Impactite

Abstract: Lunar meteorites represent a more random sampling of lunar material than the Apollo or Luna collections and, as such, lunar meteorite impact melt ages are the most important data in nearly 30 years with which to reexamine the lunar cataclysm hypothesis. Within the lunar meteorite breccias MAC 88105, QUE 93069, DaG 262, and DaG 400, seven to nine different impact events are represented with 40Ar-39Ar ages between 2.76 and 3.92 billion years ago (Ga). The lack of impact melt older than 3.92 Ga supports the concept of a short, intense period of bombardment in the Earth-moon system at approximately 3.9 Ga. This was an anomalous spike of impact activity on the otherwise declining impact- frequency curve.

Abstract: THE 200-km-diameter Chicxulub structure1–3 in northern Yucatan, Mexico has emerged as the prime candidate for the Cretaceous/Tertiary (K/T) boundary impact crater3–6. Concentric geophysical anomalies associated with enigmatic occurrences of Upper Cretaceous breccias and andesitic rocks led Penfield and Camargo1 to suspect that this structure was a buried impact basin. More recently, the discovery of shocked quartz grains in a Chicxulub breccia3, and chemical similarities between Chicxulub rocks and K/T tektite-like glasses3–6 have been advanced as evidence that the Chicxulub structure is a K/T impact site. Here we present evidence from core samples that Chicxulub is indeed a K/T source crater, and can apparently account for all the evidence of impact distributed globally at the K/T boundary without the need for simultaneous multiple impacts or comet showers. Shocked breccia clasts found in the cores are similar to shocked lithic fragments found worldwide in the K/T boundary ejecta layer7,8. The Chicxulub melt rocks that we studied contain anomalously high levels of iridium (up to 13.5 parts per 109), also consistent with the iridium-enriched K/T boundary layer9. Our best estimate of the crystallization age of these melt rocks, as determined by 40Ar/<39Ar analyses, is 65.2 ±0.4 (1σ) Myr, in good agreement with the mean plateau age of 64.98 ± 0.05 Myr recently reported10. Furthermore, these melt rocks acquired a remanent magnetization indicating that they cooled during an episode of reversed geomagnetic polarity. The only such episode consistent with40Ar/<39Ar constraints is chron 29R, which includes the K/T boundary.

Abstract: Glass produced by melting during impact events is called impact glass or impactite. Although impact crate ring produces a great variety of shock­ metamorphosed rocks, all of which are called impactites by various authors (to a differing extent), we restrict the use of this term here to material completely converted into glass. The degree of metamorphism seen in different classes of impact materials (starting with those showing simple shock effects, shock lithification, shatter cones, diaplectic glasses, coesite, stishovite, lechatelierite, impact breccias, glass) depends on the peak pressure and temperature the rock has experienced. As a result of mixing, brecciation, and similar processes, impact glasses also tend to include fragments of less shocked or less homogeneous metamorphosed rocks. We limit our discussion here, however, to the more homogeneous members of the terrestrial impactite family. [A discussion of lunar impact glasses was given by Delano et al (1981).] Another group of natural glasses are tektites, which at first glance are similar to obsidian. Most tektites are very homogeneous, often aerodynam­ ically shaped (spherically symmetric) objects that are several centimeters in size and are found in various areas (referred to as tektite strewn fields). In reflected light they are usually black, but in transmitted light they show variations of grey, brown, and green. The surface features (gouges, furrows, grooves, pits, striations) clearly distinguish them from terrestrial volcanic glasses and have probably originated from solution etching and weathering effects on Earth (Baker 1963, LaMarche et a1 1984) and from surface stresses occurring during their cooling. Tektites found in geographically adjacent

Abstract: To ascertain the progressive stages of shock metamorphism of zircon, samples from three well-studied impact craters were analyzed by optical microscopy, scanning electron microscopy (SEM), and Raman spectroscopy in thin section and grain separates. These samples are comprised of well-preserved, rapidly quenched impactites from the Ries crater, Germany, strongly annealed impactites from the Popigai crater, Siberia, and altered, variably quenched impactites from the Chicxulub crater, Mexico. The natural samples were compared with samples of experimentally shock-metamorphosed zircon. Below 20 GPa, zircon exhibits no distinct shock features. Above 20 GPa, optically resolvable planar microstructures occur together with the high-pressure polymorph reidite, which was only retained in the Ries samples. Decomposition of zircon to ZrO2 only occurs in shock stage IV melt fragments that were rapidly quenched. This is not only a result of post-shock temperatures in excess of ~1700 °C but could also be shock pressure-induced, which is indicated by possible relics of a high-pressure polymorph of ZrO2. However, ZrO2 was found to revert to zircon with a granular texture during devitrification of impact melts. Other granular textures represent recrystallized amorphous ZrSiO4 and reidite that reverted to zircon. This requires annealing temperatures >1100 °C. A systematic study of zircons from a continuous impactite sequence of the Chicxulub impact structure yields implications for the post-shock temperature history of suevite-like rocks until cooling below ~600 °C.