TEKTITES

It is commonly accepted that tektites, a word derived from the Greek "molten", formed from vaporized terrestrial rock following a hypervelocity impact on Earth. One could think of tektites as meteorites from Earth in the sense that they were blasted from the Earth through impact and shaped by aerodynamic forces as they plummeted back through the atmosphere to create defined strewn fields. Previous theories espousing an origin from volcanic eruptions on the Moon have been adequately disputed through comparative studies of the chemistry and mineralogy of lunar samples returned from the Moon by the Apollo astronauts. Hydrocode simulations of impacts (Artemieva, 2001) have demonstrated that high velocity impacts (35–40 km/s) having impact angles of 30° to 50° from the horizon, and impacting dry target material, are consistent with model constraints on tektite production.

Most tektite strewn fields have now been associated with a particular impact structure with an established age directly relating the tektite to its corresponding ejecta deposit: Moldavites with the Ries Crater in Germany, Ivory Coast tektites with the Bosumtwi crater in Ghana, and the North American tektites with the Chesapeake Bay Impact Crater. The largest known strewn field, encompassing the Austalasian tektites, has yet to be identified with a particular impact structure, but it has been suggested that this event may reflect an aerial burst similar to the event over Tunguska, Siberia, and that which produced Libyan Desert Glass in Egypt. Still, recent gravity and topography data from Seasat and Geosat has identified an ~100-km circular feature off the coast of Vietnam in the South China Sea centered at 13.6° N., 110.5° E., which could prove to be the elusive crater. Geochemical data suggest that these tektites sample typical post-Archean sedimentary, upper crustal rocks.

Lechatelierite, a form of fused quartz, and other relict mineral inclusions showing evidence of shock metamorphism, are found inside tektites, consistent with an impact origin rather than one based on igneous volcanism. Moreover, the high-pressure polymorph of quartz known as coesite has been identified in tektites, indicating pressures greater than 2 GPa. Elemental abundance patterns, along with size and shape characteristics of the mineral inclusions, suggest that the typical parent material for tektites was a terrestrial, fine-grained, sedimentary deposit similar to a graywacke or loess.

One revealing fact in the origin debate is that no tektites have ever been found in Antarctica, where meteorites from most all classes have previously been recovered, including many lunar specimens. A major point of contention held by the lunar-origin theorists (Futrell and O'Keefe, 1997) has been the very low content of water in tektites compared to the terrestrial source rock. A recent study into the physics and chemistry of impact melting and vaporization explains this lack of water as a natural outcome of the thermodynamics associated with shock pressures in excess of 100 GPa and temperatures in excess of 50,000°C. Under these conditions, volatile-containing bubbles would carry all of the water vapor out of the silicate droplets, along with free oxygen, leading to reduction of Fe and the formation of the dark green to black colors associated with tektites. Furthermore, lunar rocks contain still less water than tektites do.

Recoveries of some Indochinite tektites with signs of stretching during flight provide more evidence for an impact origin rather than an extraterrestrial origin. If these features had been the result of etching by soil acids, as some have argued, the stretch areas would have been etched to a similar degree as the rest of the tektite, which is not the case. This leads to the conclusion that these features were formed by aerodynamic sculpting instead. The internal heating demonstrated by the plastic stretching is also further evidence that the tektites did not arrive on Earth as individuals, in which case the only heating effects would be those on the exterior caused by heating during entry. Instead, these features were the result of spallation on a large, rapidly rotating body within the atmosphere.

The photo above shows four members of the ~803 t.y. old Australasian strewn field, which covers at least one-tenth of the Earth's surface. Other tektite sources likely encompasssed by this strewn field include those from Tibet, the Phillipines (Rizalite, Bikolite, Anda) and Java. Based on the spatial variation in microtektite concentrations from deep-sea drilling cores, the probable location for the source crater is determined to be in eastern Cambodia (12° N., 106° E.), in agreement with previous predictions. The diameter of the source crater is estimated to be 90–116 km.

The button/lens-shaped Australite above, which was the first type of tektite described in the literature by naturalist Charles Darwin, exhibits typical concentric ring-wave flow ridges on the forward face emanating from the stagnation point, consistent with aerodynamically stable hypervelocity ablation during descent. The Muong Nong-type layered tektites are thought to have formed close to the source crater, consistent with their lack of ablation features and a recovery location very near that predicted for the source crater. Also pictured above is a tektite-like glass, a Colombianite (or Amerikanite), which is now thought to be of terrestrial volcanic origin due to its high water content, in spite of its strong resemblance to a tektite.

