首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 265 毫秒
1.
Abstract— We have analyzed fluorine and boron in nine tektites from all four strewn fields, and in a suite of impact glasses and target rocks from the Zhamanshin and Darwin impact craters, as well as Libyan Desert Glass and Aouelloul impact glass samples. Fluorine and boron are useful indicators for the volatilization and temperature history of tektites and impact glasses. Tektites from different strewn fields show a limited range of F and B contents and have F/B ratios near unity. Most splash-form tektites have lower average F and B contents than Muong Nong type tektites, which is similar to the relation between irghizites and zhamanshinites. The F and B contents in target rocks from the Zhamanshin and Darwin impact craters are similar to normal terrestrial sediments. Fluorine in impact glasses and tektites is more depleted compared to their (known or inferred) target rocks than is boron, which is caused by the higher volatility of F. The F/B ratios therefore decrease with increasing temperature of formation (suggesting that irghizites were formed at a higher temperature than zhamanshinites, and Muong Nong type tektites at a lower temperature than splash-form tektites). Mixing of local country rocks together with partial loss of the volatiles F and B can reproduce the F and B contents of impact glasses.  相似文献   

2.
Two irghizites, three zhamanshinites and one sample each of lechatelierite, vein-quartz, Palaeogene silty clay and Palaeogene quartzite were analyzed using neutron activation analysis. A silicate analysis of the Palaeogene silty clay has also been performed, as well as an incomplete analysis of the Palaeogene quartzite from the Zhamanshin impact crater. The REE abundances of irghizites resemble those of sedimentary rocks. On the Köhler and Raaz diagram all projection points of irghizites lie inside the field of tektites, and indicate that they were derived from terrestrial sedimentary rocks. The Zhamanshin impact glasses may be divided into three types: (a) silica-rich zhamanshinites (x?SiO2 = 73.89%), (b) zhamanshinites (x?SiO2 = 54.34%), and (c) silica-poor zhamanshinites (x?SiO2 = 39.64%). These are also characterized by varying proportions of alkalis and Al. Mn and Ca contents. Irghizites and silica-rich zhamanshinites display a depletion of Eu. Zhamanshinites do not show this Eu depletion. Partial melting is assumed to be an important process in the origin of zhamanshinites.  相似文献   

3.
Abstract— One hundred and thirteen Australasian tektites from Vietnam (Hanoi, Vinh, Dalat, and Saigon areas) were analyzed for their major and trace element contents. The tektites are either of splash form or Muong Nong‐type. The splash‐form tektites have SiO2 contents ranging from 69.7 to 76.8 wt%, whereas Muong Nong‐type tektites, which are considerably larger than splash‐form tektites and have a blocky and chunky appearance, have slightly higher silica contents in the range of 74–81 wt%. Major‐element relationships, such as FeO versus major oxides, Na2O versus K2O, and oxide ratio plots, were used to distinguish the different groups of the tektites. In addition, correlation coefficients have been calculated for each tektite group of this study. Many chemical similarities are noted between Hanoi and Vinh tektites from the north of Vietnam, except that the Hanoi tektites contain higher contents of CaO than Vinh; the higher content of CaO might be due to some carbonate parent material. Both Dalat and Saigon tektites have nearly similar composition, whereas the bulk chemistries of the tektites from Hanoi and Vinh appear different from those of Saigon and Dalat. There are differences, especially in the lower CaO and Na2O and higher MgO, FeO, for the tektites of Dalat and Saigon in comparison to that of Hanoi tektites. Furthermore, the Dalat and Saigon tektites show enrichments by factors of 3 and 2 for the Ni and Cr contents, respectively, compared to those of Hanoi and Vinh. The difference in chemistry between the North Vietnam tektites (Hanoi, Vinh) to that of South Vietnam tektites (Saigon, Dalat) of this study indicate that the parent material was heterogeneous and possibly mixing between different source rocks took place. Muong Nong‐type tektites are enriched in the volatile elements such as Br, Zn, As, and Sb compared to the average splash‐form tektites of this study. The chemical compositions of the average splash‐form and Muong Nong‐type tektites of this study closely resemble published data for average splash‐form and Muong Nong‐type indochinites, indicating that they have the same source. The trace element ratios Ba/Rb (2.7), Th/U (5.2), Th/Sc (1.3), Th/Sm (2.2), and the rare earth element (REE) abundances of this study show close similarities to those of average upper continental crust.  相似文献   

