Ivory Coast Tektites Research Articles

Origin of the Ca-phosphate inclusions in Ivory Coast and Australasian Muong-Nong-type tektites

Tektites are reduced (Fe2+) glasses formed by the quenching of molten material ejected from Earth’s surface as a result of a hypervelocity impact. The vast majority of tektites are usually homogeneous glasses, but rare samples containing mineral inclusions can provide insights about the source material, sample thermal history, and tektite formation process. Tektites from two distinct strewn fields presenting Ca-phosphate inclusions detected from anomalous magnetic susceptibility were studied: one sample from the Ivory Coast tektite (ICT) field ejected at 1.07 Ma from the Bosumtwi crater (10.5 km in size) in Ghana and two Muong Nong type samples from the Australasian tektite field (MN-AAT) ejected at 0.79 Ma from a crater possibly situated in southeast Asia. In ICT, Ca-phosphate inclusions are systematically embedded in lechatelierite (SiO2 glass). In MN-AAT Ca-phosphate are either embedded in lechatelierite or in Fe-rich glass forming schlieren. Multiscale petrographic characterization using correlative microscopy associating scanning electron microscopy, microprobe and, transmission electron microscopy reveals that rounded inclusions in ivoirite are composed of acicular Ca-phosphates (merrillite) embedded in an amorphous P-rich glass. In MN-AAT, inclusions consist mostly of single droplets of Fe-Mg rich Ca-phosphate (structurally related to apatite), but few droplets often forming an emulsion texture show a complex assemblage of apatite, magnetite, pyroxene, and spinel growing from a Pt-rich nucleus. Diffusion profile around lechatelierite domains reveals maximum temperatures greater than 2200–2400 °C in the impact plume of the Australasian tektite and the Ivory coast tektite. Heating time is of the order of seconds-tens of seconds rather than minutes as previously suggested (20 s for MN-AAT and 5 s for ICT). The number, the density, and the fact that inclusions are entirely crystallized in MN-AAT support relatively slow cooling rates (<200 °C/h), in comparison with the faster cooling rates (>2000 °C/h) indicated by the precipitation of amorphous P-rich glass in ICT. In both impact events, ejecta that had been heated to high temperatures did not remain in the vapor plume for an extended period of time and landed rapidly (within tens of seconds) at a relatively high temperature (>1000 °C) on the Earth’s surface.Phosphate inclusions systematically embedded in lechatelierite in ICT provide clues about the source material. It suggests that the parent material for these silica-rich inclusions is not conventional detrital quartz. Rather, parts of lechatelierite domains may be inherited from a biogenic source that could be consistent with tropical soil (source of the phosphor) and its biomass (silica of plant origin). The reduction process that tektites record during their formation may be explained by superficial material since forests can contain a sizable mass of carbon that can reduce iron in tektites and produce platinoid-rich metallic nuclei and the Fe3+/ΣFe gradient recorded by the dendritic spinels.

Potassium elemental and isotope constraints on the formation of tektites and element loss during impacts

Potassium elemental and isotope systematics were investigated for a suite of central European tektites from three strewn sub-fields in Czech Republic and possible parent sedimentary materials from the vicinity of the Ries impact structure in SE Germany, supplemented by data for several other impact-related materials (bediasites, Ivory Coast tektites, Libyan Desert Glass). This is paralleled by computation of potential K loss and attendant isotope fractionation for physico–chemical conditions typical for formation of tektite precursor melts. These theoretical calculations indicate a <0.1% loss of K from tektite precursor melts up to 2,500 K and <0.002‰ change in the 41K/39K ratio even for a small sphere of 0.002 m at 2,500 K, precluding any significant K loss and isotope fractionation. Numerical modelling also indicates that differential velocities between surrounding gas and liquid are not sufficient to remove the gaseous boundary layer, such that the partial pressure of potassium developed around the molten moldavite beads impedes further evaporation and also contributes to back-condensation of the already evaporated potassium.Central European tektites (moldavites) are enriched in K compared to the assumed sedimentary sources from the wider Ries area whereby the latter materials do not exceed 2.9 wt.% K2O compared to 2.5–4.1 wt.% K2O in moldavites. The apparent K enrichment in moldavites may be explained by a yet unaccounted process during formation of tektite precursor melts and/or unidentified source, such as volcanoclastic deposits that were produced by large Mid-Miocene volcanic centers in the Pannonian Basin. The K isotope compositions of tektites are more variable than those of sediments from the wider Ries area but they largely overlap (δ41K from −0.78 ± 0.03‰ to −0.13 ± 0.03‰ versus −0.72 ± 0.03‰ to −0.28 ± 0.02‰, respectively). These ranges mimic 41K/39K variations reported for igneous and sedimentary portions of the upper continental crust (δ41K roughly between −0.7 and −0.1‰). They show a slight difference among the three investigated strewn sub-fields, depending on their respective distance from the impact. In detail, moldavites from the closest strewn sub-field in the Cheb Basin show predominantly heavy K isotope compositions and those from the farthest strewn sub-field in Western Moravia are uniformly isotopically light. The origin of this difference may reflect lithological heterogeneity of the target area.Potassium contents in bediasites and Ivory Coast tektites range between 1.3 and 1.8 wt.% K2O and their corresponding δ41K values vary from −0.57 ± 0.02‰ to −0.41 ± 0.03‰. Both ranges are significantly narrower than those observed for moldavites. When compared to data for possible sedimentary precursors in the Chesapeake Bay and Bosumtwi impact structure, respectively, it is apparent that these tektites were neither depleted nor enriched in potassium. The extent of their K isotope fractionation relative to plausible sources remains unconstrained. The Libyan Desert Glass displays invariant δ41K of ~ −0.57 ± 0.06‰ at ≤0.01 wt.% K2O. Given the silica-rich nature of LDG and the lack of possible parent materials, no further constraints can be placed at present to further resolve the source material or reveal details of LDG formation process.

