TEKTITES AND MICROTEKTITES - KEY FACTS AND INFERENCES

Tektites are generally small, black, rounded, silicate glass bodies found scattered over several widely separated areas of the Earth's surface called strewn fields. Four major tektite strewn fields are known: (1) Australasian, (2) Ivory Coast, (3) Czechoslovakian and (4) North American. Tektites from these four strewn fields have been dated at 0.7, 1, 15, and 34 Ma respectively. Most tektites are splash forms (i.e., spheres, teardrops, dumbbells, etc.), but some have undergone a second period of melting to produce ablated forms. Tektites with a blocky, layered appearance are called Muong Nong-types. Microtektites (microscopic tektites) belonging to the Australasian, Ivory Coast and North American strewn fields have been found in deep-sea sediments. The geographic occurrence and abundance of microtektites at each site have been used to determine the size and mass of each strewn field. The Australasian, Ivory Coast and North American strewn fields contain 100 million, 20 million and 1 billion tonnes of glass respectively. Although tektites resemble obsidian, they can be distinguished from terrestrial volcanic glasses by their composition and petrography. Tektites, for example, are drier than volcanic glasses and have higher FeO/Fe2O3 ratios. In addition, tektites, unlike volcanic glasses, contain lechatelierite particles (SiO2 glass) and are devoid of primary crystallites. Although some authors favor a lunar volcanic origin, most of the compositional and petrographic data indicate a terrestrial impact origin. The major, trace, and isotopic compositions of tektites indicate that the parent material was a terrestrial sedimentary deposit, and the presence of lechatelierite, coesite and shocked relict inclusions favors an impact origin. Furthermore, two tektite strewn fields have been associated with known impact craters, and tektites from one strewn field have been found associated with impact ejecta. Unlike the Cretaceous-Tertiary boundary layer, microtektite layers are not associated with mass extinctions or iridium anomalies. © 1990.