Ivory Coast Tektites Research Articles

Ivory Coast microtektite strewn field: description and relation to the Jaramillo geomagnetic event

During the present study the Ivory Coast microtektite layer was found in cores from five equatorial Atlantic sites, bringing the total number of Ivory Coast microtektite-bearing cores to eleven. The strewn field appears to be restricted to between 9°N and 12°S latitude. There is a general increase in the concentration of microtektites towards the Bosumtwi crater, which is generally thought to be the source of the Ivory Coast tektites. The relationship between the onset of the Jaramillo subchron and the Ivory Coast microtektite layer has been investigated in six cores. A plot of the difference in depth between the base of the Jaramillo subchron and the microtektite layer versus sediment accumulation rate was used to determine the average post-depositional remanent magnetization (PDRM) acquisition depth and the age difference between the onset of the Jaramillo subchron and the deposition of the microtektites. Assuming that the PDRM acquisition depth does not vary with sediment accumulation rate, we find that the average PDRM acquisition depth is 7 cm and that the microtektites were deposited approximately 8 ky after the onset of the Jaramillo subchron. This indicates that the impact responsible for the Ivory Coast tektites and microtektites could not be causally related to the geomagnetic reversal at the base of the Jaramillo subchron.

ZHAMANSHIN AND AOUELLOUL: CRATERS PRODUCED BY IMPACT OF TEKTITE‐LIKE GLASSES?

It is shown that the enhanced abundance of siderophile elements and chromium in tektite‐like glasses from the two impact craters of Zhamanshin and Aouelloul cannot be explained as a result of contamination of the country rock by meteorites nor, probably, comets. The pattern is, however, like that found in certain Australasian tektites, and in Ivory Coast tektites. It is concluded, in agreement with earlier suggestions by Campbell‐Smith and Hey, that these craters were formed by the impact of large masses of tektite‐like glass, of which the glasses which were studied are fragments. It follows that it is necessary, in considering an impact crater, to bear in mind that the projectile may have been a glass.

Chemical analyses of Bosumtwi crater target rocks compared with the Ivory Coast tektites

The Bosumtwi crater, Ghana, was excavated in phyllites and greywackes with subordinate microgranite dykes and quartz veins of the 2000 Ma old Lower Birimian System with a granodiorite intrusion at Pepiakese on its northeastern side. New major and trace element analyses are presented for 7 phyllites, 5 greywackes, 2 microgranites, 3 Pepiakese intrusion rocks and 1 suevite using XRF and INNA. Means and standard deviations were calculated using all available modern analyses for each element in the Bosumtwi target rocks, Bosumtwi suevite glasses and Ivory Coast tektites. Good agreements between the means were found for the three groups with the suevite glasses and tektites having more limited compositional ranges than the target rocks. Least squares mixing between target rock types shows that the best fits to the tektite and suevite glass compositions require components of about a third or a quarter from the Pepiakese intrusion and some extra silica, derived from quartz veins, as well as the metasediments. The new data provide evidence for vapour phase fractionation of P 2O 5 and Na 2O in the tektites in addition to the previously reported Pb and Rb. Evidence for a meteoritic component in the tektites was found to be equivocal since the target rocks are probably a sufficient source of the meteorite indicator elements Ni and Ir.

