Impact glasses from Zhamanshin crater (U.S.S.R.): chemical composition and discussion of origin

Abstract

Three silica-rich zhamanshinites and one irghizite from the Zhamanshin impact crater (northern Aral area, U.S.S.R.) have been analyzed for up to 40 major, minor, and trace elements. All data point to a clear distinction between these impact glasses and other tektites or impact glasses. e.g. from the Australasian strewn field. Halogens are generally enriched in the irghizites and zhamanshinites when compared to normal splash for tektites. with zhamanshinites enriched more than irghizites. The same holds also for the alkali metals and a number of other volatile elements like Sb and As. Nickel and cobalt are enriched in the irghizite sample to a considerable degree, suggesting meteoritic contamination. This view is also supported by gold and selenium data, but for quantifications other siderophile elements need to be considered. Chromium is not a valid indicator of meteoritic contamination, because smallamounts of ultra-basic igneous material may completely alter the picture. The rare earth elements do show a sedimentary pattern, consistent with two or three different source materials and a variation which is probably mostly due to dilution with silica-rich materials. The peak pressure and temperature experienced by the irghizites was lower than for australites or other splash-form tektites, and even lower for zhamanshinites, which is revealed by the content of volatile elements and lesser homogeneity.