The geochemistry of tektites: an overview

Abstract

Geochemical arguments have been used to demonstrate that tektites have been derived from terrestrial upper crustal rocks, presumably sediments. These arguments include the abundances and ratios of major- and trace-elements as well as isotope data. Major elements in tektites commonly show an inverse correlation with SiO 2. Other major-element relationships such as oxide-ratio plots, or soda-potash diagrams, are used to distinguish tektites from different strewn fields or, where applicable, to identify substrewn fields within a particular strewn field. Muong Nong-type tektites differ from normal splash-form tektites in being larger, less homogeneous, and containing higher abundances of volatile elements and water. Also, the Fe 3+ content is higher, and they do not show any signs of aerodynamic ablation or spherical symmetry. All this points to an origin at temperatures and pressures which are lower than those for splash-form tektites. In addition, Muong Nong-type tektites most probably have not travelled far from their point of origin, and are, therefore, close to the source crater. Trace-element ratios in all types of tektites are characteristic, particularly ratios such as Ba/Rb, Th/Sm, K/U and La/Sc which support the conclusion that tektites are unlikely any lunar material, but close to terrestrial upper crustal sediments in composition. Furthermore, the rare-earth elements in tektites show the distinctive pattern of terrestrial post-Archaean upper crustal sediments, thereby providing another clue to the nature of the target material. Only very low levels of platinum-group and other siderophile elements are present in tektites but these are probably the only elements from the impactor surviving the impact. The platinum-group element patterns may give hints about the nature and chemical composition of the projectile (e.g., the possibility that an achondritic projectile was responsible for the Ries moldavite event). Rb/Sr and Sm/Nd isotope data also support an upper crustal percentage for tektites, whereas K/Ar and fission-track age dating provide the time link between tektites and impact craters.