Dieter Storzer

Geochemistry and age of Ivory Coast tektites and microtektites

Abstract 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 NYbN 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.

New fission track ages of tektites and related glasses

Abstract Fission track dating is applied to a large number of australites, Muong Nong type tektites, Darwin glasses and Libyan Desert glasses. It is found that these glasses often have lowered fission track ages due to annealing effects. The same specimens have also smaller fission track etch pits. According to the diminishing percentage of etched fossil fission track diameters, lowered fission track ages are corrected. A mean age of 0.7 m.y. is found for eleven australites, six Darwin glasses and seven Muong Nong type tektites (from Laos and Thailand), indicating a related genesis of these glasses. Uncorrected fission track ages of five Libyan Desert glasses (26.6 −1.3 +6.3 m.y.) and two bediasites (33.2 1.2 +8.3 m.y.) are found to be different. With regard to these results an interrelationship between these two groups of glasses seems to be improbable.

FISSION-TRACK DATING OF VOLCANIC GLASSES AND THE THERMAL HISTORY OF ROCKS.

Abstract The fission track method was applied to get information about a samples thermal history, using Permian pitch-stone glasses from South Tyrol as an example. The fission track ages obtained are lowered due to thermal annealing. They range between 61 m.y. and 186 m.y. Going from the South to the North, the ages decrease. Size studies of the fossil fission tracks reveal that the measured ages are mixed ages, composed of a thermally unaffected and a thermally affected age. The latter part of the mixed ages can be corrected. The corrected fission track ages converge to a mean age of 256 m.y. The thermally unaffected ages date the end of the last thermal event. They range between 21 m.y. and 149 m.y. The measured and the corrected fission track ages as well as the thermal history of the pitchstones are discussed. The results are a typical example how to use fission tracks as geological thermometer.

The Quaternary impact record from the Pampas, Argentina

Abstract Loess-like deposits cover much of central Argentina and preserve a rich record of impacts since the late Miocene. The present contribution focuses on two localities containing Quaternary impact glasses: along the coastal sequences near Centinela del Mar (CdM) and from near Rio Cuarto (RC). These highly vesicular glasses contain clear evidence for an impact origin including temperatures sufficient to melt most mineral constituents (1700°C) and to leave unique quench products such as β-cristobolite. The CdM glasses occur within a relatively narrow horizon just below a marine transgression expressed by a series of coastal paleo-dunes and systematic changes in the underlying sediments. High-resolution 40Ar/39Ar dating methods yielded an age of 445±21 ka (2σ). Glasses were also recovered from scattered occurrences lower in the section but were dated to 230±40 ka. This inconsistency between stratigraphic and radiometric age is most likely related to a nearby outcrop of glass that had been exposed and locally re-deposited in coastal lagoons during the last marine transgression at 125 ka. Sediments containing the original impact glass layer are now missing due to an unconformity, perhaps related to subsequent marine transgressions after the impact (410 ka and 340 ka) and hiatuses in deposition. Two different types of impact glasses from RC yield two distinct dates. High-resolution 40Ar/39Ar dating of fresher-appearing glasses (well-preserved tachylitic sheen) indicates an age of 6±2 ka (2σ). Independent fission track analyses yielded a similar age of 2.3±1.6 ka (2σ). More weathered glasses, however, gave significantly older ages of 114±26 ka (2σ). Consequently, materials from two separate Quaternary impacts have been recovered at Rio Cuarto. The younger glasses are consistent with previously reported carbon dates for materials on the floor of one of the large elongate structures. The depths of excavation for the RC and CdM impacts are very different. While the RC glasses are largely derived from near-surface materials, the CdM glasses from the upper level contain added components consistent with Miocene marine evaporites at a depth of about 400–500 m (e.g., high CaO and P2O5). The CdM glasses also incorporated older loess-like sediments from depth based on the geochemistry. Several ratios of key trace and rare earth elements of sediments of different ages from the Miocene to the Holocene indicate a systematic compositional change through time. Such changes calibrate the observed differences in glass composition from their host sediments and further indicate incorporation of materials from depth. Consequently, the Argentine loess-like sediments preserve evidence for at least four separate Quaternary impacts. Based on foreign components in the glasses, the CdM impact very likely produced a crater (now buried or eroded) once as large as 6 km in diameter. The younger RC glasses, however, are consistent with shallower excavation consistent with an oblique impact.