Tektites

Tektites are small, generally rounded, silicate glass bodies found in several widely separated areas of the Earth's surface called strewn fields. Four strewn fields are recognized: (1) Australasian, (2) Ivory Coast, (3) Czechoslovakian, and (4) North American. Tektites from the four known strewn fields were formed approximately 0.7, 1, 15 and 35 million years (m.y.) ago, respectively. Tektites are generally black, but those from Czechoslovakia and Georgia are translucent green or brown in color. Most tektites occur as splash forms, but some are blocky with a layered structure (Muong Nong-type); and some of the splash forms have been remelted or ablated by high velocity flight down through the Earth's atmosphere. Although they superficially resemble obsidian, they can be distinguished from obsidian by their petrography and compositions. Tektites, unlike obsidians, contain ubiquitous lechatelierite (SiO2 glass) particles and are generally completely devoid of microlites or crystallites. Tektites are drier than obsidians and have higher FeO/Fe2O3 ratios. Although some researchers favor a lunar volcanic origin for tektites, most of the chemical, isotopic, and petrographic studies seem to support a terrestrial impact origin. The presence of lechatelierite, coesite, shock relict inclusions, and Ni-Fe spherules favors an impact origin; and chemical and isotopic data indicate that the parent material was a terrestrial sedimentary deposit. The Czechoslovakian and Ivory Coast tektites have been associated with the Ries crater (in Germany) and the Bosumtwi crater (in Ghana), respectively.

High precision elemental abundances for tektites and crater glasses-thorium, uranium and potassium

Thorium, uranium and potassium mean abundances for 15 Ivory Coast tektites, 23 Australites, 10 Philippinites, 25 Thailandites, 16 Bediasites and 12 Moldavites are reported. Abundances are also reported for Muong Nong type tektites, Nordlinger Ries crater glass, Bosumtwi crater glass and Darwin crater glass. Previous measurements are reviewed. The value of the present work lies in the fact that high precision instrumental neutron activation analysis was employed to measure a large number of tektites encompassing the major strewn fields. Four independent measurements of thorium were made for each specimen with two Ge(Li) gamma spectrometers. Two of the measurements were based on the 312 keV gamma of233Pa, while the other two measurements were based on the uranium\(K_{\alpha _1 }\) and\(K_{\alpha _2 }\) X-rays which accompany the decay of233Pa. Internal consistency among the four thorium measurements is high. The uranium measurements were based on the 106 keV gamma of239Np, while potassium was measured from the 1524 keV gamma of42K.

Age and provenance of the target materials for tektites and possible impactites as inferred from Sm-Nd and Rb-Sr systematics

Sm-Nd and Rb-Sr analyses of tektites and other impactites can be used to place constraints on the age and provenance of target materials which were impact melted to form these objects. Tektites have large negative ε Nd(0) values and are uniform within each tektite group while the ε Sr(0) are large positive values and show considerable variation within each group. Chemical, trace element, and isotopic compositions of tektites are consistent with production by melting of sediments derived from old terrestrial continental crust. Each tektite group is characterized by a uniform Nd model age, T CHUR Nd , interpreted as the time of formation of the crustal segment which weathered to form the parent sediment for the tektites: (1) ∼1.15 AE for Australasian tektites; (2) ∼1.91 AE for Ivory Coast tektites; (3) ∼0.9 AE for moldavites; (4) ∼0.65 AE for North American tektites, and (5) ∼0.9 AE for high-Si irghizites. Sr model ages, T UR Sr , are variable within each group reflecting Rb-Sr fractionation and in the favorable limit of very high Rb/Sr ratios, approach the time of sedimentation of the parent material which melted to form the tektites. Australasian tektites are derived from ∼0.25 AE sediments, moldavites from ∼0.0 AE sediments, Ivory Coast tektites from ∼0.95 AE sediments. Possible parent sediments of other tektite groups have poorly constrained ages. Our data on moldavites and Ivory Coast tektites are consistent with derivation from the Ries and Bosumtwi craters, respectively. Irghizites are isotopically distinct from Australasian tektites and are probably not related. Sanidine spherules from a Cretaceous-Tertiary boundary clay have initial ε Nd ∼ +2; ε Sr ∼ +5 and are not derived from old continental crust or meteoritic feldspar. They may represent a mixture of basaltic oceanic crust and sediments, implying an oceanic impact. These isotopic results are also consistent with a volcanic origin for the spherules.

