The Zhamanshin impact structure, Kazakhstan: A comparative geochemical study of target rocks and impact glasses

Abstract The Zhamanshin impact structure, which is about 1 Myr old, has a diameter of 14-km and is situated in the semi-arid region of Kazakhstan (48°24’N,60°58’E). It has a heterogeneous suite of target rocks, including predominantly crustal lithologies (e.g., clays and siltstones) with minor ultramafics. Zhamanshin is known for its unique association of impact glasses, including basic and acidic varieties of zhamanshinites and (tektite-like and few aerodynamically-shaped) irghizites. The origin of both of these impact glasses has long been debated, which is complicated by incomplete sampling of target lithologies at the Zhamanshin site and a limited number of isotopic analyses. However, such studies are a prerequisite for a comprehensive discussion of the mechanisms that formed the unique association of different impact glasses in one impact event. We present major- and trace element contents, as well as combined Sr-Nd isotope data for target rocks and impact glasses from the Zhamanshin impact structure. These data, for the first time, include Paleogene clays and siltstones from a core drilled in the vicinity of the crater and cutting through all major lithologies. The core samples represent an important source lithology for the impactites from the Zhamanshin area. Mixing calculations, based on the geochemical data and Sr-Nd isotope signatures, indicate that irghizites and Si-rich zhamanshinites can be produced from variously homogenized mixtures of mainly clays and siltstones with minor additions of ultrabasic rocks. Based on highly siderophile element (HSE) and Os isotope data (including the first analyses of the clay and siltstone lithologies) we calculated a hypothetical Os composition of the irghizite precursor, allowing us to approximate a chondritic admixture to the irghizites of roughly 1% of a chondritic component. This confirms previous suggestions about the amount of extraterrestrial components. A new HSE and Os isotope dataset for five zhamanshinites reveals, on average, crust-like HSE concentrations and Os isotope compositions confirming earlier suggestions of a lack of meteoritic admixtures to these impact glasses.