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Residual stress in meteorites – perspective through High-pressure Mössbauer spectroscopy

This paper is available in a repository.
This paper is available in a repository.

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Abstract

Pressure-induced studies on meteorites conveniently provide information regarding the residual stress due to impact processes experienced by the body. Mössbauer spectroscopy, a powerful and sensitive probe for quantitative analysis of oxidation state and local environment of Fe atom through the Mössbauer parameters-isomer shift and quadrupole splitting, was used to study Lohawat and Piplia Kalan meteorites categorized as howardite and eucrite respectively from Howardite-eucrite-diogenite (HED) clan of differentiated meteorites originated from asteroid 4-Vesta. While Lohawat meteorite was composed mainly of orthopyroxene and anorthite, Piplia Kalan meteorite was dominant with anorthite, clinopyroxene and clino-ferrosilite. In both these samples, Mossbauer spectroscopic technique could sense only Fe 2+-contained pyroxene mineral (anorthite and olivine did not contribute due to negligible Fe-content). Pyroxenes have two inequivalent distinctive octahedral M1 and M2 sites with Fe 2+ preferring larger and more distorted M2 sites. Conventional practice of assigning larger QS to M1 site is adopted during the analysis and any changes in the parameter would reflect the changes in the environment surrounding Fe atom. M1 site responded more to applied pressure for Lohawat meteorite as M2 site for Piplia Kalan sample. Anorthite, an important constituent of meteorite in general showed onset of amorphization between 10 and 14 GPa followed by complete irreversible amorphization about 22GPa. Lohawat meteorite indicated such irreversible amorphization at ~ 3 GPa (ref.1) while for Piplia Kalan meteorite onset of amorphization occurred at ~ 6.5 GPa (ref 2). The transformation occurring at lower transition pressure implied residual stress persisting in the systems. Combined with X-ray diffraction studies, the technique could be demonstrated as a powerful tool to study the residual stress and to estimate the peak shock metamorphic pressure experienced by impact ejection of meteorite from the parent body. Future Studies on meteorites recovered from Didwana, Ararki and Nathdwara (in progress) would be useful for shock grade classifications.