The last photo shown above is a mass of Libyan desert glass (LDG), melted silica glass thought to have been formed by the intense heat of a meteoric aerial burst over sand dunes of the Great Sand Sea in western Egypt, ~28.5 m.y. ago. However, a study of C and Cl in LDG indicates quenching upon impact of a carbon-rich projectile with silica-rich crustal rocks and sea-water, rather than impacting on dry land (Miura, 2009). It has been estimated that at least 10 million tons of LDG were created in a region extending over an area of ~80–25 km², of which 20 tons has been collected (Pratesi et al., 2002). A small, extensively weathered fragment of oxidized iron showing remnants of an octahedral structure (named Great Sand Sea 003) is possibly associated with this event. The energy released during this explosion is calculated to have been equivalent to that of the Tunguska event.

Similar to LDG, the high-silica Dakhleh Glass (DG) from the Dakhleh Oasis in the Western Desert of Egypt, derives from a low-altitude airburst ~120 (±40) t.y. ago into Pleistocene lacustrine sediments (Osinski et al., 2008). Many of the black to dark-gray DG specimens are vesiculated on the upper surface, sometimes exhibiting impressions of grass and reed vegetation on the underside, some containing burnt sediments. Dakhleh Glass is predominantly a mixture of glass and crystallites.

Resulting high-temperature melt-products identified in LDG glass include lechatelierite and cristobalite from quartz, and baddeleyite from zircon. Cathodoluminescence imaging has identified previously unrecognized quenched flow textures in this glass, potentially useful for characterizing the temperature of formation (Gucsik et al., 2003); temperatures in the 1700–2100°C range have now been calculated (Pratesi et al., 2002). These rare brown to bluish (from Rayleigh scattering by 60 nm-sized or smaller particles) streaks have been resolved into discrete rounded glass spherules which constitute an immiscible liquid within the silica-glass matrix. These spherules are enriched in Al, Fe, and Mg, as well as Ir, Os, Cr, Co, and Ni, elements which may reflect a significant meteoritic component. Graphite ribbons, possibly derived from the impactor, have also been identified.

Brecciated sandstone samples from the LDG area show signs of fractures, mosaicism, undulatory extinction, and cleavage. Planar deformation features and other shock features provide persuasive evidence for a hypervelocity impact origin. Geochronological data and isotopic ratios of Sr and Nd indicate that the target material was sand, derived from Precambrian crustal granitic rock, rather than Lower Cretaceous sandstones of the Nubia Group.

The photo shown below is an artifact recovered from King Tutankhamun's tomb. This pectoral fearures a yellow-green gem in the image of a scarab, carved from a piece of Libyan Desert Glass. This demonstrates the special nature of this material to the early Egyptian civilization.

Pictured below is an olive-green, filigree-like moldavite, a member of the Central European strewn field associated with the 14.68 (±0.11) m.y. old, 24-km-wide Nördlinger Ries impact crater in Germany (Laurenzi et al., 2003; Vincenzo and Skála, 2008). The 3.8-km-wide Steinheim Basin crater, located 42 km west-southwest of the Ries crater, is considered to have been formed during the same event, attesting to a binary asteroid impact. Through 3-D hydrocode simulations (Stöffler et al., 2002), it was determined that the two impact projectiles, with diameters of 1.5 km and 0.15 km, impacted at an angle of 30–50° from the horizontal at a velocity of ~20 km/s. The leading shock wave impacted the target material, consisting of both weathered and unweathered, unconsolidated, Middle Miocene silica-rich sands, with lesser amounts of clays and carbonates, generating temperatures of ~40,000°C (Skála et al., 2008). The superheated melt was distributed up to 400–500 km away in a symmetrical, ~60° fan-shaped jet, the remnant of which today is represented by the moldavite strewn subfields of South Bohemia, Moravia, the Cheb Basin, northern Austria, and Lusatia. Rapid cooling and solidification of the melt ensued, leading to the formation of tektites.

Moldavites accumulated from melt particles of variable chemical composition. The ultimate shape of moldavites is a result of etching by groundwater in an acidic, permeable environment. They are virtually water-free and incorporate cations that reflect differentiation based on ionic size, a result of their formation (fractional condensation) at plasma temperatures (von Engelhardt et al., 2005). Shock forces from the Ries impact created shattercones in the limestone layer below.

The four photos below represent tektites from the two remaining recognized strewn fields, along with an impact glass similar in composition to the LDGs:

