摘要:

Abstract—Isotopic analysis of nesquehonite recovered from the surface of the LEW 85320 H5 ordinary chondrite shows that the δ13C and δ18O values of the two generations of bicarbonate (Antarctic and Texas) are different: δ13C = +7.9‰ and +4.2‰; δ18O = +17.9‰ and + 12.1‰ respectively. Carbon isotopic compositions are consistent with equilibrium formation from atmospheric carbon dioxide at −2 ± 4 °C (Antarctic) and +16 ± 4 °C (Texas). Oxygen isotopic data imply that the water required for nesquehonite precipitation was derived from atmospheric water vapour or glacial meltwater which had locally exchanged with silicates, either in the meteorite or in underlying bedrock. Although carbonates with similar δ13C values have been identified in the SNC meteorites EETA 79001 and Nakhla, petrographic and temperature constraints argue against their simply being terrestrial

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00934.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract—The relative abundance of different compositional types and mass frequency distributions are presented for four meteorite samples (the modern falls, Antarctic finds, Yamato finds and Allan Hills Main Icefield finds). The modern falls sample represents continuous collection of a known number of falls over a short timespan, while the Antarctic samples represent a longer timespan and an unknown number of falls. The Allan Hills Main Icefield sample has many desirable collection characteristics indicating it best represents Antarctic meteorites. By retabulating the modern falls to create a sample with characteristics similar to those of the Allan Hills Main Icefield finds, we can directly compare the two.The mass frequency distributions of Antarctic samples exhibit a tail toward the larger sizes and thus differ from that of the modern falls (which approximates a normal curve). In general, normal and power law models prove to be inadequate to explain the observed mass frequency distributions, possibly because they fail to correctly account for atmospheric and collection effects. Non‐parametric statistics show that it is unlikely that the two are good samples of a single steady‐state meteoritic complex. In addition, there is an excess in numbers of small H chondrites in Allan Hills Main Icefield finds relative to modern falls which is not easy to explain given modern showerfall rates of occurrence. This supports the view that the delivery of meteoritic material to Earth might be variable over the short timescale represented by these sa

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00935.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract—Absolute and relative cratering rates on the terrestrial planets have been calculated using the same asteroidal collision model and Monte Carlo program used for previous studies of the terrestrial meteorite flux, the steady‐state number of Apollo‐Amor objects, and the orbital distribution of both meteorites and Apollo‐Amor objects. The most straightforward result is that projectiles from the asteroid belt appear to provide about one‐third the observed present‐day production of terrestrial craters larger than 10 km in diameter. When uncertainties in the calculations and observations are included, it cannot be excluded that the entire terrestrial cratering flux is asteroidal. On the other hand, assumption of an additional Apollo‐Amor source of extinct comets, in the same quantity permitted by Apollo‐Amor observations, provides better agreement with the observed cratering rate. In addition, a significant (e.g., ∼30%) terrestrial contribution from active long and short period comets is acceptable within the uncertainties of the assumptions required.The ratios of the cratering rates on the different terrestrial planets are somewhat sensitive to the assumed source. A purely asteroidal source predicts a martian cratering rate per unit area about four times that on Earth, whereas the difference is reduced to about a factor of two for the mixed asteroid‐extinct comet source. The opposite effect is found for Mercury. As discussed by previous authors, the predicted lunar cratering rate is significantly higher than that observed. It is not clear whether this is a result of scaling to impacts on a body considerably smaller than Earth, or if it indicates an increase in the cratering flux dur

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00936.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract—The Farmington ordinary L5 chondrite with its uniquely short cosmic‐ray exposure age of less than 25 000 years may have been a member of a large meteoroid swarm which was detected by the Apollo seismic network when it encountered the Moon in June 1975. The association implies that the parent body of the Farmington meteorite was in an Earth‐crossing orbit at the time the swarm was formed. This supports the idea that at least some meteorites are derived from the observable population of Earth‐crossing as

