Petrographic and Micro-Raman Study of Thermal and Shock Metamorphism in Mezőmadaras, Knyahinya and Mócs L-Chondrites

Abstract

We investigated three Hungarian L‐chondrites: Mezőmadaras, Knyahinya, and Mocs sample. In Knyahinya chondrite, two phenomena were observed as follows: the weak mosaicism (S4) and mechanical twins in pyroxene. The Mocs sample is characterized by strong shock metamorphic effects such as high density of fractures and abundant shock‐veins. Planar Deformation Features (PDFs) were identified in two olivine grains, and an olivine grain shows well‐developed Planar Fractures (PFs). In Mezőmadaras sample, there are irregular fractures (S1), and PFs (S3), and mechanical twins in pyroxene. We have reinvestigated of these meteorite samples using Raman spectroscopy for the identification of shock‐metamorphic effects, which occurred in the structure changes of olivine and pyroxene grains. The olivine doublet peak position is shifted to higher wavenumbers, due to partly disordered olivine structure. We observed that minerals with mechanical twins exhibit less interference color, compared to the non‐shocked minerals, especially pyroxene. It is important to note that, the mechanical twins were complicated to describe, only the direction of the deformation was determined. On the basis of the measured FWHM, the following structural disordering degree of the selected samples can be determined as a (growing rate): Castle Rock, Mezőmadaras, Shergotty and Mocs. The result reveals that amorphization rate is the highest in Mocs and Shergotty pyroxene grains.