North American microtektites from the Caribbean Sea and their fission track age

Over 6000 microscopic glass spherules between 125 μm and 1 mm in diameter were found in a sediment core (RC9-58) from the Caribbean Sea. These glassy objects are mostly confined to a zone ∼ 40 cm thick at a depth of ∼ 250 cm. We believe that the microscopic glass objects are microtektites belonging to the North American strewnfield, based on their geographical location, appearance, physical properties, stratigraphic age (middle Upper Eocene), fission track age (∼34.6 my) and major element compositions. The occurrence of North American microtektites in the Caribbean Sea indicates that the North American strewnfield is two to three times larger than previously indicated. An estimate on the abundance of microtektites in core RC9-58 indicates that the North American strewnfield may contain greater than 1017 g of tektite material.

Fission track ages of North American tektites

Abstract Fission track ages of North American tektites from Texas, Georgia and Marthas vineyard range widely between 0.8 my and 34.2 my with the tektites from Georgia giving the lowest ages. Size studies of the fission track etch pits reveal the thermal history of each individual tektite resulting in the partial loss of tracks. Therefore, the measured fission track ages can be understood as thermally lowered. Based on annealing experiments, correction factors for the thermally lowered fission track ages are found. The weighted mean of the corrected fission track ages is 34.9 my for the bediasites and the Marthas Vineyard tektite. On the other hand, the corrected fission track ages of the Georgia tektites are much lower resulting from a more complex thermal history. It may now be stated that North American tektites and Libyan Desert glasses cannot have had a simultaneous origin.

Fission Track Ages and Ages of Deposition of Deep-Sea Microtektites

The Australasian and Ivory Coast deep-sea microtektites have fission track ages of 0.71 and 1.09 million years, respectively. These ages are in good agreement with the ages of deposition of the microtektites determined from paleomagnetic data. Both the fission track ages and ages of deposition of the microtektites agree with the potassium/ argon and fission track ages of tektites from the respective tektite strewn fields.

Geometry factor in fission track counting

Abstract Fission track density determinations of external and internal counting surfaces (“2π”and4π geometry) have been made for different glasses, apatite, zircon, sphene, and muscovite. The ratios between the two geometries range from 0.57 and 0.89 depending on the material and the etching time. Fission track ages using a factor of 0.5 between the two geometries must be corrected.

The records of solar wind and solar flares in aubrites

A large number of individual enstatite crystals of the gas-rich aubrites Khor Temiki, Staroe Pesyanoe and Bustee was analyzed for implanted helium and for steep gradient ion tracks in order to investigate the relation between solar flare irradiation and solar wind implantation with extreme local resolution. Irradiated and non-irradiated crystals coexist within the gas-rich phases of the aubrites investigated. Statistically in a given meteorite the proportion of crystals with implanted solar wind is similar to the proportion of solar flare irradiated crystals. It varies from aubrite to aubrite in the sequence of their bulk contents of trapped rare gases. For nine enstatites, tracks and rare gases were subsequently measured within the same crystal. The results support the intimate association of solar flare tracks and implanted He. The4He-surface concentrations of irradiated crystals vary between <5 × 10−7 and 10−4 cm3 STP/cm2. The absence of saturation effects together with the low degree of elemental gas fractionation indicates very short solar wind exposure times (< 100 yr) rather than strong diffusion losses. The evidence from tracks and rare gases can be understood in terms of an early simultaneous irradiation of aubritic crystals by solar wind and solar flare particles on top of a regolith-covered parent body.

Solar flare activity: Evidence for large-scale changes in the past

Abstract An analysis of radar and photographic meteor data and of spacecraft meteoroid penetration data indicates that there probably has not been a large increase in meteoroid impact rates in the last 10 4 yr. The solar flare tracks observed in the glass linings of meteoroid impact pits on lunar rock 15205 are therefore reanalyzed assuming a meteoroid flux that is constant in time. Based on this assumption, the data suggest that the production rate of Fe-group solar flare tracks may have varied by as much as a factor of 50 on a time scale of about 10 4 yr. No independently obtained data are known to require conflict with this interpretation. Confidence in this conclusion is somewhat qualified by the experimental and analytical uncertainties involved, but the conclusion nevertheless remains the present “best” explanation for the observed data trends.