GEOCHEMICAL CONSTRAINTS ON THE ORIGIN 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: the Australasian, Ivory Coast, Central European, and North American strewn fields. For three of these four strewn fields, the source crater is known. The chemical composition and isotopic of tektites within each strewn field is closely related, and in general tektites have compositions that are identical to that of the terrestrial upper continental crust. However, the exact details of tektite formation are still not quite resolved. For the largest and youngest strewn field, no source crater has yet been identified, leading to hypotheses of (physically difficult to understand) airburst formation of tektites. It is clear that tektites form during hypervelocity impact events on Earth and their composition is identical to terrestrial surface rocks, with no or only very minor meteoritic admixture. Tektites from three of the four strewn fields contain the cosmogenic radionuclide ¹⁰Be, which forms by interaction of cosmic rays with atmospheric N and C is concentrated in the top of any sediment column. The average value of ¹⁰Be in Australasian tektites is comparable to that of near-surface source materials, e.g., soils or sediments. Aerodynamically shaped tektites (australites) found farther from the presumed impact site in southeast Asia have higher contents compared to layered (Muong Nong-type) tektites, which cannot have been transported as far.

Microtektites make an important part of 3 of the 4 strewn fields, being widely distributed and, in most cases, outlining the extent of the strewn field. The ¹⁰Be content of a composite of seven microtektites (due to their small sizes) was found to be higher than in the other tektite types. Recent improvements in analytical methods allowed the determination in 13 individual microtektites, all from the Australasian strewn field [1]. These data confirm that the average content is about 2x that of splash-form (ablated) australites, and that microtektites have the highest ¹⁰Be in a strewn field, indicating that microtektites are derived from the topmost surface of the target and that they are formed in the earliest phase of the impact process, well before crater formation.

[1] Koeberl C. et al. MAPS 50, S1, p. A165 (abs. 5187).