Secondary Changes of Rocks in Zhamanshin Meteorite Crater(Kazakhstan)

Abstract

Magnetic properties, structure, and phase composition of samples of ferruginous rocks from the Zhamanshin meteorite crater have been investigated. X-ray phase analysis, microscopy and magnetometry have revealed that studied rocks can be considered as a fine-grained polymictic sandstone which undergone significant alteration. All samples appear highly ferruginous, with cement of goethite with small particles of the matrix. Detrital grains are quartz, feldspar, rarely mica, as well as monazite, ilmenite, zircon, and cassiterite in accessory quantities. Inclusions of glass, probably of impact origin, with low silica, high alumina, and high iron content have also been encountered. A characteristic feature of goethite is the presence of Mn and a pronounced zonal structure manifesting itself in an increased Al content. Intergrowths and druses of authigenic magnetite are found in voids and cracks; magnetite however appears completely by iron (hydr)oxides. These observations generally agree with the paleogeographic data, and suggest that the crater and its vicinity experienced significant changes of the redox regime in the post-impact time.