Vesicular Olivines and Pyroxenes in Shocked Kamargaon L6 Chondrite: Implications for Primary Volatiles and Its Multiple Impacts History

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.

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