4.
Abstract— During Leg 150 of the Ocean Drilling Project (ODP), two sites (903C and 904A) were cored that have sediments of the same biostratigraphic age as the upper Eocene tektite-bearing ejecta layer at Deep Sea Drilling Project (DSDP) Site 612. Core 45X from ODP Site 904A (～4 km north of Site 612) contains a 5 cm thick tektite-bearing ejecta layer, and Core 56 from Site 903C (～8 km north-northwest of Site 904) contains a 2 cm thick layer of impact ejecta without any tektite or impact glass. Shocked quartz and feldspar grains, with multiple sets of planar deformation features (PDFs), and abundant coesite-bearing grains are present at both sites. The major oxide contents, trace element compositions, and rare earth element (REE) patterns of the Site 904 tektites are similar to those of the Site 612 tektites and to North American tektites (especially bediasites). The ?Sr and ?Nd values for one composite tektite sample from Site 904 fall within the range previously obtained for the Site 612 tektites, which defines a linear trend that, if extrapolated, would intersect the values obtained for North American tektites. The water contents of eight tektite fragments from Site 904 range from 0.017 to 0.098 wt%, and, thus, are somewhat higher than is typical for tektites. The heavy mineral assemblages of the 63–125 μm size fractions from the ejecta layers at Sites 612, 903, and 904 are all similar. Therefore, we conclude that the ejecta layer at all three sites is from the same impact event and that the tektites at Sites 904 and 612 belong to the North American tektite strewn field. Clinopyroxene-bearing (cpx) spherules occur below, or in the lower part of, the main ejecta layer at all three sites. At all three sites, the cpx spherules have been partly or completely replaced with pyrite that preserved the original crystalline textures. Site 612, 903, and 904 cpx spherules are similar to those found in the Caribbean Sea, Gulf of Mexico, central equatorial Pacific, western equatorial Pacific, and eastern Indian Ocean. The cpx event appears to have preceded the North American tektite event by 10–15 ka or less. The fining-upward sequence at all three sites and concentration of the denser, unmelted impact ejecta at the top of the tektite layer at Sites 612 and 904 suggest that the tektite-bearing ejecta layers are not the result of downslope redeposition and that the unmelted ejecta landed after the glass. Geographic variations in thickness of the tektite-bearing ejecta layer, the lack of carbonate clasts in the ejecta layer, and the low CaO content of the tektite glass suggest that the ejecta (including the tektite glass) were derived from the Chesapeake Bay structure rather than from the Toms Canyon structure. A sharp decline in microfossil abundances suggests that local environmental changes caused by the impact may have had adverse effects on benthic foraminifera, radiolaria, sponges, and fish as well as the planktic foraminifera.  相似文献   

5.
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 (<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 (<0.05 wt%) of such glasses can be considered good evidence for an origin by impact.  相似文献   

6.
Abstract— The composition of surface deposits on vesicle walls in irghizites (i.e., impact glasses at site) from the Zhamanshin meteorite crater were studied using time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS). The cavity walls are unique interfaces for condensation of gases from the superheated, high‐silica melt during the impact. Initially, signals from the cavity wall are dominated by hydrocarbon fragments whereas the glass fracture face surrounding the cavity gave only signals corresponding to glass components. After 12 h in ultra high vacuum (UHV), signals from the cavity wall are dominated by peaks corresponding to fragments normally measured from organosilanes and organosiloxanes with the majority of the hydrocarbon signals markedly reduced. Characteristic hydrocarbon fragments are now observed on the glass fracture surface next to the cavity in an annulus around the cavity perimeter. There are also minor signals in this region from organosilanes and organosiloxanes. In contrast, four tektites (Australites) (i.e., glassy distal ejecta) gave no organosilane or organosiloxane signals after the same preparation and vacuum evaporation procedure. These species appear to be formed only at the impact site where higher levels of organic material are likely to be present in soil and are trapped before evaporation. This appears to be the first report of naturally occurring silicon‐organic compounds.  相似文献   

7.
Abstract— Tektites are natural glasses formed from terrestrial material that was melted and displaced by the impact of an extraterrestrial body. The surface and near-surface compositions of tektite glass results from fractionation during impact and ejection, and/or postsolidification weathering. The first goal of this study was to characterise the surface and near-surface (in the order of tens of angstroms) chemical composition of two tektites by x-ray photoelectron spectroscopy (XPS), and to estimate the importance of weathering vs. fractionation during flying. In order to separate the chemical modification due to weathering from that due to fractionation during ballistic flight, we studied two samples from the Australasian tektite strewn field. One of them was collected in a hot desert area (Nullarbor Plain, Australia) and the other, in a humid climate (Thailand). Our study reveals the presence of well-developed leached layers in both tektites. In the Australian tektite, Si is depleted in the topmost layers (a few tens of angstroms). A more complex chemical zoning is defined in the tektite from Thailand. These leached layers are comparable to those observed in weathered glasses, and therefore we conclude that weathering is responsible for the chemical composition of the surface and near-surface compositions. The second goal was to investigate the chemical environment of O, N and C in the glass. The O peak was resolved into two bridging O components (Si-O-Si and Al-O-Si) that are comparable to O environments in artificial glasses. The binding energy of the C1s electron is typical for C-C and C-H bonds in hydrocarbons; minor organic acid components are also present. Nitrogen is only observed on the surface of the Thailand tektite. The binding energy of N1s is comparable to that of ammonia, and the surface enrichment in N is interpreted as due to sorption related to interactions between glass and fluid buffered by the organic material in the soil.  相似文献   

8.
Abstract— We have the elemental abundances and isotopic compositions of noble gases in Muong Nong‐type tektites from the Australasian strewn field by crushing and by total fusion of the samples. We found that the abundances of the heavy noble gases are significantly enriched in Muong Nong‐type tektites compared to those in normal splash‐form tektites from the same strewn field. Neon enrichments were also observed in the Muong Nong‐type tektites, but the Ne/Ar ratios were lower than those in splash‐form tektites because of the higher Ar contents in the former. The absolute concentrations of the heavy noble gases in Muong Nong‐type tektites are similar to those in impact glasses. The isotopic ratios of the noble gases in Muong Nong‐type tektites are mostly identical to those in air, except for the presence of radiogenic 40Ar. The obtained K‐Ar ages for Muong Nong‐type tektites were about 0.7 Myr, similar to ages of other Australasian tektites. The crushing experiments suggest that the noble gases in the Muong Nong‐type tektites reside mostly in vesicles, although Xe was largely affected by adsorbed atmosphere after crushing. We used the partial pressure of the heavy noble gases in vesicles to estimate the barometric pressure in the vesicles of the Muong Nong‐type tektites. Likely, Muong Nong‐type tektites solidified at the altitude (between the surface and a maximum height of 8–30 km) lower than that for splash‐form tektites.  相似文献   

9.
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.  相似文献   

10.
Abstract— To test different hypotheses of moldavite formation, a major and trace-element study of 25 moldavite tektites and Sm-Nd isotope measurement of three moldavite tektites was completed. The samples were selected from the classical substrewnfields and the newly described locations in Lusatia (Saxony, Germany). Samples with unusual bulk composition were also included. The results confirm earlier studies that the variation in the chemical composition can be explained by single impact and through incomplete mixing of at least three lithographical components dominated by one of the three minerals or mineral groups: dolomite, clay minerals and quartz. An additional endmember, possibly a rare Earth's mantle component, containing high Co, Cr and Ni concentrations is also needed to explain the observed variations in compatible elements of some tektites. Volatile element abundances are low but not necessarily the result of selective volatilization.  相似文献   

11.
Tasmanian Darwin glass has a fusion age sensibly identical with that of Australasian tektites and it is reasonable to assume all were produced in the same event. Recently a number of new Darwin glass localities and an associated crater have been discovered. The glass stewnfield covers at least 400 km2 and there is a strong positive correlation between glass fragment size and abundance and proximity to the crater. The glass was distributed from some point near the crater, with the smallest pieces traveling furthest. This structure is apparently an impact crater of rather unusual configuration and fortuitous location. Our gravity survey reveals a closed sedimentary basin about 1000 meters in diameter. A centrally located drill hole penetrated 60 meters of lacustrine clays and 40 meters of mixed clay, sand and rock fragments. The hole was terminated at 100 meters in loose sand containing sand-sized fragments of Darwin glass and lechetelierite. The 100 meters of cored sediments accounts for only about half of the observed 3.5 milligal negative anomaly and there must be a substantial additional thickness of low density material at depth. Further drilling is essential to confirm an impact origin and to delineate the subsurface crater configuration. This information would be of great calibration value for theoretical modeling studies of explosive cratering. The Darwin strewnfield characteristics support the theory that the distribution of Australasian tektites was aided by an impact-generated, atmospheric blast wave (or waves). The stratigraphic position of glass below 100 meters of lake sediments is strong evidence that the postulated stratigraphic age of the Australian land tektites is incorrect.  相似文献   

12.
The Fe oxidation state and coordination number of 29 impact glass spherules recently recovered from the Transantarctic Mountains (Antarctica) have been determined by X‐ray absorption near edge structure (XANES) spectroscopy. Based on geochemical, isotopic, and fission track data, these spherules are considered as microtektites from the Australasian tektite/microtektite strewn field. Their find location is the farthest so far discovered from the possible source crater region, and their alkali content is the lowest compared with other published data on Australasian microtektite glasses. The Fe3+/(Fe2++Fe3+) ratio, determined from the analysis of the pre‐edge peak energy position and integrated intensity, is below 0.1 (±0.04) for all the samples, and is comparable to that of most tektites and microtektites from the Australasian strewn field. Also, the pre‐edge peak integrated intensity, which is sensitive to the average Fe coordination geometry, is comparable to that of other Australasian microtektites reported in the literature. The agreement of the Fe oxidation state and coordination number, between the Transantarctic Mountain microtektites (TAM) and the Australasian tektites and microtektites, further confirms the impact origin of these glass spherules and provides an independent suggestion that they represent a major extension southeastward of the Australasian strewn field. The fact that similar redox conditions are observed in tektites and microtektites within the Australasian strewn field regardless of the distance from the source crater area (up to approximately 11000 km) could be an important constraint for better understanding the different processes affecting microtektite formation and transport. The fact that the Fe oxidation state of microtektites does not increase with distance, as in the case of North American microtektites, means that thermal and redox histories of Australasian and TAM microtektites could differ significantly from those of North American microtektites.  相似文献   

13.
Abstract— Heavy noble gas concentrations in tektites (splash-form type) are considerably lower than those in impact glasses. This can not be explained only by high formation temperatures for tektites, as might be expected from low concentrations of water and most volatile elements in tektites, and indicates that tektites solidified in an atmosphere with an ambient pressure of much less than 1 atm. The heavy noble gas concentrations may be an indicator of the height to which tektites were carried by the impact before they solidified.  相似文献   

14.
Abstract— Late Eocene tektite material from DSDP site 612 is composed of angular to spherical tektites and microtektites containing abundant vesicles and a few unmelted to partially melted mineral inclusions. The major element compositions of the 612-tektites are generally comparable to those of North American tektites, but the physical features suggest that the DSDP-612 tektites were formed by less severe shock melting. The 87Sr/86Sr and 143Nd/144Nd compositions of 612-tektites: a) show much wider ranges than the tightly constrained group of North American tektites and microtektites, and b) are significantly different from those of other groups of tektites. The existence of large isotopic variations in tektites from DSDP site 612 requires that they were formed from a chemically and isotopically heterogeneous material in a regime that is distinctive from that of other groups of tektites. TNDCHUR and TSrUR model ages of the 612-tektites indicate that they were formed from a crustal source of late Precambrian mean age (800–1000 Ma) which in middle Palaeozoic time (?400 Ma) was further enriched in Rb/Sr during sedimentary processes. These source characteristics suggest that the impact which produced the 612-tektites occurred in rocks of the Appalachian orogeny or sediments derived from this orogenic belt. Potential source materials for both 612-tektites and North American tektites are present on the eastern and southeastern part of the North American continent and its adjacent shelf. The distinct isotopic differences between 612-tektites and North American tektites indicate that the two groups of tektites were either formed by the impact of more than one bolide in the same general area, or by a single impact event that sampled different layers.  相似文献   

15.
The meteorite which fell near Messina, Italy, on 16 July 1955 is a typical olivine-hypersthene (L-group) chondrite. Its mineralogical composition is: olivine (Fa24), orthopyroxene (Fs20) with some polysynthetically twinned clynopyroxene, plagioclase (An10) and merrillite. Opaque phases present are: copper, kamacite, taenite, plessite, chalcopyrrhotite, mackinawite, troilite and chromite. The stone contains abundant chondrules. The matrix consists chiefly of broken chondrules with tiny fragments of crystals and rare amorphous material. Chondrules form more than 42% of the meteorite by volume. Some unusual features of the fabric of this meteorite include silicate grains showing deformation; silicates with fusion spots of dark glass containing blebs of metallic iron; iron and troilite with marginal fusion yielding globules and droplets sometimes showing flow structures. The classification of this chondrite is confirmed by bulk chemical analysis.  相似文献   