• (top) Georgiaite, 15.9 g, a member of the North American strewn field with an age, location, and source rock composition consistent with its origin from the 35.5 m.y. old, ~40-km-wide Chesapeake Bay impact structure.
• (top center) Bediasite, 26.7 g, along with Georgiaites, these are the oldest tektites known. While Georgiaites are found in 17 counties in east central Georgia, Bediasites are found in 9 counties in southeast Texas and are characterized by their darker shade of green and their deeply grooved and etched surfaces which resemble the Rizalites of the Philippines. Bediasites were propelled 1,300 miles from the impact site, landing in Eocene sediments where they are now being slowly eroded out along a narrow band in central Texas.
• (bottom center) Ivory Coast, 14.6 g, a member of the Ivory Coast strewn field, from the 1.07 m.y. old, 10.5-km-wide Bosumtwi crater in Ghana. These have the lowest water contents of all tektites measured. Osmium isotopic ratios within these tektites are much higher than continental crust values, evidence of a relict meteoritic component. Low rhenium isotopic ratios are the result of fractionation that occurred during the impact.
• (bottom) Irghizite, 1.8 g, an impact glass from the ~0.8 m.y. old, 13.5-km-wide Zhamanshin crater in Kazakhstan. These glasses were formed at the impact site as flows and incorporate both terrestrial and meteoritic components. Examination of vesicles within Irghizites has revealed the presence of organosilane and organosiloxane, compounds never before found in nature. They are thought to have formed under intense heat and reducing conditions through interactions of a high-silica melt with hydrocarbons. Also associated with the Zhamanshin crater are the Zhamanshinites, but they are larger and have a more blocky shape than their Irghizite cousins. In contrast to Irghizites, Zhamanshinites are thought to have formed in the cooler melt regions at the outer rim and did not retain a meteoritic component.

Still other impact glasses have been found to be associated with craters:

• (top) Aouelloul glass, 0.95 g, associated with a 3.1 m.y. old, 390-m-wide crater in the Adrar region of the Western Sahara Desert in Mauritania. Lechatelierite and baddeleyite are present in the glass as well as an extraterrestrial component. Through BSE and CL imaging it was found that the degree of homogenization attests to a lower temperature formation of Aouelloul glass than of Libyan desert glass (Gucsik et al., 2004).
• (bottom) Darwin glass, 1.64 g, with an 40Ar/Ar39 age of ~816 t.y., associated with a 1.2 km diameter impact crater in southwest Tasmania, Australia. On a geologic time scale, the Darwin impact occurred coincident with the Australasian impact (~803 t.y.), but the two events were separated by several thousand km. Relative to its size, Darwin crater is the source of the largest volume of glass compared to other impact craters worldwide (Howard, 2009). This prodigious amount of glass is a direct result of the enhanced volatility caused by the high water content in the target stratigraphy. This vesiculated, layered, fragmental glass ranges in color from white (5%, located closest to the crater), light green (31%), dark green (53%), and black (11%, located farthest from the crater). In addition, five different shape categories have been distinguished: irregular, ropy, elongate, droplet, and spheroid. The light green, irregular-shaped sample shown below represents the most common shape (74%) among all colors. Heterogeneity among samples is indicative of rapid quenching of the melt, and metal enrichment in some specimens likely reflects contamination by the impactor (Howard, 2008).

In addition to these, the Monturaqui glasses are associated with a ~100 t.y. old, 370-m-wide crater located in the Atacama Desert, Chile, and at least seven distinct horizons of impact glass have been identified in loess-like deposits of the Pampeano Formation of Argentina (Schultz et al., 2002, 2004):

1. Rio Cuarto impact glass dated to the Holocene Epoch at 6 (±2) t.y. ago
2. impact glass sites dated to the Pleistocene Epoch at 114 (±26) t.y. ago
3. impact glass sites dated to the Pleistocene Epoch at 230 (±30) t.y. ago
4. glass impactites dated to the Pleistocene Epoch from Centinela del Mar at 445 (±11) t.y. ago
5. impact glass sites dated to the Pleistocene Epoch near Necochea at 445 (±21) t.y. ago
6. impact glass near Mar del Plata dated to the Pliocene Epoch at 3.27 (±0.08) m.y. ago
7. impact glass near Buenos Aires Province dated to the Miocene Epoch at 5.33 (±0.05) m.y. ago
8. impact glass near Chasico dated to the Miocene Epoch at 9.23 (±0.09) m.y. ago, which is associated with a concentric structure measuring 15 km in diameter

Furthermore, three specimens of a 24 m.y. old impact glass have been recovered in Western Siberia near Novy Urengoi, known as Urengoites, and some unusual glass nodules have been discovered in Tikal, Guatemala, which are thought to be impact related, possibly belonging to the Australites (A. Hildebrand), but perhaps actually part of the distant Ivory Coast strewn field (S. Webb, pers. comm.). In addition, a single 6.2 m.y. old piece of impact glass has been found near Magnitogorsk in the Urals, named South-Ural glass. Besides their petrographic characteristics, these glasses contain very low water contents compared to glasses of volcanic origin, and are therefore suspected to be of impact origin. Finally, bottle-green microtektites have been found in a core sample from the South Tasman Rise in the Indian Ocean, which have been dated to 4.6–12.1 m.y. ago (D. Kelly and L. Elkins-Tanton, 2004). These microtektites have been geochemically associated with impact glasses from Bahía Blanca, Argentina, dated at ~5.28 (±0.04) m.y. ago, and indicate that a large impact event occurred. Basaltic clasts analysed from this impact-melt glass show close similarities to known angrites (Harris and Schultz, 2009).

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