ZINC ISOTOPES AND THE FORMATION AND FRACTIONATION OF TEKTITES

Tektites are a rare type of impact glass; they are found in only four distinct and geographically extended strewn fields, 0.8 to 35 My in age. For three of these four strewn fields, the source crater is known. Also, from 3 of the 4 fields, microtektites are known in addition to normal, centimeter-sized specimens. Tektites differ from “normal” impact glasses in that they were derived from the very surface of the target area (as is indicated by their high content of the cosmogenic radioisotope Be-10) and may have formed and ejected before the main crater excavation phase even begun. As most tektites are homogeneous glass, they must have experienced extremely high formation temperatures – certainly higher than that of impact glasses that formed directly within impact craters during later crater formation phases. Earlier research did not indicate any isotopic fractionation of elements such as B, K, and Si. However, Zn isotopes are strongly fractionated during volatilization from impact processes in lunar soils and meteorites (d^66/64Zn>6), but are hardly fractionated during igneous processes on Earth, with a very limited range of variation in terrestrial igneous rocks (0.2<d^66/64Zn<0.4). Therefore, Zn isotope ratios are very good tracers of volatile history of samples. We have recently started to measure the Zn isotopic composition of terrestrial rocks associated with impact on Earth. We have shown that Zn (and Cu) is isotopically fractionated by volatilization in tektites (d^66/64Zn>2). Most tektites (except the Muong Nong-type) are enriched in the heavy isotopes of Zn (and Cu) in comparison to the terrestrial crust with no clear distinction between the different groups. To explain these data we developed a model of evaporation of Zn from a molten sphere that shows that evaporation is limited by the diffusion of Zn within the sphere. The study of unconventional stable isotopes provides interesting clues regarding formation, differentiation, and deposition of tektites.