The quest for an extraterrestrial component in Muong Nong-type and splash-form Australasian tektites from Laos using highly siderophile elements and Re-Os isotope systematics

Abstract Extremely low and variable concentrations of osmium (Os) and other highly siderophile elements (HSE) in most tektites make it challenging to establish direct links between these impact-related materials and their possible extraterrestrial contribution. New Os concentrations (2–43 ppt) and 187Os/188Os ratios (0.131–0.68) in a suite of fifteen well-characterized Australasian tektites from Laos (Muong Nong and splash-form types) with variable Ni enrichment indicate a maximum of ∼0.005% addition of a chondritic impactor. This is similar to some Australasian tektites from Vietnam with similarly low siderophile contents, but significantly lower than found in previous studies of more Ni-rich Australasian splash-form tektites and microtektites from different parts of the Australasian strewn field (e.g., Indonesia, South China Sea). The contents of HSE and Re–Os isotopic compositions of layered Muong Nong-type Australasian tektites are highly variable, suggesting mingling of crustal-derived (siderophile element-poor) and extraterrestrial (siderophile element-rich) materials. The absence of a direct correlation between HSE and Ni contents is interpreted to result from a fractionation process related to their different vaporization/condensation temperatures. The low Os abundance in most of the analyzed Australasian tektites, combined with non-radiogenic 187Os/188Os far below average upper continental crust, may provide a direct test to distinguish continental versus seawater impact scenario. In the absence of any specific low-Os target, a particular process of Os loss following impact is required. We envisage a scenario where evaporative loss of >>90% Os in the form of Os oxides from the overheated tektite melt is aided by volatile species derived from dissociated seawater and/or saline pore water embedded in sediments off-shore Indochina, consistent with elevated contents of halogens in Australasian tektites. This water-assisted Os loss could also play significant role for Central European tektites, while the continental surface with limited amount of water would prevent from more efficient HSE loss as could be the case for Ivory Coast tektites.

Lithium in tektites and impact glasses: Implications for sources, histories and large impacts

Lithium (Li) abundances and isotope compositions were determined in a representative suite of tektites (moldavites, Muong Nong-type tektites and an australite, Ivory Coast tektites and bediasites), impact-related glasses (Libyan Desert Glass, zhamanshinites and irghizites), a glass fragment embedded in the suevite from the Ries impact crater and sedimentary materials in order to test a possible susceptibility of Li to fractionation during hypervelocity impact events and to de-convolve links to their potential parental sources. The overall data show a large spread in Li abundance (4.7–58 ppm Li) and δ 7Li values (−3.2‰ to 26.0‰) but individual groups of tektites and impact glasses have distinctive Li compositions. Most importantly, any significant high-temperature Li isotope fractionation can be excluded by comparing sedimentary lithologies from central Europe with moldavites. Instead, we suggest that Li isotope compositions in tektites and impact-related glasses are probably diagnostic of the precursor materials and their pre-impact geological histories. The Muong Nong-type tektites and australite specimen are identical in terms of Li concentrations and δ 7Li and we tentatively endorse their common origin in a single impact event. Evidence for low-temperature Rayleigh fractionation, which must have operated prior to impact-induced melting and solidification, is provided for a subset of Muong Nong-type tektites. Although Li isotope variations in most tektites are broadly similar to those of the upper continental crust, Libyan Desert Glass carries high δ 7Li ⩾24.7‰, which appears to mirror the previous fluvial history of parental material that was perhaps deposited in lacustrine environment or coastal seawater. Lithium isotopes in impact-related glasses from the Zhamanshin crater define a group distinct from all other samples and point to melting of chemically less evolved mafic lithologies, which is also consistent with their major and trace element patterns. Extreme shock pressures and the related extreme post-shock temperatures alone appear not to have any effect on the Li isotope systematics; therefore, useful information on parental lithologies and magmatic processes may be retrieved from analyses of Martian and lunar meteorites. Moreover, lack of significant Li depletion in tektites provides further constraints on the loss of moderately volatile elements during the Moon-forming impact.

