Tektite origin by hypervelocity asteroidal or cometary impact: The quest for the source craters

Tektites are natural glasses that are chemically homogeneous, often spherically symmetrical objects several centimeters in size, and occur in four known strewn fields on the surface of the Earth: the North American, moldavite (or Central European), Ivory Coast, and Australasian strewn fields. Tektites found within such strewn fields are related to each other with respect to their petrological, physical, and chemical properties as well as their age. A theory of tektite origin needs to explain the similarity of tektites in respect to age and certain aspects of isotopic and chemical composition within one strewn field, as well as the variety of tektite materials present in each strewn field. In addition to tektites on land, microtektites (which are generally less than 1 mm in diameter) have been found in deep-sea cores. Tektites are classified into three groups: (1) normal or splash-form tektites, (2) aerodynamically shaped tektites, and (3) Muong Nong-type tektites (sometimes also called layered tektites). The aerodynamic ablation results from partial remelting of glass during atmospheric passage after it was ejected outside the terrestrial atmosphere and quenched from a hot liquid. Aerodynamically shaped tektites are known mainly from the Australasian strewn field where they occur as flanged-button australites. The shapes of splash-form tektites (spheres, droplets, teardrops, dumbbells, etc., or fragments thereof) are the result of the solidification of rotating liquids in the air or vacuum. Mainly due to chemical studies, it is now commonly accepted that tektites are the product of melting and quenching of terrestrial rocks during hypervelocity impact on the Earth. The chemistry of tektites is in many respects identical to the composition of upper crustal material

Sampling Mars: Analytical requirements and work to do in advance

Sending a mission to Mars to collect samples and return them to the Earth for analysis is without doubt one of the most exciting and important tasks for planetary science in the near future. Many scientifically important questions are associated with the knowledge of the composition and structure of Martian samples. Amongst the most exciting questions is the clarification of the SNC problem- to prove or disprove a possible Martian origin of these meteorites. Since SNC meteorites have been used to infer the chemistry of the planet Mars, and its evolution (including the accretion history), it would be important to know if the whole story is true. But before addressing possible scientific results, we have to deal with the analytical requirements, and with possible pre-return work. It is unlikely to expect that a possible Mars sample return mission will bring back anything close to the amount returned by the Apollo missions. It will be more like the amount returned by the Luna missions, or at least in that order of magnitude. This requires very careful sample selection, and very precise analytical techniques. These techniques should be able to use minimal sample sizes and on the other hand optimize the scientific output. The possibility to work with extremely small samples should not obstruct another problem: possible sampling errors. As we know from terrestrial geochemical studies, sampling procedures are quite complicated and elaborate to ensure avoiding sampling errors. The significance of analyzing a milligram or submilligram sized sample and putting that in relationship with the genesis of whole planetary crusts has to be viewed with care. This leaves a dilemma on one hand, to minimize the sample size as far as possible in order to have the possibility of returning as many different samples as possible, and on the other hand to take a sample large enough to be representative. Whole rock samples are very useful, but should not exceed the 20 to 50 g range, except in cases of extreme inhomogeneity, because for larger samples the information tends to become redundant. Soil samples should be in the 2 to 10 g range, permitting the splitting of the returned samples for studies in different laboratories with variety of techniques

Volcanic ash layers in blue ice fields (Beardmore Glacier Area, Antarctica): Iridium enrichments

Dust bands on blue ice fields in Antarctica have been studied and have been identified to originate from two main sources: bedrock debris scraped up from the ground by the glacial movement (these bands are found predominantly at fractures and shear zones in the ice near moraines), and volcanic debris deposited on and incorporated in the ice by large-scale eruptions of Antarctic (or sub-Antractic) volcanoes. Ice core studies have revealed that most of the dust layers in the ice cores are volcanic (tephra) deposits which may be related to some specific volcanic eruptions. These eruptions have to be related to some specific volcanic eruptions. These eruptions have to be relatively recent (a few thousand years old) since ice cores usually incorporate younger ice. In contrast, dust bands on bare blue ice fields are much older, up to a few hundred thousand years, which may be inferred from the rather high terrestrial age of meteorites found on the ice and from dating the ice using the uranium series method. Also for the volcanic ash layers found on blue ice fields correlations between some specific volcanoes (late Cenozoic) and the volcanic debris have been inferred, mainly using chemical arguments. During a recent field expedition samples of several dust bands found on blue ice fields at the Lewis Cliff Ice Tongue were taken. These dust band samples were divided for age determination using the uranium series method, and chemical investigations to determine the source and origin of the dust bands. The investigations have shown that most of the dust bands found at the Ice Tongue are of volcanic origin and, for chemical and petrological reasons, may be correlated with Cenozoic volcanoes in the Melbourne volcanic province, Northern Victoria Land, which is at least 1500 km away. Major and trace element data have been obtained and have been used for identification and correlation purposes. Recently, some additional trace elements were determined in some of the dust band samples, including Ir. Iridium determinations were made using INAA, with synthetical and natural (meteorite) standards. These findings are discussed

The Cretaceous-Tertiary (K/T) impact: One or more source craters?

The Cretaceous-Tertiary (K/T) boundary is marked by signs of a worldwide catastrophe, marking the demise of more than 50 percent of all living species. Ever since Alvarez et al. found an enrichment of IR and other siderophile elements in rocks marking the K/T boundary and interpreted it as the mark of a giant asteroid (or comet) impact, scientists have tried to understand the complexities of the K/T boundary event. The impact theory received a critical boost by the discovery of shocked minerals that have so far been found only in association with impact craters. One of the problems of the K/T impact theory was, and still is, the lack of an adequate large crater that is close to the maximum abundance of shocked grains in K/T boundary sections, which was found to occur in sections in Northern America. The recent discovery of impact glasses from a K/T section in Haiti has been crucial in establishing a connection with documented impact processes. The location of the impact-glass findings and the continental nature of detritus found in all K/T sections supports at least one impact site near the North American continent. The Manson Impact Structure is the largest recognized in the United States, 35 km in diameter, and has a radiometric age indistinguishable from that of the Cretaceous-Tertiary (K/T) boundary. Although the Manson structure may be too small, it may be considered at least one element of the events that led to the catastrophic loss of life and extinction of many species at that time. A second candidate for the K/T boundary crater is the Chicxulub structure, which was first suggested to be an impact crater more than a decade ago. Only recently, geophysical studies and petrological (as well as limited chemical) analyses have indicated that this buried structure may in fact be of impact origin. At present we can conclude that the Manson crater is the only confirmed crater of K/T age, but Chicxulub is becoming a strong contender; however, detailed geochemical, geochronological, and isotopic data are necessary to provide definitive evidence

The origin of tektites: A geochemical discussion

Tektites are a group of natural glasses occurring in four different strewn fields on earth. They are generally small, brownish to black, partly transparent, spherically symmetric, and sometimes aerodynamically ablated. Strewn fields are geographically restricted areas on earth where tektites are found, usually in association with microtektites. Microtektites are spherules of up to about 1mm in diameter and are retrieved from deep sea sediments. Recently microtektites have been found to co-occur with tektite fragments. Besides chemical, isotopical, and age arguments this gives final proof for a genetic relationship between tektites and microtektites. Chemically tektites are Si-rich glasses (SiO_2 between 65 and 85wt%), not unlike some well known terrestrial impact glasses. The major element chemistry allows the distinction of tektites of different strewn fields, and the use of some geochemical diagrams allows the further distinction between different sub-strewn fields. Within one strewn field, we can distinguish between splash-form tektites (normal tektites) and so called Muong Nong tektites, which differ from normal tektites in respect to a higher volatile content and greater inhomogeneity, besides being of generally larger size. The trace element chemistry reveals a close similarity of tektites to terrestrial rocks, especially to surface sediments. No similarity with lunar rocks or terrestrial mantle derived rocks can be observed. Trace element ratios and rare earth element patterns are especially useful in further delineating the type of precursor rocks for tektite production. The study of isotopes, like Rb/Sr, Sm/Nd, or the lead isotopes, adds further credibility to the connection between tektites and upper crustal sediments. For two of the four strewn fields a clear connection, based on chemical, isotopical, and age considerations, between tektites and impact craters, has been established. This is the Ries crater and moldavite and the Bosumtwi crater and Ivory Coast tektite connection. The consideration of various theories of tektite origin in view of the known facts leads to the conclusion that only the production of tektites during an impact on earth is consistent with the data. Analogous studies of impact glasses associated with terrestrial impact craters (like the Zhamanshin crater) give a picture which is in accordance with the impact model for tektites. Some of these glasses show an enrichment of the platinum group and other cosmic fingerprint elements compared to the terrestrial sedimentary background, these elements being the only ones from the impacting body which may survive the impact. Tektite analyses point in the same direction, and it may be possible to get further clues on the nature of the projectile by studying these elements

Trace element geochemistry of lunar meteorites Yamato-791197 and -82192

The lunar meteorites Yamato-791197 and -82192 have been analyzed for up to 36 minor and trace elements. Bulk compositional data are reported as well as data for grain size fractions and individual clasts separated from both meteorites. The data shows clearly that there are some significant chemical differences between Y-791197 and -82192,and also to ALHA81005. Y-82192 shows no sign of a KREEP component at all, and shows a slightly different REE (and incompatible element) pattern than the other two meteorites. Siderophile elements (Ir, Ni, Co, Au) and Se show an admixture of about 1.5% C1 component, but the patterns are quite different for all three lunar meteorites. Y-791197 shows an enrichment in some volatile elements (like Se and Au) already in the bulk composition. Both meteorites contain some volatile-rich clasts, but only Y-791197 has also a volatile rich bulk composition. This is in favor of an earlier suggestion of volcanic contaminations in the lunar source region for Y-791197. The trace element data seems to indicate that there have been at least three individual impacts responsible for the known lunar meteorites

Early Archean spherule beds of possible impact origin from Barberton, South Africa: A detailed mineralogical and geochemical study

The Barberton Greenstone belt is a 3.5- to 3.2-Ga-old formation situated in the Swaziland Supergroup near Barberton, northeast Transvaal, South Africa. The belt includes a lower, predominantly volcanic sequence, and an upper sedimentary sequence (e.g., the Fig Tree Group). Within this upper sedimentary sequence, Lowe and Byerly identified a series of different beds of spherules with diameters of around 0.5-2 mm. Lowe and Byerly and Lowe et al. have interpreted these spherules to be condensates of rock vapor produced by large meteorite impacts in the early Archean. We have collected a series of samples from drill cores from the Mt. Morgan and Princeton sections near Barberton, as well as samples taken from underground exposures in the Sheba and Agnes mines. These samples seem much better preserved than the surface samples described by Lowe and Byerly and Lowe et al. Over a scale of just under 30 cm, several well-defined spherule beds are visible, interspaced with shales and/or layers of banded iron formation. Some spherules have clearly been deposited on top of a sedimentary unit because the shale layer shows indentions from the overlying spherules. Although fresher than the surface samples (e.g., spherule bed S-2), there is abundant evidence for extensive alteration, presumably by hydrothermal processes. In some sections of the cores sulfide mineralization is common. For our mineralogical and petrographical studies we have prepared detailed thin sections of all core and underground samples (as well as some surface samples from the S-2 layer for comparison). For geochemical work, layers with thicknesses in the order of 1-5 mm were separated from selected core and underground samples. The chemical analyses are being performed using neutron activation analysis in order to obtain data for about 35 trace elements in each sample. Major elements are being determined by XRF and plasma spectrometry. To clarify the history of the sulfide mineralization, sulfur isotopic compositions are being determined

Gabbroic lunar mare meteorites Asuka-881757(Asuka-31) and Yamato-793169:Geochmical and mineralogical study

Asuka-31 (Asuka-881757) is a new type of gabbroic mare basalt which is similar to VLT mare basalts from Apollo 17 and Luna 24. The small lunar meteorite Yamato-793169 was described to have petrologic characteristics that are similar to those of A-881757; this similarity is confirmed here for trace element compositions. We studied bulk (powder) samples and mineral and rock fragments of A-881757. The mineralogy and mineral chemistry of the separates from A-881757,111 is that of a coarse-grained gabbroic rock of VLT composition; the texture of the mesostases suggests some metamorphic recrystallization with highly unequilibrated mineral compositions. As for mineral phases, we found plagioclase, pyroxene, fayalite, Fe-Ni metal, silica, apatite, and some trace minerals. In addition, we found ulvospinels associated with Na-rich plagioclase, fayalite, apatite, ilmenite, SiO_2,Fe-sulfide, and a rare metal (0.3wt% Ni, 2.0wt% Co) which has a composition unknown so far from any lunar rocks. The trace element contents found in the mineral separates and fragments correspond closely to the mineralogical findings. The chondrite normalized REE patterns for the bulk samples of A-881757 and Y-793169 are relatively flat, with some light REE depletion that is typical of mare basalts. The plagioclasedominated subsamples A31-CL, A31-CL2,and A31-YC show a distinct positive Eu anomaly. The bulk trace element composition of Y-793169 is almost identical to that of A-881757. Solar-wind dervved noble gas and exposure age studies of A-881757 and Y-793169 have indicated marked differences between the two meteorites, indicating that they might have been ejected in different events. These two new gabbroic, VLT-like, mare basalts are therefore valuable new additions to the lunar meteorite collection

Lunar meteorite Yamato-86032: Mineralogical, petrological, and geochemical studies

Yamato-86032 is a shock-lithified anorthositic fragmental breccia. It consists mainly of highly feldspathic meta-breccias and meta-meltrocks and possibly contains a small contribution from mare lithologies, but there is no indication of a KREEP component. In many respects Y-86032 is similar to the previously described lunar meteorites Y-82192/3,but there are some notable differences. We have analyzed about 40 major and trace elements in bulk matrix, impact melt, and clast samples from two chips of Y-86032. The abundances of most lithophile and incompatible elements are lower in Y-86032 than in Y-82192 (which contains very low abundances compared to normal lunar highland rocks). The REE abundances are comparable to those of Y-82192. The elements Sc, Cr, Mn, Fe and Co have significantly lower abundances than in Y-82192,and the siderophile element pattern is also different. Since cosmic ray exposure data indicate pairing of Y-86032 with Y-82192/3,the source region for these meteorites on the moon must have been fairly heterogeneous

Lunar meteorite Yamato-793274: Mixture of mare and highland components,and barringerite from the Moon

Two small samples of the new lunar meteorite Yamato-793274 have been studied for mineralogical, petrological, and geochemical composition. The meteorite has a coarse grained texture and consists of a dense breccia that contains relatively large and abundant mineral fragments (clinopyroxene, plagioclase, olivine, ilmenite), rare fine-grained granulitic (poikilitic) breccias, and some mostly brownish devitrified glass. The matrix is abundant, dense, and consists of mineral fragments and interstitial mostly recrystallized glass. One large recrystallized melt breccia of anorthositic-noritic-troctolitic (ANT) composition was found. All plagioclase fragments are highly anorthitic, and olivine compositions range from Fo_. The occurrence of these breccias and plagioclases, as well as the chemistry of some matrix glass, is consistent with an origin from the lunar highlands. However, some glasses have a considerably more mafic composition and show admixture of a low mg-component. Pyroxenes are unusually abundant when compared with other lunar meteorites. They usually show exsolution lamellae, are heavily shocked, and their compositions show a bimodal distribution, with low-mg pyroxenes most probably of mare origin. Among opaque phases, kamacite, and a Co-rich taenite were found, and, for the first time in lunar rocks, the rare higher phosphide barringerite, (Fe, Ni)_2 P. The bulk major and trace element composition is unlike the anorthositic lunar highland meteorites (e. g., ALHA81005,Y-791197,Y-86032,MAC88104/5), but somewhat similar to the newly identified mare meteorite EET87521. The mineral compositions as well as the major and trace element compositions of the bulk show a close similarity to certain VLT mare basalts, e. g., the Luna 24 ferrobasalts. This is obvious, for example, in a plot of molar Mg/(Mg+Fe) vs. TiO_2 content. The lithophile trace element abundances in Y-793274 are similar to EET87521 and Apollo 17 and Lunar 24 VLT\u27s. The REE patterns show higher abundances (about 20-10×Cl) than the anorthositic meteorites and a small negative Eu anomaly. They are similar to EET87521 and some Apollo 14 green volcanic glasses. From the mineralogical and chemical data, pairing with any other lunar meteorite is very unlikely. Y-793274 is a shock lithified fragmental breccia containing a minor regolith component and numerous mafic mineral fragments and glasses. It is a mixture of about two thirds mare material and one third highland component, and therefore different from all previously known lunar meteorites