Identification of the projectile at the Brent crater, and further considerations of projectile types at terrestrial craters

Impact melt samples from drill hole B1-59 at the 3.8 km diameter Brent crater (Ontario) have been analysed for siderophile trace elements indicative of meteoritic contamination. Samples from the basal melt zone at 823–857 m depth are enriched in Ir, Os, Pd, Ni, Co, Cr and Se over basement, with the abundance pattern suggesting a chondritic projectile for Brent. From a Ni-Cr correlation of 10 melt samples an L or LL chondrite is inferred. The contribution of an ultramafic country rock (alnoite) in the melt is too small to significantly influence its NiCr ratio. Glass-rich breccias from the allochthonous breccias filling the crater also contain a meteoritic component. Interelement ratios (e.g. NiCr) are, however, fractionated relative to the melt zone samples. This, as well as the low Au content of all Brent samples, is probably a product of alteration.Additional data on impact melts from the 65 km diameter crater Manicouagan still did not reveal a meteoritic component, as also for the Mistastin crater (28 km diameter) where Cr analyses set an upper limit of 1% of an achondritic projectile component in the melt. Irghizites (tektite like glasses) from the Zhamanshin impact structure have been found to contain high Ni and Co concentrations, and our data show that Ir is also enriched. It is however not possible to define the projectile-type. Enrichment of an Ivory Coast tektite in Ir is confirmed. There are large differences in siderophile element concentrations among tektites, with otherwise similar chemical composition.There are now four known craters formed by chondrites (Clearwater East, Lapparjärvi, Wanapitei, and Brent), with Brent being the smallest of these. For smaller craters the projectiles appear to be limited to iron or stony-iron meteorites, because of atmospheric destruction of relatively small stony meteorites. It appears, however, that all major classes of meteorites are represented among the projectiles at terrestrial impact craters.

Tektites: Size estimates of their source craters and implications for their origin

From estimates of the total masses of tektites in three strewnfields, calculations by Orphal et al. (1980) of the amount of melt that could be ejected from impact craters, and equations relating kinetic energy of impact to crater diameter, it is possible to calculate minimum diameters of lunar craters capable of ejecting the liquid masses that could have formed the various tektite strewnfields. No lunar craters of the requisite sizes have been found that are young enough to correlate with the dates of formations of the strewnfields and it seems clear that the Moon must be eliminated as a source of tektites on the Earth. It is concluded that the associations of the Ivory Coast tektites with the Bosumtwi crater and the moldavites with the Rieskessel are real and the tektites are of terrestrial origin. It follows that if the Ivory Coast tektites came from the 10.5-km-wide Bosumtwi crater, the larger masses in the Australasian and North American strewnfields came from craters 17 km in diameter and between 33 and 65 km in diameter, respectively. No crater has yet been proven to be the parent of the Australisian tektites. The large crater that formed the North American tektites may not yet have been found, although the Mistastin Lake Crater may eventually be proven to be the source.

Tektites probably wholly terrestrial and related to continental movement

SummaryThe four groups of tektites are now widely regarded as the product of splash from a meteoritic or cometary impact melt of terrestrial rock. They include two groups (Moldavites and Ivory Coast tektites) attributed to the Nordlinger Ries and Bosumtwi craters. These craters seem unlikely to be of impact origin, though hyper-explosive. Tektites are, therefore, more probably all wholly terrestrial. For much the largest group, those of the combined Southeast Asian-Australian strewnfield, no satisfactory source is known. An origin in Southeast Asia, even by impact, is improbable. A possible source lies in Central Asia. The hyper-explosive activity is regarded as an extreme form of volcanism, a consequence locally and rarely of the continual combined vertical and ‘horizontal’ movements of continents, here termedtribulation.