Single crystal U–Pb zircon age and Sr–Nd isotopic composition of impactites from the Bosumtwi impact structure, Ghana: Comparison with country rocks and Ivory Coast tektites

The 1.07Myr old Bosumtwi impact structure (Ghana), excavated in 2.1–2.2Gyr old supracrustal rocks of the Birimian Supergroup, was drilled in 2004. Here, we present single crystal U–Pb zircon ages from a suevite and two meta-graywacke samples recovered from the central uplift (drill core LB-08A), which yield an upper Concordia intercept age of ca. 2145±82Ma, in very good agreement with previous geochronological data for the West African Craton rocks in Ghana. Whole rock Rb–Sr and Sm–Nd isotope data of six suevites (five from inside the crater and one from outside the northern crater rim), three meta-graywacke, and two phyllite samples from core LB-08A are also presented, providing further insights into the timing of the metamorphism and a possibly related isotopic redistribution of the Bosumtwi crater rocks. Our Rb–Sr and Sm–Nd data show also that the suevites are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite). A comparison of our new isotopic data with literature data for the Ivory Coast tektites allows to better constrain the parent material of the Ivory Coast tektites (i.e., distal impactites), which is thought to consist of a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of phyllite (and shale) than the suevites (i.e., proximal impactites). When plotted in a Rb/Sr isochron diagram, the sample data points (n=29, including literature data) scatter along a regression line, whose slope corresponds to an age of 1846±160Ma, with an initial Sr isotope ratio of 0.703±0.002. However, due to the extensive alteration of some of the investigated samples and the lithological diversity of the source material, this age, which is in close agreement with a possible “metamorphic age” of ∼1.8–1.9Ga tentatively derived from our U–Pb dating of zircons, is difficult to consider as a reliable metamorphic age. It may perhaps reflect a common ancient source whose Rb–Sr isotope systematics has not basically been reset on the whole rock scale during the Bosumtwi impact event, or even reflect another unknown geologic event.

Isotopic fractionation of zinc in tektites

Tektites are terrestrial natural glasses produced during a hypervelocity impact of an extraterrestrial projectile onto the Earth's surface. The similarity between the chemical and isotopic compositions of tektites and terrestrial upper continental crust implies that the tektites formed by fusion of such target rock. Tektites are among the driest rocks on Earth. Although volatilization at high temperature may have caused this extreme dryness, the exact mechanism of the water loss and the behavior of other volatile species during tektite formation are still debated. Volatilization can fractionate isotopes, therefore, comparing the isotope composition of volatile elements in tektites with that of their source rocks may help to understand the physical conditions during tektite formation. For this study, we have measured the Zn isotopic composition of 20 tektites from four different strewn fields. Almost all samples are enriched in heavy isotopes of Zn compared to the upper continental crust. On average, the different groups of tektites are isotopically distinct (listed from the isotopically lightest to the heaviest): Muong-Nong type indochinites ( δ 66/64Zn = 0.61 ± 0.30‰); North American bediasites ( δ 66/64Zn = 1.61 ± 0.49‰); Ivory Coast tektites ( δ 66/64Zn = 1.66 ± 0.18‰); the Australasian tektites (others than the Muong Nong-type indochinites) ( δ 66/64Zn = 1.84 ± 0.42‰); and Central European moldavites ( δ 66/64Zn = 2.04 ± 0.19‰). These results are contrasted with a narrow range of δ 66/64Zn = 0–0.7‰ for a diverse spectrum of upper continental crust materials. The elemental abundance of Zn is negatively correlated with δ 66/64Zn, which may reflect that isotopic fractionation occurred by evaporation during the heating event upon tektite formation. Simple Rayleigh distillation predicts isotopic fractionations much larger than what is actually observed, therefore, such a model cannot account for the observed Zn isotope fractionation in tektites. We have developed a more realistic model of evaporation of Zn from a molten sphere: during its hypervelocity trajectory, the molten surface of the tektite will be entrained by viscous coupling with air that will then induce a velocity field inside the molten sphere. This velocity field induces significant radial chemical mixing within the tektite that accelerates the evaporation process. Our model, albeit parameter dependent, shows that both the isotopic composition and the chemical abundances measured in tektites can be produced by evaporation in a diffusion-limited regime.

