in rocks, depending on the material strength. At higher impact velocities shock compression is the dominating mechanism for melting. Here, the conventional peak shock pressure method provides similar results compared with our new model. Our method serves as a powerful tool to accurately determine impact-induced heating in particular at relatively low-velocity impacts.

¹Kishan Tiwari,¹Sujoy Ghosh,²Masaaki Miyahara,³Dwijesh Ray
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2022JE007420]
¹Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, India
²Graduate School of Advanced Science and Engineering, Hiroshima University, Higashihiroshima, Japan
³Planetary Sciences Division, Physical Research Laboratory, Ahmedabad, India
Published by arrangement with John Wiley & Sons

Abundant vesicles are found in terrestrial rocks, basaltic meteorites, and carbonaceous chondrites which testify to the presence of significant quantities of volatiles and favorable conditions for vesiculation. Furthermore, vesicular olivine has been reported in terrestrial rocks and carbonaceous chondrites. However, vesicles in the ordinary chondrites are rare due to their low volatile content and obliteration by the late impact events. Here, we report the first evidence of vesicular olivine (Fo76) and pyroxene (En73–81Fs17–26Wo01–02) in an ordinary chondrite. The vesicular texture in the shocked Kamargaon L6 chondrite possibly formed due to localized melting during a shock event and subsequent degassing of volatiles after decompression. We propose three possible mechanisms for vesicle formation in the Kamargaon meteorite: (a) liberation of S2 vapor, (b) evaporation of moderately volatile elements (MVEs) like Na and Mn, and (c) vaporization of olivine and pyroxene, by constraining the primary abundance of volatiles based on the observed volume of vesicles. We suggest that all three or any combination of these mechanisms could be responsible for vesicle formation. Segmented texture in olivine is also observed in the shock vein (SV) of the Kamargaon chondrite. The segmentation has developed due to the formation of sub-grain boundaries during the recovery process when grains were subjected to localized shear stress. Average shock pressure and temperature conditions in the SV are ∼24–25 GPa and ∼2310–2633K, respectively. The thermal model of the SV cooling gives the crystallization time of ∼50 ms and shock pulse duration of ∼2s.