The Ivory Coast microtektite strewnfield: new data

Ivory Coast microtektites have recently been found in three piston cores from the Atlantic Ocean. In two of these cores, the microtektites occur in a layer above the base of the Jaramillo geomagnetic event indicating that they fell approximately 0.95 m.y. ago. Major element compositions have been obtained for forty-one of the microtektites by electron microprobe and energy-dispersive X-ray analysis. The geographic location, stratigraphic age and major element compositions indicate that these microtektites are part of the Ivory Coast tektite strewnfield. Present data suggest that the Ivory Coast strewnfield may cover an area of about 4.5 million square kilometers and contain about 20 million metric tons of microtektites.

Meteoritic material at five large impact craters

Sixteen crater samples were analyzed by radiochemical neutron activation analysis for Ge, Ir, Ni, Os, Pd and Re. Two impact melt rock samples from Clearwater East (22 km) showed strong, uniform enrichments in all elements except Ge, corresponding to 7.4% C1 chondrite material. Interelement ratios suggest that the meteorite was a C1 (or C2) chondrite, not an iron, stony iron, or chondrite of another type. An Ivory Coast tektite (related to the 10 km Bosumtwi crater) was enriched in Ir + Os and Ni to about 0.04 and 1.6% of C1 chondrite levels, but in the absence of data on country rocks, the meteorite cannot yet be characterized. Impact melt rock samples from Clearwater West (32km), Manicouagan (70km), and Mistastin (28 km) showed no detectable meteoritic component. Upper limits, as Cl chondrite equivalent, were Os ≤ 2 × 10 −3% (~0.01 ppb), Ni ≤ 2 × 10 −1% (~20 ppm). Possible causes are high impact velocity and/or a chemically inconspicuous meteorite (achondrite, Ir,Os-poor iron or stony iron). However, a more likely reason is that some fraction of the impact melt remains meteorite-free, especially at craters with central peaks. Clearwater East is the first terrestrial impact crater found to be associated with a stony meteorite. Apparently the consistent absence of stony projectiles at small craters (< 1 km diameter) reflects their destruction in the atmosphere, as proposed by Öpik.

Rare earth elements in tektites

Four moldavites and one sample each of an australite, billitonite, indochinite, philippinite, thailandite, Ivory Coast tektite, bediasite and a georgianite were analyzed using neutron activation analysis for La, Ce, Sm, Eu, Tb, Yb and Lu. The REE abundances resemble those of sedimentary rocks. Most of the tektites display a depletion of Eu, a characteristic feature of mature Phanerozoic continental sedimentary rocks. However, the Ivory Coast tektite and georgianite are relatively enriched in Eu, possibly due to the presence of plagioclase-rich source rocks.

