High-velocity impact ejecta: Tektites and martian meteorites

Abstract

Earth retains the poorest record of impact craters through geologic time. Important clues of the occurrence of large impact events through Earths geologic history come from the presence of preserved distal ejecta layers. Distal ejecta comprise a small but essential fraction of material ejected in impacts; it was crucial in the recognition of the end-Cretaceous impact event (Alvarez et al. 1980). This chapter discusses specific types of distal ejecta, which are characterized by substantial shock compression, and high ejection velocity, and are best represented by the enigmatic tektites and Martian-lunar meteorites. Tektites are naturally occurring glasses, generally a few centimeters in diameter, currently found in four distinct strewn fields (Table 1). Microtektites (≤1 mm in diameter) have been found in deep-sea cores of three of the four strewn fields (Glass 1972). Tektites of a given strewn field are related to each other by their chemistry, age, and petrologic and physical characteristics (see reviews by Glass 1990; Koeberl 1990, 1994). There is numerous evidence that tektites were used by ancient civilizations. However, their scientific study began with Charles Darwins description of Australites in Geology of the Voyage of the Beagle (1851). Early hypotheses of their origin (volcanic glass, impact of glassy asteroid, ablation of high-velocity cosmic body in the Earths atmosphere, and even artifacts of ancient glass-makers) are clearly unrealistic. The first celestial mechanics deduction was that tektites must come from some source no farther than the Moon (Urey 1955). The result was confirmed by the small amounts of cosmogenic isotopes (26Al and 10Be) in Australian-Asian tektites (Viste and Anders 1962). At the same time the huge extent of some of the known strewn fields implies a powerful launch mechanism: the only two in nature are volcanism and impact. By the mid-1960s, four hypotheses emerged: high-velocity impacts on Earth or the Moon, and volcanic activity on these bodies. Terrestrial volcanic glasses (obsidians) are similar to tektites in many aspects. On the other hand, typical ejection velocities during a volcanic eruption never exceeds 700 ms?1, which is not enough to create a large tektite strewn field similar to the Australian-Asian. The extremely young age of all tektites (≤35 Myr) is in disagreement with lunar chronology and the early extinction of lunar vulcanism. Geochemical studies of Apollo lunar samples demonstrated that the silica content of lunar rocks is not ≥50%, whereas tektites usually have ≥60% silica. It is worth mentioning that the first real samples of lunar rocks reaching the Earth, lunar meteorites, were identified on Earth only toward the end of the 1980s (Eugster 1989). A very interesting historical description of tektite exploration can be found in OKeefe 1976. This chapter concentrates on the latest hypothesis: Tektites are high-velocity molten ejecta originating in terrestrial craters. © 2008 Springer.