Abstract (eng)
The 1 Ma, 14-km-diameter, Zhamanshin impact structure, situated in a semi-arid region of Kazakhstan, has a heterogeneous suite of target rocks. There are a variety of impact glasses (zhamanshinites) and tektite-like objects (irghizites), both subclassified into Si-poor and Si-rich varieties. One drill core and four surface target rocks, as well as five impactite samples (two irghizites and three zhamanshinites), were analyzed in this study for their petrography, geochemical composition, as well as Sr, Nd, and Os isotopic compositions in order to study the relationship between target rocks and impactites and to search for a possible meteoritic components within the irghizites and zhamanshinites. Based on the Re/Os ratios, a general bimodal trend can be observed between impactites and most target rocks. Impactites in general show lower Re/Os ratios than the investigated target rocks. Even though the concentrations of selected highly siderophile elements (Re,Os,Ir, and Pt) and the corresponding interelement patterns of most analyzed impactites are comparable to those of the target rocks (which, on average, mirror the trend defined by the upper continental crust), one of the analyzed (Si-rich) irghizites and one of the analyzed (Si-rich) zhamanshinites exhibit highly siderophile element concentrations up to two orders of magnitude higher and a less fractionated and nearer to chondritic interelement pattern compared to the upper continental crust. The irghizite sample also shows the most unradiogenic 187Os/188Os isotope ratio of all analyzed samples. Interpreted in terms of a meteoritic component, and after excluding a possible contamination from ultramafic target rocks that occur in the Zhamanshin area (which could simulate such an extraterrestrial signal), the presence of about 0.1% of a chondritic component within this irghizite and this zhamanshinite is postulated.