Tektites: A post-Apollo view

Prior to the receipt of the lunar samples, it was the scientific consensus that tektites were melted and splashed material formed during large cometary or meteorite impact events. Whether the impact took place on the Earth or the Moon was the topic of a long-standing scientific debate, which raged with particular intensity during the decade previous to the lunar landings. Four definite and separate tektite-strewn fields are known: bediasites (North America, 34 m.y.); moldavites (Czechoslovakia, 14 m.y.); Ivory Coast (1.3 m.y.); and Southeast Asian and Australian fields (0.7 m.y.). A fifth possible occurrence, of high-Na australites, possibly 3–4 m.y. old, remains to be substantiated. The age of infall of the australites is not agreed upon. Radiometric and fission track dates agree with the magnetic stratigraphy for deep-sea core microtektite occurrences at about 0.7 m.y. Terrestrial stratigraphic evidence favours a recent (30,000 years) date. The chemistry of tektites appears to reflect that of the parent material, and losses during fusion appear to be restricted to elements and compounds more volatile than cesium. Terrestrial impact glasses provide small-scale analogues of tektite-forming events, and indicate that only the most volatile components are lost during fusion. The Apollo lunar missions provide critical evidence which refutes the hypothesis of lunar origin of tektites. Tektite chemistry is totally distinct from that observed in lunar maria basalts. These possess Cr contents which are two orders of magnitude higher than tektites, distinctive REE patterns with large Eu depletions, high Fe and low SiO 2 contents, low K/U ratios and many other diagnostic features, none of which are observed in the chemistry of tektites. The lunar uplands compositions, as shown by Apollo 14, 15 and 16 samples and the μ-ray and XRF orbiter data, are high-Al, low-SiO 2 compositions totally dissimilar to those of tektites. The composition of lunar rock 12013 shows typical lunar features and is distinct from that of tektites. The small amounts of lunar K-rich granitic material found in the soils have K/Mg and K/Na ratios 10–50 times those of tektites. The ages of the lunar maria (3.2–3.8 aeons) and uplands (> 4.0 aeons) are an order of magnitude older than the parent material of the Southeast Asian and Australian tektites, which yield Rb-Sr isochrons indicating ages of the order of 100–300 m.y. The lunar lead isotopic compositions are highly radiogenic whereas tektites have terrestrial Pb isotopic ratios. Lunar δ 18 O values are low (< 7 per mil) compared with values of +9.6 to +11.5 per mil for tektites. In summary, a lunar impact origin for tektites is not compatible with the chemistry, age or isotopic composition of the lunar samples. A lunar volcanic origin, recently revived by O'Keefe (1970) encounters most of the same problems. Recent lunar volcanism (< 50 m.y.), if the source of tektites, should contribute tektite glass to the upper layers of the regolith. None has been found. The presence of meteoritic components in tektites, and the high pressure phase coesite, are more readily interpreted as evidence of impact. The element abundances and inter-element variations in tektites do not resemble those in terrestrial igneous rocks, but show a close similarity to terrestrial sandstones. The composition of the Southeast Asian tektites, australites and moldavites resembles that of micaceous sandstones or subgreywackes, the Ivory Coast tektite composition is similar to that of greywacke, and the bediasite chemistry is analogous to that of arkose. No suitable terrestrial impact site has been identified for the bediasites, Southeast Asian tektites and australites. It is suggested that a search for the source of these latter strewnfields be made using satellite photographs to look for wide shallow craters produced by super-Tunguska type events on areas of Mesozoic sandstones. The moldavites were possibly formed during the Ries Crater event but, if so, the precise source of the material remains to be identified. The Ivory Coast tektites are linked by chemistry, isotope and age evidence to the Bosumtwi Crater, Ghana. The overall evidence now supports the origin of tektites by cometary (or meteorite) impact on terrestrial sedimentary rocks.

New data on selected Ivory Coast tektites

Fourteen Ivory Coast tektites exhibit a range of bulk indices of refraction of 1.5156 to 1.5217 plus or minus 0.0004 and of bulk specific gravities of 2.428 to 2.502 plus or minus 0.005. Seven of these Ivory Coast (IVC) tektites were analyzed for major and minor element content. Compared to tektites from other strewn fields, their SiO2 content is low (67.2-69.1%), Al2O3 relatively high (15.8-16.8%), and total iron relatively high but with a more restricted range (6.3-6.8% as FeO). Their lime content is low (0.71-1.35%) compared to Australasian tektites but their MgO/CaO ratio (about 3.1) is unusually high. All other tektite groups have Na2O/K2O ratios less than unity, but the Na2O/K2O ratio of the IVC tektites is slightly greater than unity. Their K/Rb ratios range from 200 to 256 and average 227.

Are Microtektites the Result of Cometary Impacts with the Earth ?

Durrani and Khan1 suggest that microtektites obtained from ocean sediments near Australia and the Ivory Coast may be the result of impacts of comets with the Earth. They measured the ages of the microtektites obtained from the two regions and determined the ages at 0.71 ± 0.1 and 0.88 ± 0.13 m.y., respectively, for Australasian and Ivory Coast tektites. These ages agree closely with the dates of two of the reversals of the geomagnetic field, and this close agreement is interpreted by them as suggesting that the microtektites and magnetic reversals have resulted from impacts of comets with the Earth.

