WEATHERING AND POLYMERIZATION OF TEKTITES - AN X-RAY PHOTOELECTRON-SPECTROSCOPY (XPS) INVESTIGATION

Tektites are natural glasses formed from terrestrial material that was melted and displaced by the impact of an extraterrestrial body. The s urface and near-surface compositions of tektite glass results from fra ctionation during impact and ejection, and/or postsolidification weath ering. The first goal of this study was to characterise the surface an d near-surface (in the order of tens of angstroms) chemical compositio n of two tektites by x-ray photoelectron spectroscopy (XPS), and to es timate the importance of weathering vs. fractionation during flying. I n order to separate the chemical modification due to weathering from t hat due to fractionation during ballistic flight, we studied two sampl es from the Australasian tektite strewn field. One of them was collect ed in a hot desert area (Nullarbor Plain, Australia) and the other, in a humid climate (Thailand). Our study reveals the presence of well-de veloped leached layers in both tektites. In the Australian tektite, Si is depleted in the topmost layers (a few tens of angstroms). A more c omplex chemical zoning is defined in the tektite from Thailand. These leached layers are comparable to those observed in weathered glasses, and therefore we conclude that weathering is responsible for the chemi cal composition of the surface and near-surface compositions. The seco nd goal was to investigate the chemical environment of O, N and C in t he glass. The O peak was resolved into two bridging O components (Si-O -Si and Al-O-Si) that are comparable to O environments in artificial g lasses. Tile binding energy of the C-ls electron is typical for C-C an d C-H bonds in hydrocarbons; minor organic acid components are also pr esent. Nitrogen is only observed on the surface of the Thailand tektit e. The binding energy of N-ls is comparable to that of ammonia, and th e surface enrichment in N is interpreted as due to sorption related to interactions between glass and fluid buffered by the organic material in the soil.