[en] Various types of meteorites have experienced processes of condensation, accretion, metamorphism, differentiation, brecciation, irradiation and fragmentation. A typical view of meteorite formation has been that the processes following accretion take place in a few asteroidal-sized (approximately 100 km) objects. Discovery of decay products of now extinct Al-26 and Pd-107 in meteorites, discovery of isotopic heterogeneity among meteorite types, re-analysis of meteorite cooling rates, and continuing study of meteoritic compositions have led some meteoriticists to conclude that meteorites obtained their chemical, isotopic, and some textural characteristics in objects initially less than 10 km in diameter. Such a scenario, which is described in this paper, raises the possibility that some of these small planetesimals may have been 'condensation nuclei' for the formation of comets as well as the precursors of asteroids

[en] Pb-Pb and Rb-Sr internal isochrons have been determined in the Saint Severin amphoterite. The Sr age and initial ratio are, respectively, 4.61 +- 0.15 b.y. (lambda =1.39 X 10^-11yr^-1) and 0.69903 +- 0.00020 (2sigma errors). Evidence is presented for acceptance of radiogenic Sr from whitlockite, possibly from troilite, during events younger than 3.1 b.y., thus giving too high initial ratios. The Pb-Pb age, determined from plagioclase, whole rock and very radiogenic whitlockite is 4.543 +- 0.019 b.y. (2sigma errors), using the new U decay constants. Both U-Pb and Th-Pb ages of whitlockite are concordant at 4.54 b.y. The range of the Th/U ratio of whitlockite is 9-11, in agreement with the value of Crozaz [Earth Planet. Sci. Lett. 23(1974) 164-169]. The results are compared with the other Rb-Sr and U-Pb data on chondrites and achondrites. (Auth.)

[en] The research of Smith et al. (1978) and Oliver et al. (1981) on the Abee meteorite's possible negative tellurium anomaly is extended. Two sets of measurements of the tellurium isotopic composition of Abee are reported, and the meteoritic data are compared with a terrestrial tellurium standard. No isotopic anomalies can be distinguished within the error limits. However, further work on the isotopic composition of Te in residues from the Allende meteorite need to be pursued by accurate mass spectrometric analysis. 23 references

[en] Different sets of initial conditions for impacts of stray objects on solid surfaces of planetary bodies or asteroids have been computed for cooling times of chondrules. The results of the calculations indicate cooling times shorter than 60 seconds for condensed matter from 10 to the 4th to 10 to the 15th g generated by impact and initial temperatures of the gas phase formed up to presumed degeneracy limit of 150,000 K. To retain solid impact products originated that way on meteorite parent bodies by gravitational forces, a target diameter of at least 800 km is required. It is shown that chondrule formation by impact seems to be an unlikely event. 29 references

[en] A neutron-irradiated bulk sample of the Murray (C-2) carbonaceous chondrite was etched with H₂O₂ and then divided into colloidal and non-colloidal fractions. The H₂O₂ treatment removed approx. 80% of the trapped Xe and greatly increased variations in the ¹²⁹Xe/¹³²Xe ratio measured in stepwise heating. The colloid showed very little excess ¹²⁹Xe, but the anti-colloid gave a fairly good I-Xe correlation corresponding to formation 3.7 +- 2.1 m.y. after Bjurboele. (orig./ME)

[en] Two hypotheses are proposed for the aqueous alteration of carbonaceous chondrites within their parent bodies, in which respectively the alteration occurs (1) throughout the parent body interior, or (2) in a postaccretional surface regolith; both models assume an initially homogeneous mixture of ice and rock that is heated through the decay of Al-26. Water is seen to exert a powerful influence on chondrite evolution through its role of thermal buffer, permitting substitution of a low temperature aqueous alteration for high temperature recrystallization. It is quantitatively demonstrated that liquid water may be introduced by either hydrothermal circulation, vapor diffusion from below, or venting due to fracture. 104 refs

[en] The ferromagnesian chondrules in Allende follow a trend in the oxygen three-isotope plot that diverges significantly from the 16-O mixing line defined by light and dark inclusions and the matrix of the meteorite. The trend probably results from isotopic exchange with an external gaseous reservoir during the process of chondrule formation sometime after the establishment of the isotopic compositions of the inclusions and matrix. The Allende chondrules approach, but do not reach, the isotopic compositions of chondrules in unequilibrated ordinary chondrites, implying exchange with a similar ambient gas, but isotopically different solid precursors for the two types of meteorite. 8 references

No abstract available

[en] True relative Sr isotopic compositions, determined by the double-spike technique, are reported for 8 olivine chondrules from Allende and a single chondrule from Richardton. The Richardton chondrule has an Sr composition identical with the whole meteorite, but the Allende chondrules are up to 1.4% per mass unit light-isotope enriched, closely similar to Ca-Al inclusions (CAI) from the same individual stone. The correspondence of the patterns for chondrules and CAI suggests that both groups of objects derived their fractionated Sr in similar ways. The lack of any detectable non-linear Sr isotopic anomaly in the objects suggests that their Sr compositions did not have some exotic or extrasolar origin, but were derived from normal system Sr by mass fractionation. The consistent light-Sr enrichment of Allende objects may be explained by several schemes, and all are heavily model-dependent. Most plausible to the author is that the CAI and chondrules derived their fractionated Sr from a region of the nebula made isotopically light by partial kinetic mass separation of elements in the vapour phase. Later, the solid objects may have moved to an isotopically more normal region, where the Allende matrix accreted. (orig.)

[en] Noble gases in five recently found ureilites were analyzed by conventional noble gas mass spectrometry and, in one case, by closed-system combustion mass spectrometry of an acid-resistant residue. From analyses of the Hajmah ureilite we derive a composition for trapped neon of ²⁰Ne/²²Ne=10.4±0.3, ²¹Ne/²²Ne=0.027±0.003. Since this differs from all other known components of trapped neon, we propose the name Ne-U for this composition the occurrence of which may not be restricted to ureilites alone. Its composition is compatible both with being fractionated solar Ne and being a mixture of solar Ne and Ne-E. The associated He has a ratio 3He/4He<5x10^-4, within the range of other trapped He compositions. Closed-system combustion data demonstrate the presence of an easily combustible fraction with low ³⁶Ar/¹³²Xe ratio, in accordance with our earlier suggestion for the existence of a second gas component in addition to that residing in diamonds. Attempts to characterize this phase in the Nilpena ureilite which had been reported to be diamond-free met with little success, because it turned out that diamonds are present. In the case of ALPHA 78019, more work is required