16.
Abstract— Three samples of Muong Nong tektites have been studied for N and noble gases. The isotopic composition of noble gases is airlike. The noble gas amounts are much higher in Muong Nong tektites than in splash-form tektites. As compared to air, He and Ne have been enriched, most likely due to inward diffuion from ambient air, subsequent to glass formation. Nitrogen contents range from 0.3 to 1.34 ppm, with a non-atmospheric δ15N ranging from 8 to 17%. The release pattern of δ15N clearly shows the presence of two N components. Higher N/36Ar values than those of air, together with positive δ15N, show that a major portion of N in Muong Nong tektites is a remnant from the sedimentary source material.  相似文献   

17.
Abstract— We have analyzed the potassium isotopic composition of four tektites from the Australasian strewn field, spanning a wide diversity of thermal histories, inferred from textures and volatile element contents. Our results indicate no isotopic differences between tektites and terrestrial crustal rocks, placing stringent limits of ≤2% loss of potassium during the brief duration of high temperature heating experienced by these samples. This confirms that the chemical composition of tektites is entirely a reflection of source rock composition and has not been modified by the tektiteforming process for elements less volatile than potassium. Losses of more volatile components, e.g., the halogens and water, are not precluded by the present data. Coupling a radiative cooling temperature‐time path with potassium vapor pressure data indicates that tektite melt drops are not likely to develop bulk elemental fractionation during the brief heating episodes of tektites for peak temperatures <2273 K. The extent of K isotopic fractionation is independent of droplet size but dependent on peak heating temperature. The exact peak temperature depends on the choice of vapor pressure data used for K, which need to be better constrained.  相似文献   

18.
Abstract— Fifty-odd years of tektite research are reviewed, proceeding from the discovery of the first North American tektites in 1936. This included the early recognition that tektites were terrestrial objects rather than meteorites and that the glassy particles in tektites were fused quartz (lechatelierite). Later, during National Science Foundation-supported research, it was found that some tektites appeared to have formed as puddles of melt, that the content and character of bubbles in lechatelierite can be used as a relative temperature scale, that rayed bubbles in tektites formed from hydrous minerals, that bubbles in tektites formed chiefly from water which was absorbed into the walls of the bubbles leaving vacuums, and that “fingers” in the surficial part of some tektites may have formed by differential volatilization. Some unpublished observations and adventures are briefly reported.  相似文献   

19.
Abstract— We measured noble gases and Ne isotopic compositions of five tektites collected from three different strewn fields. The elemental abundance patterns of noble gases in all samples show anomalous Ne enrichments relative to air. Ne isotopic compositions in tektites are in good agreement with that of atmospheric Ne, suggesting that Ne has diffused in from the atmosphere. It is conceivable that the high relative Ne abundance is essentially an equilibrium effect, i.e., storage of Ne in vesicles rather than the glass itself, facilitated by the relatively high diffusion coefficient of Ne.  相似文献   

20.
Abstract— Five indochinites from Hainan Island and the Leizhou Peninsula, China were analyzed for noble gas abundances and isotopic ratios. These splash‐form tektites show vesiculation ranging from 0.4 vol% to 8 vol%, as determined by digital image analysis (software SXM®) on thin section photographs. To study the distribution of noble gases in vesicles and in glass, the gases were extracted by heating and by crushing, respectively, on 2 aliquots of the same sample. The results show that 5 to 53% of the total measured 20Ne resides in vesicles. The calculated concentration of neon dissolved in the glass is higher (0.7‐1 times 10?7 cm3STP/g) than that expected from solubility equilibrium (1 times 10?8 cm3STP/g), assuming solubility data from MORB glasses. The neon concentration of splash‐form tektites, those analyzed in this work and those from other strewn‐fields worldwide, is correlated with the SiO2 content of glass and with the nonbridging oxygen per tetrahedral cation in the melt (hereafter NBO/T ratio), the latter being an index of the free‐volume in the silica network where neon could be dissolved. These correlations suggest that the glass structure of tektite has a larger free‐volume available for dissolving noble gases than MORB glasses.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号