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00937.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract—A meteorite shower fell at Torino, Italy on 18 May 1988. Petrographic studies indicate that the stone is an H6 chondrite having features of moderate to severe shock. Chemical analyses of the meteorite are reported. Cosmic ray produced3He,21Ne and126Xe yield an exposure age of about 48 Ma. The cosmic ray track densities in three fragments range between 1.8 to 5 × 105/cm2suggesting about 99% mass ablation in the atmosphere.Twelve radioisotopes with half lives ranging between 5.6 days to 7.3 × 105years have been measured with high precision (2 to 10%). Marginal signals were observed for several short‐lived nuclides and upper limits were obtained for the activity levels of eight radionuclides (24Na,48Cr,57Ni,47Sc,47Ca,59Fe,42Ar and44Ti) some of which have not been hitherto detected in fresh falls. The data are generally consistent with the nuclide production by galactic cosmic rays when modulation due to the solar cycle is taken into consideration.The preatmospheric radius of the chondrite is estimated to be 20 cm, consistent with track densities and activity levels of60Co,26Al and other radionuc

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00938.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract—Yamato 82042 is an unusual CM2 chondrite consisting mainly of phyllosilicates, a few olivines and carbonates, very minor sulphides and trace metal. Olivine occurs: (1) as isolated grains dispersed in the phyllosilicate matrix, (2) as constituents of mineral aggregates or accretionary fragments associated with abundant phyllosilicates and minor sulphides, and (3) as objects which resemble barred olivine chondrules also associated with phyllosilicates. Olivine, from all occurrences, ranges in composition from 0.26 to 22.6 weight % FeO, but generally contains less than 1.25 wt.% FeO. Minor element contents, particularly Ca, Al, and Cr, are relatively high and are generally correlated, as reported for olivines in other carbonaceous chondrites. However, we report here uncorrected trends for the same minor elements which occur in distinct areas (volumes) within the same olivines. These compositional trends may be due to condensation of olivine from a vapor of non‐solar composition and partial mobilization of Ca during later annealing. If this is the case, the data may be used to trace changes in the Ca/Al ratio of the parent medium during the formation of these olivines, provided that it is possible to distinguish the effects of any post‐formation annealing which could have redistributed the minor elements. Some isolated olivines show distinctive minor element zoning which severely limits the possibility of any post‐formation redistribution of these elements. Accordingly, these isolated olivines indeed retain evidence of early condensation processes in the solar nebula, though non‐classic conditions are implied for their

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00939.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract—Submicron platey Sn‐rich grains are present in chondritic porous interplanetary dust particle (IDP) W7029*A and it is the second occurrence of a tin mineral in a stratospheric micrometeorite. Selected Area Electron Diffraction data for the Snrich grains match with Sn2O3and Sn3O4. The oxide(s) may have formed in the solar nebula when tin metal catalytically supported reduction of CO or during flash heating on atmospheric entry of the IDP. The presence of tin is consistent with enrichments for other volatile trace elements in chondritic IDPs and may signal an emerging trend towards non‐chondritic volatile element abundances in chondritic IDPs. The observation confirms small‐scale mineralogical heterogeneity in fine‐grained chondritic porous interplane

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00940.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract—Olivines in chondrules of the Allende CV3 carbonaceous chondrite contain two types of micron sized silicate melt inclusions: clear glass (type I) and devitrified glass (type II) inclusions. Many of the type I inclusions contain a gas bubble of variable size. Type II inclusions can be transparent (IIa), with or without a gas bubble, and brown (type IIb), with a gas bubble. A number of inclusions were measured with the Raman microprobe to detect possible presence of carbon. Carbon in the form of graphite was detected only in type II inclusions. Compositions of 11 inclusions were determined with the electron microprobe and proton microprobe in search for possible explanation of this preference of carbon for devitrified inclusions. All of the measured inclusions are rich in Si, Al and Ca. No significant differences between the compositions of the two types of inclusions were found. The data suggest that the inclusions formed from the melt trapped in growing olivine crystals, which themselves crystallized from a silicon rich, gas bearing melt. There is no coherent relation between the occurrence of graphite and the gas abundance in the original melt, as indicated by the sizes of gas bubbles. Therefore, carbon was not combined in a gaseous species (e.g., CO). It must have been preferentially dissolved in some domains of the mel

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00941.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Abstract—The Wilder chondrite, a single stone of 1970 g, was found in southwestern Idaho in 1982. A classification of H5 is indicated by the mean compositions of olivines (Fa18.4) and orthopyroxenes (Fs16.3), and the narrow range in mineral compositions, recrystallized nature of the matrix, and mean Wo content of the orthopyroxenes (Wo1.34

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1989.tb00942.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.tb00943.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY