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Genesis of chondritic meteorites

1966; Wiley; Volume: 4; Issue: 2 Linguagem: Inglês

10.1029/rg004i002p00113

8755-1209

A. E. Ringwood,

High-pressure geophysics and materials

The abundances of elements in the major classes of chondrites are discussed, and the major abundance patterns defined. The chemical compositions of type I carbonaceous chondrites are uniquely related to those of other classes of chondrites, the compositions of which can be obtained solely by the removal of appropriate amounts of trace and minor elements from type I carbonaceous chondrites by appropriate chemical and physical processes. The chemical and mineralogical constitution of type I carbonaceous chondrites shows that they have experienced a simpler chemical and thermal history than other classes of chondrites. Their chemical composition agrees well with estimates of primordial elemental abundances based on nucleosynthetic arguments. Furthermore, their composition agrees well with the composition of the solar atmosphere. Previous claims of the existence of a gross discrepancy between the solar and the chondritic iron abundances are shown to be unfounded. The above relationships, when considered in conjunction with the highly oxidized state of type I carbonaceous chondrites and their high content of volatiles, strongly suggest that these chondrites are extremely primitive in nature and may be closely related to the primordial dust of the parental solar nebula. This leads to the hypothesis that other classes of chondrites have been derived from parental material resembling type I carbonaceous chondrites by various complex chemical and physical fractionation mechanisms. Several fractionation mechanisms are discussed. Ordinary and enstatite chondrites may have been formed when primitive material similar in composition to type I carbonaceous chondrites was subjected to autoreduction at high temperatures accompanied by loss of volatiles. Under such conditions, extensive chemical fractionations would be caused by selective volatility, by selective solubility of elements in supercritical fluids of H 2 O, CO 2 , H 2 S, and H 2 , and by physical fractionation of metal particles from silicates. The mineralogy of the different classes of chondrites is reviewed. Types II and III carbonaceous chondrites represent a physical mixture of high‐temperature minerals with primitive material similar to type I carbonaceous chondrites. The significance of these mineral assemblages is discussed. Compositional relationships between the olivines, pyroxenes, and metal phases of ordinary chondrites indicate a fairly close approach to chemical equilibrium, and they also provide information on the rates and temperatures of crystallization. The ordinary chondrites possess appreciable amounts of both oxidized and metallic iron and are thus intermediate in oxidation state between carbonaceous and enstatite chondrites. The mineralogy of the enstatite chondrites is indicative of an intense degree of reduction at high temperatures. They are rich in metal, which also contains some silicon in solid solution, and they contain no oxidized iron. A detailed review of oxidation‐reduction relationships in chondrites is given, particularly with respect to ‘Prior's rule.’ Chondrites are observed to display a wide range of oxidation states, the degree of oxidation being qualitatively related to the amount of metal present and to the nickel content of the metal. The range in oxidation states is not continuous. There is a hiatus between enstatite chondrites and ordinary chondrites and also between the ordinary H and L chondrites. Furthermore, there has been an additional independent fractionation of metal with respect to silicates both within and between groups. This is most apparent in the ordinary L chondrites which exhibit a metal deficiency of about 5%. The validity of Prior's rule is established, in the sense that it expresses a qualitative relationship among metal contents, metal compositions, and redox states of the coexisting silicates. However, because of metal‐silicate fractionation, Prior's rule cannot be applied quantitatively. The present constitution of ordinary and enstatite chondrites is due to a chemical reduction process operating upon primitive oxidized material. The nature of the reduction process is discussed. It is concluded from mineralogical and chemical evidence that the metal in chondrites was produced by a carbon‐reduction process operating on primitive oxidized material in an essentially condensed system at high temperatures rather than by hydrogen reduction in a highly dispersed system. Hypotheses relating to the origin of chondrules and chondritic structures are reviewed. Chondrules may have formed during volcanic processes on a parent body or by impact phenomena during collisions of planetesimals with one or more parent bodies. A recent suggestion that chondrules condensed as liquid droplets from a high‐temperature gas of solar composition is considered. According to this hypothesis, the chondrules were originally highly reduced, and the oxidized iron now present in ordinary chondrites was introduced during later metamorphism. Numerous observations on recrystallization and metamorphism in chondrites effectively contradict this hypothesis, however. Recent evidence concerning shock‐wave phenomena in chondrites and their possible role in trapping primordial rare gases is also discussed. The question of the location of the parent bodies of chondrites remains open. Much recent evidence has suggested that chondrites are derived from the moon. However, earlier views that chondrites are formed by collisions in the asteroidal belt cannot yet be discarded. If the collision theory should prove to be correct, it appears probable that the parent bodies were larger than the present asteroids (but smaller than the moon). There is evidence that chondrites have evolved in a substantial gravitational field. Three theories of origin of chondrites are considered. The hypothesis that chondrules formed by direct condensation as liquid droplets from a gas phase is criticized on numerous grounds. According to another hypothesis, chondrites evolved on parent bodies initially of type I carbonaceous chondrite composition. Internal heating of these bodies by extinct radioactivities caused autoreduction in the interior leading to the formation of a metal phase. The metal phase was followed by a form of volcanism at the surface, leading to formation of chondrules. This hypothesis is shown to be satisfactory in many important respects. Nevertheless, it possesses some serious drawbacks, which must be resolved before the hypothesis can be accepted. Finally, a new hypothesis is suggested, according to which chondrites formed by impact phenomena when planetesimals of type I carbonaceous chondrite composition fell on one or more parent bodies. This hypothesis is highly speculative in its principal aspects. Nevertheless, if subsequent experiments should prove them feasible, this hypothesis would possess many attractive properties, since it offers possible explanations of many phenomena not readily explained by other theories.