Appendix H
In lieu of an abstract, here is a brief excerpt of the content:

Tektites are natural glasses quenched from superheated melts, produced and ejected at relatively large velocities, by impacts on the surface of the earth. Some of their most obvious petrologic characteristics (e.g., reduction, volatile depletion, and lack of crystallites) are a consequence of this superheating. To date, tektites have been reported from five strewn fields, in some cases associated with known source craters. However, tektites are probably produced in all sufficiently large terrestrial impacts. Unusual glass nodules discovered by archaeological studies in the Maya city of Tikal, Guatemala, show the superheated signatures characteristic of tektites and may represent products of an as yet unknown impact. Provenance During sorting and classification of obsidian artifacts found in excavations of the Maya ruins at Tikal, Guatemala, eleven unworked nodules were separated from the obsidian artifacts on the basis of their shape and size (Fig. B: 120a), and color (clear brownish green). The nodules were found in general excavations widely scattered through the city, mostly in small structure groups that are presumed to be residential. We assume that the nodules were collected at points unknown and transported by the Maya to the city of Tikal; no other similar nodules have yet been reported from other Maya archaeological sites. Petrography The three nodules studied to date are composed of clear glass lacking any phenocrysts, microlites, or schlieren visible to optical or scanning electron microscopes . On this basis, a possible impact origin had been previously suggested (Moholy-Nagy and Nelson 1990: 75; Essene et al. 1987). A scattering of spherical vesicles ranging up to ~0.5 mm does occur, similar in abundance and size to those found in philippinites. Although the surfaces are pitted from presumed dissolution , no surficial alteration rims were obvious in cross section. Composition The tektites’ composition has been studied by electron microprobe, Instrumental Neutron Activation Analysis (INAA), Inductively Coupled Plasma-Mass Spectrometry, and X-Ray Fluorescence. All three samples yield similar compositions of major, minor, and trace elements and results are consistent with previous work (Essene et al. 1987). The silica abundance is ~62 percent, a value lower than those exhibited by most tektites except those of Cretaceous/Tertiary (K/T) age. Indeed, the major and minor element abundances are similar to those of the K/T tektites, causing us to speculate that the Tikal tektites might have been transported from K/T boundary sections that outcrop near Tikal. However, trace elements, such as the Rare Earth Elements, revealed patterns distinct from those displayed by the K/T tektites Appendix H Report on the Tektites Found at Tikal Alan R. Hildebrand [revised 6 June 1994] APPENDIX H 101 (Koeberl and Sigurdsson 1992; Hildebrand et al. 1992). In general, all lithophile incompatible elements are depleted relative to abundances found in the other four tektite groups, consistent with the depletion in silicon, although an upper crustal affinity is evidenced. Water Content Tektites are depleted in all volatiles relative to other terrestrial glasses; water contents are of the order of 100 ppm, which is more than an order of magnitude less than found in the driest volcanic glasses (Koeberl and Beran 1988). The water content of the three samples was studied using infra-red (IR) spectrometry, yielding water contents of 60 to 80 ppm, values typical of tektites. Ferric/Ferrous Ratios Because of superheating, tektites display reduced chemistry such as Fe+3/Fe+2 ratios approaching zero, in contrast to terrestrial volcanic glasses that exhibit ratios of near unity. Mössbauer studies of one of the samples detected no Fe+3, which is consistent with extreme reduction. Similarly, magnetic studies of all three samples reveal magnetic susceptibilities, magnetizations, and Curie constants generally consistent with reduction levels found in other tektites (Thorpe and Senftie 1964). However, the temperature independent susceptibility is higher, indicating the presence of completely reduced Fe0, which possibly indicates higher formation temperatures or longer durations of heating. Measurements of internal vs. external pieces of one sample indicate that oxidation of Fe0 has occurred. Age The age of the tektites was expected to be 10 to 100 million years, so an Ar-Ar study was planned with an irradiation optimized for an age of this order. However, a much younger age of 800,000 ± 100,000 years (two sigma) was found, based on an isochron defined by the three samples. This experiment will be repeated with an irradiation optimized for this younger age and larger samples to provide more radiogenic 40Ar. Conclusions The nodules recovered at Tikal are tektites, based on their petrologic character...


pdf