摘要:

The orbital evolution of asteroidal fragments with diameters ranging from 10 cm to 20 km, injected into the 3:1 Kirkwood gap at 2.50 A.U., has been investigated using Monte Carlo techniques. It is assumed that this material can become Earth‐crossing on a time scale of 106years, as a result of a chaotic zone discovered by Wisdom, associated with the 3:1 resonance. This phenomenon, as well as close encounter planetary perturbations, the v6secular resonance, and the ablative effects of the Earth's atmosphere are included in the determination of the orbital characteristics of meteorites impacting the Earth derived by fragmentation of this asteroidal material. It is found that the predicted meteorite orbits closely match those found for observed ordinary chondrites, and the total flux is in approximate agreement with the observed fall rate of ordinary chondrites. About 10% of the predicted impacting bodies are meteorite‐size bodies originating directly from the asteroid belt. The remainder are obtained by subsequent fragmentation of larger (∼1 m to 20 km diameter) Earth‐crossing asteroidal fragments. The largest of these fragments are observable as Apollo‐Amor objects. Thus the apparent paradox between the orbital characteristics of observed ordinary chondrites and those predicted from Apollo object sources is reconciled. Both appear to be complementary aspects of the same phenomena. No other asteroidal resonance is found to be satisfactory as a source of ordinary chondrites. These meteorites are therefore most likely to be derived from S asteroids in this limited region of the asteroidal belt, the largest of which are 11 Parthenope, 17 Thetis, and 29 A

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1985.tb00842.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

We review our procedures for selecting, preparing and analyzing meteorite samples, present new analyses of 16 ordinary chondrites, and discuss variations of Fe, S and Si in the L‐group. A tendency for Fe/Mg, S/Mg and Si/Mg to be low in L chondrites of fades d to f testifies that post‐metamorphic shock melting played a significant role in the chemical diversification of the L‐group. However, these ratios also vary widely and sympathetically in melt‐free chondrites, indicating that much of the L‐group's chemical variation arose prior to thermal metamorphism and is in that sense primary. If all L‐chondrites come from one parent body, type‐correlated chemical trends suggest: 1) that the body had a traditional “onion skin” structure, with metamorphic intensity increasing with depth; and 2) that it formed from material that became more homogeneous, slightly poorer in iron, and significantly richer in sulfur as ac

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1985.tb00843.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

Uranium was measured by fission track analysis in meteoritic troilite and graphite. The distribution is extremely heterogeneous, with a few high‐U grains dominating the total abundances. U/He ages cannot be estimated from such a distribution pattern, and therefore previously reported excessive ages are not vali

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1985.tb00844.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

Poorly‐graphitised carbon particles are formed during manufacture of sample substrates (holey carbon films) for Analytical Electron Microscopy studies of small particles. The particles form during heat treatment of cellulose acetobutyrate at about 975 °C and 1050 °C. In AEM studies of fine‐grained carbonaceous extraterrestrial materials, these particles are easily recog

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1985.tb00845.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

Similarities among impact structures on different planets and satellites suggest that the cratering process transcends variations in both target and impactor. In particular, impact may control the spacing of concentric rings, if not their actual emplacement. In at least four respects the scaled horizontal dimensions of complex meteorite‐impact structures on Earth resemble those of multi‐ring basins and large craters on the Moon, Mars, Mercury, and some outer satellites: (1) Base diameter of the (topographic) central peak is a constant 20% to 25% of the rim diameter in small complex craters; (2) it averages only half as much in large structures that also have concentric rings; (3) the inner ring of a two‐ring crater lacking a central peak is half the diameter of the outer ring; (4) adjacent rings of complex craters that have more than two concentric rings are spaced at a constant interval of about (2.0 ± 0.2)0.5D, both inside and outside the main ring. Two minor differences in morphology suggest that uniquely terrestrial conditions may control some horizontal dimensions of meteorite craters: (1) the inner ring of a two‐ringed structure that also has a central peak is 0.5X the diameter of the outer, not 0.4X as it is for peak‐plus‐ring basins on the planets; and (2) two‐ring and multi‐ring meteorite craters occupy the same size range, whereas on planets most two‐ring basins are smaller than

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1985.tb00846.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

