摘要:

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 theremovalof 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 H2O, CO2, H2S, and H2, 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

ISSN:8755-1209    

DOI:10.1029/RG004i002p00113

年代:1966

数据来源: WILEY

摘要:

We propose a model to explain a number of features of the differential rigidity spectra of the nuclear component of the primary cosmic radiation. A supernova explosion is assumed to result in the acceleration of nuclei to a pure power law rigidity spectrum and a composition relatively rich in heavy and medium weight nuclei. The supernova remnant into which these particles are injected is characterized as a large‐scale trapping region resulting from predominantly closed magnetic field lines that partially isolate the region from the surrounding galactic space. This feature of the supernova region is represented by a dipole‐like field. We assume that, superimposed on this field, magnetic irregularities produce a certain amount of distortion of the field lines in the interior of the source, and that irregularities along the periphery of the source lead to local distortions of the field and, hence, a mixing of galactic and source field lines in the neighborhood of these irregularities.The observed large‐scale regularity of the magnetic field in the Crab leads us to treat the particle motion within the source region in the guiding center approximation (except at very high rigidities), so that these particles are capable of guiding outward to the source boundary and returning inward to the interior along the source's predominantly closed magnetic lines of force. The well defined inward field gradient and field curvature, however, cause the particles to drift longitudinally as they spiral along the periphery of the source region, and accordingly they have a certain probability of drifting into a neighborhood of field mixing and onto a galactic field line which guides them out of the source region.During their confinement within the source region, the particles necessarily undergo nuclear interactions and ionization loss, and, therefore, the differential rigidity spectrum of particles escaping from the source boundary into the galaxy is jointly determined by their injection spectrum, their escape probability, and the extent to which they suffer loss processes. A continuity equation combining these various processes is constructed and then averaged over the entirety of active sources in the galaxy. The solutions of the averaged continuity equation for each charge group then give us the differential rigidity spectra with which the cosmic radiation is injected into the galactic medium.The additional effects of intragalactic motion on the cosmic‐ray spectrum are treated in the approximation that the particles escaping from source regions undergo magnetic diffusion in the galactic volume with a rigidity independent scattering mean free path. The differential rigidity spectra of the various charge groups incident on the solar system are thereby obtained. The effects of solar modulation on the galactic cosmic‐ray spectrum are treated in accordance with the solar wind theory. The resulting differential rigidity spectra are derived and compared with data taken near solar minimum, and it is found that the calculated spectra, as well as the rigidity dependence of various abundance ratios, are in reasonable agreement with the avail

ISSN:8755-1209    

DOI:10.1029/RG004i002p00177

年代:1966

数据来源: WILEY

摘要:

The occurrence of graphite as a common accessory mineral in meteorites and in terrestrial metamorphic and igneous rocks gives particular importance to the study of equilibrium between graphite and a coexisting gas phase. By using a simplified model in whichT,Pgas, andfO2are independently specified for the system C‐H‐O, values ofPCO2,PCO,PH2O,PH2andPCH4in a gas phase in equilibrium with graphite have been calculated for a wide range of geologically possible conditions by means of a high‐speed computer. The numerical results support the following general conclusions: (1) The assumption thatPgas=PH2O+PCO2is significantly in error for many graphite‐bearing mineral assemblages. (2) Methane, CH4, may be a significant to dominant constituent of the gas phase in many possible geological environments involving moderate reduction; in particular, the occurrence of graphite with reduced minerals such as fayalite, wüstite, and iron is indicative of a methane‐rich gas phase. (3) Under metamorphic conditions, pure water is not stable with graphite, but graphite can coexist with a gas phase rich in CO2. (4) Original graphite in a sediment will stabilize increasingly reduced mineral assemblages during progressive thermal metamorphism. (5) The presence or absence of even small amounts of graphite can explainPO2gradients observed over short distances or between adjacent layers in metamorphic rocks. (6) It is possible that the terrestrial atmosphere could have evolved by conversion of original methane to water and CO2by reaction with graphite and other accessory minerals within the primordial earth at temperatures of 600° to 1000°C. Material requirements for such a conversion are not unreasonable, and the process itself is consistent with many proposed models for the origin

ISSN:8755-1209    

DOI:10.1029/RG004i002p00223

年代:1966

数据来源: WILEY

摘要:

The results of ionospheric observations by means of a variety of techniques are summarized to show that magnetic field‐aligned irregularities exist in the ionosphere. The irregularities appear in patches having horizontal dimensions up to 1000 km in the east‐west direction and several hundred kilometers in the north‐south direction. Size of the irregularities is typically about 1 km transverse to the magnetic field and elongated along the field line. Irregularities associated with spreadFoccur in the altitude range from 250 km up to the height ofFmax. Correlations between ionogram spreadFand other ionospheric observations indicate that spread echoes are closely related to the field‐aligned irregularities. A critique of theories explaining why the irregularities cause spread echoes is given. Statistical studies of the geographic and temporal variations in spread‐Foccurrence are summarized. The fundamental question of how irregularities causing spreadFare produced in the ionosphere is

ISSN:8755-1209    

DOI:10.1029/RG004i002p00255

年代:1966

数据来源: WILEY

ISSN:8755-1209    

DOI:10.1029/RG004i002p00301

年代:1966

数据来源: WILEY