Platinum group elements provide no indication of a meteoritic component in ICDP cores from the Bosumtwi crater, Ghana

Abstract— In an attempt to identify the type of projectile, 14 samples from the Bosumtwi crater in Ghana were analyzed for platinum group element (PGE) concentrations by nickel sulfide fire assay inductively coupled plasma—mass spectrometry (ICP‐MS). The majority of the samples come from the impactite material recovered by cores LB‐07A and LB‐08A, which were drilled by the International Continental Scientific Drilling program (ICDP). One sample originates from the fallback material found at the contact between the impactite and the overlying lake sediment in core LB‐05B. No clear signature of a meteoritic contamination was identified in the 13 impactite samples. The target rock apparently dominates the PGE contribution in the impactites. These results agree with the PGE concentrations reported for the suevites collected at the crater rim and in other parts of the Bosumtwi ICDP cores. However, based on Cr and Os isotopic signatures, a meteoritic component could be present in the sample of fallback material, supporting the reports of the existence of meteoritic material in the Ivory Coast tektites. Further analyses of the fallback material from the Bosumtwi drill cores should confirm (or not) this first result.

Search for a meteoritic component in drill cores from the Bosumtwi impact structure, Ghana: Platinum group element contents and osmium isotopic characteristics

Abstract— An attempt was made to detect a meteoritic component in both crater‐fill (fallback) impact breccias and fallout suevites (outside the crater rim) at the Bosumtwi impact structure in Ghana. Thus far, the only clear indication for an extraterrestrial component related to this structure has been the discovery of a meteoritic signature in Ivory Coast tektites, which formed during the Bosumtwi impact event. Earlier work at Bosumtwi indicated unusually high levels of elements that are commonly used for the identification of meteoritic contamination (i.e., siderophile elements, including the platinum group elements [PGE]) in both target rocks and impact breccias from surface exposures around the crater structure, which does not allow unambiguous verification of an extraterrestrial signature. The present work, involving PGE abundance determinations and Os isotope measurements on drill core samples from inside and outside the crater rim, arrives at the same conclusion. Despite the potential of the Os isotope system to detect even small amounts of extraterrestrial contribution, the wide range in PGE concentrations and Os isotope composition observed in the target rocks makes the interpretation of unradiogenic, high‐concentration samples as an impact signature ambiguous.

Geochemistry of impactites and basement lithologies from ICDP borehole LB‐07A, Bosumtwi impact structure, Ghana

Abstract— In 2004, a drilling project by the International Continental Scientific Drilling Program (ICDP) at the Bosumtwi impact crater, Ghana (1.07 Myr old and 10.5 km in diameter), obtained drill core LB‐07A, which sampled impactites and underlying metasediments in the crater moat surrounding the small central uplift of the structure. The LB‐07A core consists of three sequences: 82.29 m of an upper impactite sequence of alternating polymict lithic and suevitic impact breccias overlying 54.88 m of so‐called lower impactite of monomict impact breccia with several suevite intercalations, and 74.53 m of meta‐graywacke and altered shale of the basement, also containing a number of suevite intercalations. Major‐ and trace‐element characteristics of all three sequences have been determined to investigate breccia formation and the role of the respective basement lithologies therein. Compositions of polymict impact breccias of the crater fill revealed by core LB‐07A are compared with the compositions of the Ivory Coast tektites and the fallout suevites. The impactites of the LB‐07A borehole appear well homogenized with respect to the silicate component, and little change in the ranges of many major‐ and trace‐element differences is seen along the length of the borehole (except for Fe2O3, MgO, and CaO contents). Much scatter is observed for a number of elements, and in many cases this increases with depth. It is proposed that any variability in composition is likely the function of clast population differences (i.e., also of relatively small sample sizes). No systematic compositional difference between polymict lithic and suevitic impact breccias is evident. An indication of carbonate enrichment due to hydrothermal alteration is observed in samples from all lithologies. The impactites of the borehole generally show intermediate compositions to previously defined target rocks. The fallout suevites have comparable major element abundances, except for relatively lower MgO contents. The Ivory Coast tektites are generally similar in composition to the LB‐07A suevites, but broader ranges in MgO and CaO contents are observed for the LB‐07A suevites.

Uppermost impact fallback layer in the Bosumtwi crater (Ghana): Mineralogy, geochemistry, and comparison with Ivory Coast tektites

Abstract— In 2004, an International Continental Scientific Drilling Program (ICDP) drilling project at the Bosumtwi impact crater, Ghana (10.5 km in diameter, 1.07 Myr old), was performed to study the sediments that fill the lake as well as the underlying impactites. In one (LB‐05) of 16 cores drilled into the lake sediments, the zone between the impact breccias and the post‐impact sediments was penetrated, preserving the final, fine‐grained impact fallback layer. This ∼30 cm thick layer contains in the top 10 cm “accretionary” lapilli, microtektite‐like glass spherules, and shocked quartz grains. Glass particles—mostly of splash form less than 1 mm size—make up the bulk of the grains (∼70–78% by number) in the coarser size fraction (&gt;125 μm) of the top of the fallback layer. About one‐third of all quartz grains in the uppermost part of the layer are shocked, with planar deformation features (PDFs); almost half of these grains are highly shocked, with 3 or more sets of PDFs. K‐feldspar grains also occur and some show shock deformation. The abundance of shocked quartz grains and the average shock level as indicated by the number of sets of PDFs, for both quartz and K‐feldspar, decrease with depth into the layer. The well‐preserved glass spherules and fragments are chemically rather homogeneous within each particle, and also show relatively small variations between the various particles. On average, the composition of the fallback spherules from core LB‐5B is very similar to the composition of Ivory Coast tektites and microtektites, with the exception of CaO contents, which are about 1.5 to 2 times higher in the fallback spherules. This is a rare case in which the uppermost fallback layer and the transition to the post‐impact sediments has been preserved in an impact structure; its presence indicates that the impactite sequence at Bosumtwi is complete and that Bosumtwi is a very well‐preserved impact crater.

An international and multidisciplinary drilling project into a young complex impact structure: The 2004 ICDP Bosumtwi Crater Drilling Project—An overview

Abstract— The Bosumtwi impact crater in Ghana, arguably the best‐preserved complex young impact structure known on Earth, displays a pronounced rim and is almost completely filled by Lake Bosumtwi, a hydrologically closed basin. It is the source crater of the Ivory Coast tektites. The structure was excavated in 2.1–2.2 Gyr old metasediments and metavolcanics of the Birimian Supergroup. A drilling project was conceived that would combine two major scientific interests in this crater: 1) to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago, at a near‐equatorial location in Africa for which very few data are available so far, and 2) to obtain a complete record of impactites at the central uplift and in the crater moat, for ground truthing and comparison with other structures.Within the framework of an international and multidisciplinary drilling project led by the International Continental Scientific Drilling Program (ICDP), 16 drill cores were obtained from June to October 2004 at six locations within Lake Bosumtwi, which is 8.5 km in diameter. The 14 sediment cores are currently being investigated for paleoenvironmental indicators. The two impactite cores LB‐07A and LB‐08A were drilled into the deepest section of the annular moat (540 m) and the flank of the central uplift (450 m), respectively. They are the main subject of this special issue of Meteoritics &amp; Planetary Science, which represents the first detailed presentations of results from the deep drilling into the Bosumtwi impactite sequence. Drilling progressed in both cases through the impact breccia layer into fractured bedrock. LB‐07A comprises lithic (in the uppermost part) and suevitic impact breccias with appreciable amounts of impact melt fragments. The lithic clast content is dominated by graywacke, besides various metapelites, quartzite, and a carbonate target component. Shock deformation in the form of quartz grains with planar microdeformations is abundant. First chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and Mg, but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values. Core LB‐08A comprises suevitic breccia in the uppermost part, followed with depth by a thick sequence of graywacke‐dominated metasediment with suevite and a few granitoid dike intercalations. It is assumed that the metasediment package represents bedrock intersected in the flank of the central uplift. Both 7A and 8A suevite intersections differ from suevites outside of the northern crater rim.Deep drilling results confirmed the gross structure of the crater as imaged by the pre‐drilling seismic surveys. Borehole geophysical studies conducted in the two boreholes confirmed the low seismic velocities for the post‐impact sediments (less than 1800 m/s) and the impactites (2600–3300 m/s). The impactites exhibit very high porosities (up to 30 vol%), which has important implications for mechanical rock stability. The statistical analysis of the velocities and densities reveals a seismically transparent impactite sequence (free of prominent internal reflections). Petrophysical core analyses provide no support for the presence of a homogeneous magnetic unit (= melt breccia) within the center of the structure. Borehole vector magnetic data point to a patchy distribution of highly magnetic rocks within the impactite sequence.The lack of a coherent melt sheet, or indeed of any significant amounts of melt rock in the crater fill, is in contrast to expectations from modeling and pre‐drilling geophysics, and presents an interesting problem for comparative studies and requires re‐evaluation of existing data from other terrestrial impact craters, as well as modeling parameters.

Beryllium-10 concentrations of tektites from the Ivory Coast and from Central Europe: Evidence for near-surface residence of precursor materials

By using accelerator mass spectrometry, we measured 10Be ( T 1/2 = 1.5 Ma) concentrations in nine Ivory Coast (IVC) tektites, in six soil samples collected near the Bosumtwi impact crater, the likely source region, and in a depth profile taken through a 23 g moldavite. In the core of the moldavite sample we also measured an upper limit on the 36Cl ( T 1/2 = 0.3 My) concentration. The average 10Be concentration in IVC tektites of (22 ± 11) × 10 6 atom/g exceeds reasonable limits for a meteoritic component or cosmic-ray production in situ after tektite formation. The 10Be must be meteoric, which implies that IVC tektites formed from soils or sediments. Corrected to the time of formation (ToF) 1.07 Ma ago and for a small in situ component, the average 10Be concentration of (35 ± 7) × 10 6 atom/g (1 − σ mean) is considerably lower than those of contemporary Bosumtwi soils, ∼250 × 10 6 atom/g, or of Australasian tektites at their ToF, 0.8 Ma B.P. near Lake Bosumtwi today the soil column is only ∼1 m thick. If the landscape was similar 1.07 Ma ago, then the total thickness of the tektite formation zone probably did not exceed 10 m. With increasing depth below the surface of the moldavite, the 10Be concentrations decrease rapidly owing to the presence of a surface component, probably of recent origin. The main interior mass of the sample contains ∼0.8 × 10 6 atom 10Be/g and fewer than 0.1 × 10 6 atom 36Cl/g, little of which can be meteoritic. Although not definitive, consideration of several possible cosmic-ray exposure histories suggests that about half the interior 10Be has a meteoric origin, which if corrected to the time of formation yields a concentration compatible with those measured in typical contemporary soils. The observations are consistent with the formation of three of the four main tektite groups from surface soils or sediments.

Bosumtwi impact structure, Ghana: Geochemistry of impactites and target rocks, and search for a meteoritic component

Abstract— Major and trace element data, including platinum group element abundances, of representative impactites and target rocks from the crater rim and environs of the Bosumtwi impact structure, Ghana, have been investigated for the possible presence of a meteoritic component in impact‐related rocks. A comparison of chemical data for Bosumtwi target rocks and impactites with those for Ivory Coast tektites and microtektites supports the interpretation that the Bosumtwi structure and Ivory Coast tektites formed during the same impact event. High siderophile element contents (compared to average upper crustal abundances) were determined for target rocks as well as for impactites. Chondrite‐normalized (and iron meteorite‐normalized) abundances for target rocks and impactites are similar. They do not, however, allow the unambiguous detection of the presence, or identification of the type, of a meteoritic component in the impactites. The indigenous siderophile element contents are high and possibly related to regional gold mineralization, although mineralized samples from the general region show somewhat different platinum‐group element abundance patterns compared to the rocks at Bosumtwi. The present data underline the necessity of extensive target rock analyses at Bosumtwi, and at impact structures in general, before making any conclusions regarding the presence of a meteoritic component in impactites.

Petrology and geochemistry of target rocks from the Bosumtwi impact structure, Ghana, and comparison with Ivory Coast tektites

The 10.5 km diameter Bosumtwi crater in Ghana, West Africa, is the most likely source crater for the Ivory Coast tektites, as the tektites and the crater have the same age (1.07 Ma), and there are close similarities between the isotopic and chemical compositions of the tektites and crater rocks. The crater is excavated in 2.1–2.2 Ga old metasediments and metavolcanics of the Birimian Supergroup. Here we present the first integrated petrographic and geochemical study of rocks from the Bosumtwi impact crater. A variety of target rocks from the Bosumtwi impact structure were selected to represent the major rock types that have been described before, resulting in four groups: shale, phyllite-graywacke, and two different types of granites (from dispersed dikes and from the so-called Pepiakese intrusion at the northeastern side of the crater). These rocks were analyzed for their major and trace element composition and their petrographic characteristics. In addition, representative samples were also analyzed for their O, Sr, and Nd isotopic compositions. The target rocks do not show any unambiguous evidence of shock metamorphism (i.e., planar deformation features, PDFs). Distinct impact-characteristic shock effects (PDFs) were identified only in clasts within suevite-derived melt fragments. The compositional range of the target rocks is significantly wider than that of the Ivory Coast tektites, but overlap the tektite compositions. A best-fit line for the Bosumtwi crater rocks in a Rb-Sr isotope evolution diagram yields an “age” of 1.98 Ga, and an initial 87Sr/ 86Sr ratio of 0.701, which is close to results previously obtained for granitoid intrusions in the Birimian of Ghana. Our Nd isotopic data yield depleted mantle model ages ranging from 2.16 to 2.64 Ga, and ϵ Nd values of −17.2 to −25.9‰. Harmonic least-squares (HMX) mixing calculations were able to reproduce the composition of Ivory Coast tektites from a mixture of Bosumtwi country rocks that include about 70% phyllite-graywacke, 16% granite dike, and 14% Pepiakese granite. The oxygen isotopic composition of the metasedimentary rocks and granite dikes (δ 18O = 11.3–13.6‰) and the tektites (δ 18O = 11.7–12.9‰, also this work) agree fairly well, while the Pepiakese granites have lower values (δ 18O = 8.6–9.0‰), indicating that only minor amounts of these rocks were incorporated in the formation of the Ivory Coast tektites. The large variation in Sr and Nd isotopic compositions of the target rocks do not allow the unambiguous identification of distinct endmember compositions, but in both a 87Sr/ 86Sr vs. 1/Sr plot and an ϵ Sr vs. ϵ Nd diagram, the tektites plot within the area occupied by the metasedimentary and granitic Bosumtwi crater rocks. Thus, our data support the interpretation that the composition of the Ivory Coast tektites is similar to that of rocks exposed at the Bosumtwi impact structure, indicating formation during the same impact event.

Detailed structural analysis of the rim of a large, complex impact crater: Bosumtwi Crater, Ghana

The 1 Ma Bosumtwi Crater in Ghana is an 11-km-diameter, presumably complex, well-preserved impact structure that is associated with the Ivory Coast tektite strewnfield. Detailed structural geologic studies along a complete traverse through the northwestern rim section indicated four zones characterized by distinct deformation styles from just outside of the crater rim to near the crater floor. Zone 1 is dominated by thick deposits of lithic impact breccia, intercalated in places with products of local mass wasting. Zone 2 contains inward-dipping thrust planes, conjugate radial fractures, isoclinal folding, and overturned stratigraphic sequences. Zone 3 represents a megabreccia zone, in which block size decreases upward and outward toward the rim crest. The innermost zone 4 is dominated by intense thrust faulting of multiple orientations, resulting in complex duplex- and lens-shaped bodies. These deformation styles generally correspond to those previously reported from the rims of simple bowl-shaped meteorite-impact craters and appear to be characteristic of impact structures in general.

Geochemistry and age of Ivory Coast tektites and microtektites

Ivory Coast tektites were first reported in 1934 from a geographically restricted area at Ivory Coast, West Africa. Although some additional specimens have been found later, the total number remains small (a few hundred). The Bosumtwi impact crater in Ghana is most likely the source crater for the Ivory Coast tektites, based on the finding that the tektites and the crater have the same age as well as similar isotopic and chemical compositions. In addition to tektites on land, microtektites were found in (so far) eleven deep-sea cores off the West African coast, between about 9°N and 11°S and 0° and 23°W, defining the extent of the Ivory Coast tektite strewn field. In this study we analyzed eleven Ivory Coast tektites for their major and trace element composition, studied their petrographical characteristics, provided major element data for 111 microtektites, and major and trace element data for four microtektites. We determined the 40Ar 39Ar step-heating age of five Ivory Coast tektites and four microtektites and obtained fission-track dates for ten tektites and one Bosumtwi impact glass. The tektites have very small intersample and intrasample variations of their major and trace element composition. 111 Ivory Coast microtektites from eleven cores were analyzed for their major element compositional range. Their compositional range is significantly wider than that of the Ivory Coast tektites, but the majority of all microtektites have compositions very similar to those of the tektites (within a factor of 1.2). Trace element compositions of the tektites also show little variation between samples. The samples do not show any distinct Eu anomaly in the REE patterns. This characteristic, as well as the high absolute REE abundances and La N Yb N ratios of about 8, indicate that Archean rocks are plausible source rocks. The major and trace element contents of four individually analyzed Ivory Coast microtektites show compositions that are very similar to those of the Ivory Coast tektites. However, the microtektites contain >20 rel% higher abundances of some of the lithophile and siderophile trace elements, such as Sc, Cr, Co, Ni, Sr, Zr, Ba, Hf, Ta, Th, and the REEs. These differences are probably due to incorporation of a higher abundance of accessory trace minerals with the microtektite-forming melt. The Ivory Coast microtektites also have a uniform internal composition. Duplicate 40Ar 39Ar step-heating age analyses were performed on five tektites. The best age estimate for the formation age of the tektites was calculated by taking a weighted average of the ages from the plateau portions of the runs, resulting in an age of 1.1 ± 0.05 Ma. We also tried to date four microtektites by 40Ar 39Ar age analyses, but their young age and small sample size makes it impossible to assign a reliable age to the microtektites. One run yielded satisfactory results that were similar to the tektite age. In addition, we determined the fission-track ages for ten individual Ivory Coast tektite samples and for one impact glass sample from the Bosumtwi crater. The track-size corrected ages for the Ivory Coast tektites ranged from 0.91 to 1.18 Ma, resulting in an average fission-track age of 1.05 ± 0.11 Ma. This age is, within errors, identical to that of the Bosumtwi impact glass at 1.03 ± 0.11 Ma, and to the 40Ar 39Ar age of 1.1 ± 0.05 Ma. The preferred age of the Ivory Coast tektite event is 1.07 Ma.

Water in tektites and impact glasses by fourier‐transformed infrared spectrometry

Abstract— To improve the scarce data base of H2O content in tektites and impact glasses, we analyzed 26 tektites from all four strewn fields and 25 impact glass samples for their H2O content. We used the fourier‐transformed infrared (FTIR) spectrometry method, which permits measurement of areas of ∼40 μm in diameter. Our results show that the tektites have H2O contents ranging from 0.002 to 0.030 wt% (average 0.014 ± 0.008 wt%). Ivory Coast tektites have the lowest H2O abundances (0.002–0.003 wt%), and Muong Nong‐type indochinites and some North American tektites having the highest contents (up to ∼0.03 wt%). Impact glass samples (from the Zhamanshin, Aouelloul, and Rio Cuarto craters) yielded H2O contents of 0.008 to 0.13 wt% H2O. Typical impact glasses from the Aouelloul and Zhamanshin craters have low H2O contents (0.008 to 0.063 wt%). Libyan Desert Glasses and Rio Cuarto glasses have higher H2O contents (∼0.11 wt%). We also analyzed glasses of unknown origin (e.g., urengoites; glass fragments from Tikal), which showed very low H2O contents, in agreement with an origin by impact. Our data confirm that all tektites found on land have very low H2O contents (&lt;0.03 wt% H2O), while impact glasses have slightly higher H2O contents. Both glass types are very dry compared to volcanic glasses. This study confirms that the low H2O contents (&lt;0.05 wt%) of such glasses can be considered good evidence for an origin by impact.

Krypton and xenon fractionation in North American tektites

Abstract— Elemental and isotopic compositions of the noble gases have been determined in six North American tektites (4 bediasites and 2 georgiaites) and one Ivory Coast tektite. Radiogenically produced 4He may explain the large 4He/36Ar ratios measured relative to air, despite significant diffusive losses. The Ne isotopic composition is enriched in 20Ne consistent with a single stage mass fractionation process. The enormous 20Ne/36Ar enrichments observed in all tektite samples, similar to those reported from other tektites and impact glasses, are attributed to atmospheric diffusion into the samples following solidification. The North American tektites show a systematic increase in 84Kr/36Ar and 132Xe/36Ar relative to air, with enrichments greater than those determined for any other tektite group or terrestrial samples other than shales. These enrichments are inconsistent with existing models of dissolving Kr and Xe in tektite glass without elemental fractionation at atmospheric pressures equivalent to ∼40 km altitude. The Kr and Xe isotopic compositions are indistinguishable from atmospheric within experimental uncertainty.

40Ar/39Ar age constraints for the Jaramillo Normal Subchron and the Matuyama‐Brunhes geomagnetic boundary

Our mid‐Pleistocene 40Ar/39 Ar age recalibration of the geomagnetic polarity timescale is nearly in accord with the oxygen isotope, climate record calibration of the astronomical timescale proposed by Johnson (1982) and Shackleton et al. (1990). 40Ar/39Ar ages of a normally magnetized rhyolite dome in the Valles caldera, northern Mexico, yielded a weighted‐mean age of 1.004±0.019 Ma. A K‐Ar age of 0.909±0.019 Ma for this rock by Doell and Dalrymple (1966) was the linchpin for the recognition and calibration of the Jaramillo Normal Subchron (JNS). Other 40Ar/39Ar ages from the Valles caldera and 40Ar/39Ar ages of Ivory Coast tektites indicate that the JNS began at about 1.11 Ma and ended before 0.92 Ma, probably near 0.97 Ma. The Matuyama‐Brunhes boundary occurred between 0.79 Ma and 0.76 Ma on the basis of 40Ar/39Ar sanidine ages from (1) three reversely magnetized rhyolite domes of the Valles caldera (0.793±0.018 Ma, 0.794±0.007 Ma, and 0.812±0.023 Ma) and pumice (0.789±0.006 Ma) from the reversely magnetized Oldest Toba Tuff of Sumatra and (2) pumice (0.764±0.005 Ma and 0.757±0.009 Ma) from the lower and upper units of the normally magnetized Bishop Tuff. The age of the boundary may be close to 0.77 Ma as deduced from rates of sedimentation in ancient Lake Bonneville, Utah.

Detection of a Meteoritic Component in Ivory Coast Tektites with Rhenium-Osmium Isotopes

Measurement of rhenium (Re) and osmium (Os) concentrations and Os isotopic compositions in Ivory Coast tektites (natural glasses with upper crustal compositions that are ejected great distances during meteorite impact) and rocks from the inferred source crater, Lake Bosumtwi, Ghana, show that these tektites incorporate about 0.6 percent of a meteoritic component. Analysis of elemental abundances of noble metals alone gives equivocal results in the detection of meteoritic components because the target rocks already have relatively large amounts of noble metals. The Re-Os system is ideally suited for the study of meteorite impacts on old continental crust for three reasons. (i) The isotopic compositions of the target rocks and the meteoritic impactor are significantly different. (ii) Closed-system mixing of target rocks and meteorites is linear on Re-Os isochron diagrams, which thus permits identification of the loss of Re or Os. (iii) Osmium isotopic compositions are not likely to be altered during meteorite impact even if Re and Os are lost.