摘要:

Abstract—We study the observational data relating to 44 events recorded by the Canadian fireball camera network between 1971 and 1984. Each event is believed to have dropped meteorites, with main masses in the range from 0.1 to 11 kg. Median values are given for 20 parameters describing the atmospheric behavior and orbital elements. A typical duration for a meteoritic fireball is 4 seconds, reaching an end height of 31 km and a velocity of 8 km s−1at the end of the luminous path. The peak brightness is typically −9 magnitude but varies from −7 to −15. Meteorites may survive from relatively unspectacular fireballs.Numerous correlations among parameters are investigated. The strong correlations of brightness with initial mass and of duration with slope of the trajectory could be anticipated. Moderate correlations also exist between: (1) initial mass and end height; (2) initial mass and duration of luminosity; (3) initial velocity and beginning height; (4) initial velocity and the fraction of the mass that survives as the largest fragment; (5) initial velocity and the perihelion distance of the orbit. Ablation limits the survival of small masses while fragmentation is usually severe for masses larger than 10 kg. The fractional survival may peak for entry masses between 1

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00946.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract—Three augite‐bearing ureilites from Antarctica, Y74130, MET78008 and ALH82106, have been studied by electron microprobe, scanning electron microscope (SEM), and analytical transmission electron microscope (TEM). The first two belong to the low‐16O subgroup of Clayton and Mayeda (1988) and are closely related; ALH82106 belongs to the high‐16O subgroup. MET78008 is an augite‐olivine ureilite, similar to the augite‐bearing part of the Y74130. Augites poikilitically include ellipsoidal pigeonites, and low‐Ca pyroxene poikilitically encloses ellipsoidal olivine and augite. The temperature of last equilibration deduced from an orthopyroxene‐pigeonite‐augite assemblage is above 1200 °C. The ALH82106 pigeonite contains irregular augite inclusions produced by decomposition. Augites in Y74130, MET78008 and ALH82106 (decomposed one) all show similar spinodal decomposition textures on the TEM scale. Cooling rates estimated from an experimentally calibrated diagram for the wavelengths of spinodal decomposition versus cooling rates are about 20 °C/hr. The calcic trend for the low‐16O subgroup including Y74130 and MET78008 shows large Ca variations in pyroxene with similar Mg/Fe ratio; differentiation involving reduction was not an important process. Augite in ALH82106 is a minor phase and contains lower Na2O contents than Y74130 and MET78008, confirming that ALH82106 is not dir

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00947.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract—Small terrestrial hypervelocity impact craters have a bowl‐shaped form and are partially filled by an interior breccia lens, roughly parabolic in cross‐section, of allochthonous material. This interior breccia volume is geometrically modelled as the volume of material slumped off the interior wall of the transient cavity during late stage crater modification. This model is tested by comparing the estimated volume of the breccia lens based on observational data with the calculated volume of slump material based on known dimensional parameters. The model fits well for Meteor Crater and Brent and is highly sensitive to changes in input parameters (e.g., a 10% increase in the input diameter for Meteor Crater produces an almost 200% increase in the model breccia lens volume). Further testing of the model with less constrained data from West Hawk Lake and Lonar leads to reasonable fits, given the sensitivity of the model to input parameters. Fits to other craters: Aouelloul, Tenoumer and Wolf Creek, where previous depth data are constrained only by gravity data, are unsatisfactory. However, revised depths can be obtained that fit both the gravity data and the model. While these tests do not provide unqualified support for the model, they do suggest that it may represent a good first order approximation. More and better quality dimensional data are required for more rigorous te

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00948.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

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. The87Sr/86Sr and143Nd/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. TNDCHURand TSrURmodel 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 sampl

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00949.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract—The elemental compositions of 200 interplanetary dust particles (IDPs) collected in the stratosphere have been determined by energy dispersive X‐ray (EDX) analysis. The results reasonably define the normal compositional range of chondritic interplanetary dust particles averaging 10 micrometers in size, and constitute a database for comparison with individual IDPs, meteorites, and spacecraft data from comets and asteroids. The average elemental composition of all IDPs analyzed is most similar to that of CI chondrites, but the data show that there are small yet discernable differences between mean IDP composition and the CI norm. Individual particles were classified into broad morphological groups, and the two major groups show unambiguous compositional differences. The “porous” group is a close match to bulk CI abundances, but the “smooth” group has systematic Ca and Mg depletions, and contains stoichiometric “excess” oxygen consistent with the presence of hydrous phases. Similar depletions of Ca and Mg in CI and CM matrix have been attributed to leaching, and by analogy we suggest that particles in the smooth group have also been processed by aqueous alteration. The occurrence of carbonates, magnetite framboids, and layer silicates provides additional evidence that at least a significant number of the smooth‐class IDPs have been substantially processed by aqueous activity. The presence or absence of aqueous modification in members of a particle sub‐class is an important clue to the origin. Although it cannot be proven, we hypothesize that extensive aqueous activity only occurs in asteroids and that, accordingly, the smooth class of IDPs has an asteroidal origin. If both comets and asteroids are major sources of interplanetary dust, then by default the porous particles are inferred to be dominated b

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00950.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract—Several experimentally and naturally shocked silicate samples were analyzed for noble gas contents to further characterize the phenomenon by which ambient gases can be strongly implanted into silicates by shock and to evaluate the possible importance of this process in capturing planetary atmospheres in naturally shocked samples. Gas implantation efficiency is apparently mineral independent, as mono‐mineralic powders of oligoclase, labradorite, and diopside and a powdered basalt shocked to 20 GPa show similar efficiencies. The retentivity of shock‐implanted gas during stepwise heating in the laboratory is defined in terms of two parameters: activation energy for diffusion as determined from Arrhenius plots, and the extraction temperature at which 50% of the gas is released, both of which correlate with shock pressure. These gas diffusion parameters are essentially identical for radiogenic40Ar and shock‐implanted40Ar in oligoclase and labradorite shocked to 20 GPa, suggesting that the two40Ar components occupy analogous lattice sites. Our experiments indicate that gas implantation occurs through an increasing production of microcracks/defects in the lattice with increasing shock pressure. The ease of diffusive loss of implanted gas is controlled by the degree of annealing of these microcracks/defects.Identification of a shock‐implanted component requires relatively large concentrations of implanted gas which is strongly retained (i.e., moderate activation energy) in order to separate implanted gas from surface adsorbed gases. Literature data on shocked terrestrial samples indicate only weak evidence for shock‐implanted gases, with an upper limit for40Ar of ∼ 10−6cm3STP/g. New analyses of shocked samples from the Wabar Crater indicate the presence of shock‐implanted Ar, having concentrations (∼ 10−4cm3STP/g) and activation energies for diffusive loss which are essentially that expected from experimental studies. Lack of sufficient target porosity or the presence of ground water may explain the sparse evidence for shock‐implanted gas at other terrestrial craters. Although Wabar Crater may represent an unusually favorable environment on Earth for shock‐implanting gases, surfaces of other planetary bodies, such as Mars, may frequently provide such environments. Analyses of returned samples from old Martian terraines may document temporal changes in earlie

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00951.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Book reviewed in this article:Meteorite, Urmaterie aus dem interplanetaren Raum by Rolf W. Bühler, Birkhäuser Verlag, Basel, Boston, Berlin, 1988, 192 pp.Kleine Meteoritenkunde by F. Heide. 3rd, totally revised edition by F. WlotzkaMeteorites and the Early Solar System edited by John F. Kerridge and Mildred S. Matthew

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00952.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00953.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY