I. Gyollai

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

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, espe...

Characterization and 10Be content of iron carbonate concretions for genetic aspects – Weathering, desert varnish or burning: Rim effects in iron carbonate concretions

The research investigated three iron carbonate (siderite) sedimentary concretions from Nagykovácsi, Úri and Délegyháza, Hungary. To identify possible source rocks and effects of the glaze-like exposed surface of the concretions, we carried on comparative petrological, mineralogical, geochemical and isotopic studies. The samples were microbially mediated siderite concretions with embedded metamorphous and igneous mineral clasts, and had specific rim belts characterized by semi-concentric outer Fe-oxide layers, fluffy pyrite-rich outer belts and siderite inner parts. We investigated the cross section of the Fe-carbonate concretions by independent methodologies in order to identify their rim effects. Their surficial oxide layers showed evidence of degassing of the exposed surface caused most probably by elevated temperatures. The inner rim pyrite belt in the concretions excluded the possibility of a prolonged wet surface environment. Microtextural and mineralogical features did not support desert varnish formation. 10Be nuclide values of the Nagykovácsi and Uri concretions were far above the level of terrestrial in-situ cosmogenic nuclides, but they were consistent with the lowest levels for meteorites. Though the data were not conclusive to confirm any kind of known origin, they are contradictary, and open possibilities for a scenario of terrestrial meteorite origin.