The relationship of nickel and chromium in tektites: New data on the Ivory Coast tektites

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Geology and Geophysics, 1968.

Correlations between O18/O16ratios and chemical compositions of tektites

The limits of oxygen isotope variation in tektites have been determined by analyzing 33 tektites that cover the widest possible spectrum of chemical compositions. δO18 values were obtained for 7 australites (8.9 to 11.0), 4 javaites (9.4 to 10.0), 3 indochinites (9.3 to 10.6), 5 philippinites (9.8 to 10.6), 8 moldavites (10.7 to 11.8), and 6 Ivory Coast tektites (13.0 to 13.6), as well as 2 samples of Darwin glass (14.4 to 14.9). Various tektite groupings based on chemical composition and geographic occurrence all show a systematic increase in O18 with decreasing SiO2 content, similar to that previously found for the bediasites. These systematic correlations must arise either by (1) vapor fractionation of tektite material during impact melting or (2) mixing of a SiO2-rich igneous component and a low-SiO2 component formed at a much lower temperature; perhaps the mixing occurred during weathering at the earth's surface or during hydrothermal alteration of silicic igneous rocks either on the earth or the moon.

Chemical investigation of Australasian tektites

Chemical analysis of Australasian tektites inferring extraterrestrial igneous origin, noting similarity to preimpact parent rock

Chemical composition of Ivory Coast microtektites

The chemical composition of twenty-eight microscopic glassy objects, found in a deep-sea sediment core taken near the Ivory Coast, has been determined by electron microprobe analysis. Like the Australasian microtektites, these glassy objects can be divided into two main groups. One group is composed of individuals with chemical compositions similar to tektites. These are referred to as “normal” microtektites. The other group is composed of bottle-green glassy objects with low silica contents (<53 %) and high MgO contents (16–21 %). The “normal” Ivory Coast microtektites are similar in composition to Ivory Coast tektites and like Ivory Coast tektites, they differ from the Australasian tektites and microtektites by having a high MgO CaO ratio (>3) and an average Na 2O K 2O ratio greater than unity. Although the bottlegreen objects differ from the “normal” microtektites in their appearance, physical properties and chemical compositions, they are nevertheless found in association with them and are probably therefore related to them. Both the Ivory Coast and Australasian microtektites have higher MgO and lower Na 2O contents than igneous glasses with comparable SiO 2 contents. The chemical data presented in this paper are interpreted as confirming a previous suggestion that the glassy objects found in deep-sea sediments near the Ivory Coast are microtektites and that they represent a portion of the Ivory Coast tektite strewn field.

Isotopic comparison of lead in tektites with lead in earth materials

Lead samples from moldavites and from Ries glass are isotopically similar, except that the Ries glass tends to be slightly enriched in the radiogenic lead isotopes relative to moldavites. The lead in these materials is similar to the lead found in oceanic sediments and falls on the growth curves typical of most terrestrial rock and ore lead samples. This is true of lead in other tektite groups, with one exception. Lead from Ivory Coast tektites does not fall on the growth trend of modern terrestrial lead. Lead samples from Bosumtwi Crater dark glass and from the gray phyllite (country rock) at Bosumtwi Crater are similarly displaced from the typical growth trend. These features may be explained in terms of lead isotopic development over long periods of time in environments which differ somewhat from the normal in terms of relative U, Th, and Pb abundances, which suggests that the tektites as well as the impact glass were derived from the Precambrian rocks at Bosumtwi Crater. High U Pb ratios ( U 238 Pb 204 > 20 ) were found in all tektites analysed, indicating enrichment of U relative to Pb either at the time of tektite formation or within a time period not exceeding a few hundred million years prior to tektite formation. Lower U Pb ratios were found in the impact glasses. Lead samples from a bediasite and a Georgia tektite are the same, isotopically, and similar to lead from other tektite groups (except the Ivory Coast tektites).