Noble‐gas systematics show that Brachino is not a member of the SNC‐group of meteorites. The whole‐rock K‐Ar gas retention age is (3.11 ± 0.07) AE as compared to the 1.3 AE solidification ages of SNCs; the content of radiogenic129Xe* of (3.47 ±. 15) × 10−10cm3STP/g is about two orders of magnitude higher, and the129Xe/132Xe ratio (11.0), the ratio of radiogenic129Xe* to fissiogenic136Xef(300), and the ratio36Ar/132Xe in the trapped gases are about one order of magnitude higher than observed for SNCs. The same evidence argues strongly against any simple genetic relationship with eucrites. The noble‐gas abundance pattern resembles closely that in silicate inclusions from the iron meteorites Campo del Cielo and Udei Station. Abundances of cosmic‐ray produced3He and21Ne (5.7 and. 99 × 10−8cm3STP/g, resp.) indicate an exposure age of ∼ 2.4 Ma. Irradiation conditions appear to have been perfectly normal except for an unaccountably low content of spallogenic38Ar. Losses by diffusion of radiogenic4He are severe; they must have occurred at or before the onset of the exposure of the meteoroid to

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1985.tb00847.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

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 analy

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1985.tb00848.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

Bulk chemical analyses of six E‐chondrites (Daniel's kuil, Khairpur, Kota Kota, Saint‐Sauveur, South Oman and St Mark's) are given, together with partial analyses of a further five (Blithfield, Hvittis, Indarch, Jajh deh Kot Lalu and Pillistfer). The distribution of some normally lithophile elements (Al, Ca, Cr, K, Mg, Na, P and Ti) between silicate and sulphide groups of minerals was determined using the selective attack by dry chlorine (350°C) on magnetically separated fractions. Subdivision of the E‐chondrites into types I and II (Yavnel;, 1963; Anders, 1964) is accepted and it is shown using chemical data that St Mark's and Saint‐Sauveur should be included in type I.Sulphides contribute an unexpectedly high proportion of several elements to the bulk: e.g. Ca (av. 88.5% type I, 66.3% type II); Ti(av. 77.1% type I, 84.8% type II) and P as phosphide (av. 44.4% type I,>83.2% type II). The proportion of Ti contributed to the bulk composition by the sulphides in types I and II increases with increae in ‘thermal metamorphic effect’ (Easton, 1983b) within each type.There is marked variability in the relative abundances of metal, phosphide, silicate and sulphide among the members of each type in keeping with their aggregate nature.The chemical composition of the ‘silicate’ and ‘sulphide’ in type IE‐chondrites differs from that in type II (e.g. CaO in the silicates, Mg in the bulk sulphides) which therefore precludes the isochemical evolution of all E‐chondrites from a common parent material. Partition of Ti between silicate and sulphide groups of minerals indicates that types I and II E‐chondrites originated in separate,

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1985.tb00849.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

Estimates of the volume percents of chondrules in petrographic thin sections are notoriously unreliable, ranging widely among different observers for the same meteorite or even for the same thin section. The set of visual‐estimation guides presented herein should help narrow the uncertainties in such estimates and lead to a more consistent picture of chondrule abundances than is now apparent from the literature.In generating this set of guides other quantitative measures were also retained and examined. First, the expected error in the volume abundance of chondrules, due solely to sampling of homogeneous rocks by random thin sections, decreases as the inverse of the volume abundance of the chondrules. Second, our experiments of “softly” settled chondrules could not exceed chondrule abundances of 60 vol %. If chondrule‐bearing rocks are found to exceed this value, the physical environment where they accreted must have had a greater gravity field than is available on asteroid‐si

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1985.tb00850.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

The Bocaiuva iron contains 10 to 15% by volume of silicate inclusions which are surrounded by kamacite (6.5 wt % Ni). The metal shows a Widmanstätten pattern in metal areas devoid of silicates; taenite evolved in plessite fields. The silicate inclusions occur as nodules, and as irregular or chain‐like aggregates in which olivine may be rounded or faceted. The magnesian silicates (forsterite, enstatite, diopside) are similar in composition to those of the group IAB irons, whereas the interstitial plagioclase is much more calcic (An 50) than that usually found. Iron sulfide occurs as pyrrhotite and contains 1–2 wt% Cu. Chromite and euhedral magnetite are accessory phases always associated with pyrrhotite. Some patches of pyrrhotite enclose rounded chromite and small plagioclase crystals displaying compositions different from those of the ground mass of the inclusions. Schreibersite shows a compositional variability.This preliminary study underlines the unusual nature of Ms iron and raises several questions concerning the genetic relations between silicates, sulfide and metal, and the thermal history of the whole mate

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1985.tb00851.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY