Sikhote Alin' meteoroid: A contribution to the story of its fragmentation and fragment scattering

The correlation of the size and spatial distribution of fragments from the Sikhote Alin' meteoroid, 1947, was examined.With Frost's logarithmic rule of spatial sorting of meteorite fragments, supported by an ad hoc developed averaging procedure, a new approach to the size distribution was made possible.It was found that the meteoroid has been originally broken into several - presumably 5–6 - large fragments unequal in size, which further fragmented with various efficiency and speed.The conclusions are compatible with the reports of the witnesses of the fall of the meteorite.     

Pressure-induced structural modifications in Mg2GeO4-olivine: A Raman spectroscopic study

Raman spectra of Mg₂GeO₄-olivine were obtained from ambient pressure up to 34 GPa at ambient temperature. Under quasi-hydrostatic pressure conditions, the following modifications in the Raman spectra occur as pressure increases: 1) near 11 GPa, two sharp extra bands appear in the 600–700 cm^?1 frequency range, and increase in intensity with respect to the olivine bands; 2) above 22 GPa, these two bands become very intense, and the number, position and relative intensity of the other vibrational bands drastically change; 3) the intensity of sharp bands progressively decreases above 25 GPa. The transformation occurs at lower pressures under non-hydrostatic conditions. During decompression to atmospheric pressure, the high-pressure phase partially reverts to olivine. These observations can be interpreted as the progressive metastable transformation from the olivine structure to a crystalline phase with four-fold coordinated Ge, in which the GeO₄ tetrahedra are polymerized. We propose that the metastable high-pressure phase is a structurally disordered spinelloid close to the hypothethical ω- or ?^*-phase, and forms by a shear mechanism assisted by the development of a dynamical instability in the olivine structure. Implications for the transformations undergone by olivines under far-from-equilibrium conditions (e.g. in subducting lithospheric slabs and in shocks) are discussed.     

Selected solar wind parameters at 1 AU through two solar activity cycles

In situ measurements of the solar wind largely cover more than two solar magnetic activity cycles, namely 20 and 21. This is a very appealing opportunity to study the influence of the activity cycle on the behaviour of the solar wind parameters. As a matter of fact, many authors so far have studied this topic comparing the long-term magnetic field and plasma averages. However, when the average values are evaluated on a data sample whose duration is comparable with (or even longer than) the solar rotation period we lose information about the contribution due to the fast and the slow solar wind components. Thus, discriminating in velocity plays a key role in understanding solar cycle effects on the solar wind. Based on these considerations, we performed a separate analysis for fast and slow wind, respectively. In particular, we found that: (a) fast wind carries a slightly larger momentum flux density at 1 AU, probably due to dynamic stream-stream interaction; (b) proton number density in slow wind is more cycle dependent than in fast wind and decreases remarkably across solar maximum; (c) fast wind generally carries a magnetic field intensity stronger than that carried by the slow wind; (d) we found no evidence for a positive correlation between velocity and field intensity as predicted by some theories of solar wind acceleration; (e) our results would support an approximately constant divergence of field lines associated with corotating high-velocity streams.     

Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics

In order to explore mantle heterogeneity through geological time, Pb isotopic compositions have been determined for 8 differentiated-layered intrusions whose ages are between 2.7 and 0.05 b.y.. The Pb-Pb ages of these intrusions and the U-Pb characteristics of their parent sources are discussed.The Pb-Pb dating method is found to be applicable for this type of basic or ultrabasic rocks and agrees satisfactorily with other available methods. Significant differences are found between the calculated values for the parent bodies of these intrusions. This could reflect either mantle heterogeneity since Archean times, or contamination of some of the bodies by continental crust. Discrimination between these hypotheses can be proposed from the positions of the initial Sr ratios of these massives in respect to the supposed “terrestrial” evolution line. The two intrusions which plot on that closed system Sr evolution line (Muntsche Tundra, U.S.S.R., and Skaergaard, Greenland) belong also to a simple two-stage evolution model for Pb, with a low first-stage μ value of 7.8. If these two bodies are considered as pieces of a “primitive” closed-system mantle, a4.55 ± 0.01 age of the earth can be calculated from their Pb initial ratios.     

X-ray determination and equilibrium composition of clinopyroxenes in the system CaO-MgO-Al2O3-SiO2

Ternary clinopyroxenes have been synthesized in the plane Di-CaTs-En. The variation of their crystallographic parameters has allowed the development of three determinative grids, which utilize b-, 2 _22¯1–2 ₃₁₀ and 2 ₃₃₀ –2 ₂₀₂ respectively. These grids show significant differences in comparison with the previously proposed ones. Present results have been used to review some data on clinopyroxenes equilibria in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS) (Biggar 1969; Bruno and Facchinelli 1978; Herzberg 1978; O'Hara and Schairer 1963; Boyd 1969). In particular the petrogenic grid correlating P, T and CaTs content of clinopyroxenes in spinel-lherzolite assemblage (Herzberg 1978) has been revised, and consequently equilibrium temperatures rise by one hundred degrees.     

Discovery of the largest impact crater field on Earth in the Gilf Kebir region, Egypt

Using orbital imaging radar, we have detected a large number of circular structures in the southwestern Egyptian desert, covering more than 4500 km² close to the Gilf Kebir plateau in sandstones of Upper Cretaceous. Fieldwork confirmed that it is a new impact crater field: 13 craters from 20 m to 1 km in diameter were studied. The impact origin is confirmed by the observation of shock-related structures, such as shatter cones and planar fractures in quartz grains of breccia. Considering the extension of the crater field, it was possibly created by several meteorites that broke up when entering the atmosphere. To cite this article: P. Paillou et al., C. R. Geoscience 336 (2004).

Résumé

À partir d'images satellites issues de systèmes radar, nous avons détecté un grand nombre de structures circulaires dans le Sud-Ouest du désert égyptien, s'étendant sur plus de 4500 km² à l'est du plateau du Gilf Kebir, dans des grès du Crétacé supérieur. Une étude sur le terrain a permis de vérifier qu'il s'agit d'un champ d'impacts météoritiques jusque là inconnu : 13 structures, d'un diamètre compris entre 20 m et 1 km, ont été reconnues comme cratères d'impact. La présence d'un grand nombre de cônes de percussion et de brèches dans lesquelles des quartz choqués à structures planaires ont été observés confirment l'hypothèse de l'impact. Considérant l'extension du champ d'impact, il a probablement été créé par plusieurs météorites qui se sont fragmentées dans l'atmosphère terrestre. Pour citer cet article : P. Paillou et al., C. R. Geoscience 336 (2004).     

The Northern Cordilleran Mid-Cretaceous Plutonic Province: Ilmenite/Magnetite-series Granitoids and Intrusion-related Mineralisation

Abstract. Twenty-five Early and mid-Cretaceous (145–90 Ma) plutonic suites and belts are defined across Alaska and Yukon, in the northern North American Cordillera, on the basis of lithological, geochemical, isotopic, and geochronometric similarities. These features are combined with aeromagnetic characteristics, magnetic susceptibility measurements, and whole-rock ferric: ferrous ratios to ascertain the distribution of magnetite- and ilmenite-series plutonic belts. Magnetite-series plutonic belts are dominantly associated with the older parts of the plutonic episode and comprise subduction-generated metaluminous plutons that are distributed preferentially in the more seaward localities dominated by primitive tectonic elements. Ilmenite-series plutonic belts comprise slightly younger, slightly peraluminous plutons in more landward localities in pericratonic to continental margin settings. They were likely initiated in response to crustal thickening following terrane collision. The youngest plutonic belt forms a small, but significant, magnetite-series belts in the farthest inboard position, associated with alkalic plutons that were emplaced during weak extension.
Intrusion-related metallogenic provinces with distinctive metal associations are distributed, largely in accord with classical redox-sensitive granite-series. Copper, Au, and Fe mineralisation are associated with magnetite-series plutons and tungsten mineralisation associated with ilmenite-series plutons. However, there are some notable deviations from expected associations, as intrusion-related Ag-Pb-Zn deposits are few, and significant tin mineralisation is rare. Most significantly, many gold deposits and occurrences are associated with ilmenite-series plutons: these form the basis for the newly recognized reduced intrusion-related gold deposit model.     

A kinetic model for thermally induced hydrogen and carbon isotope fractionation of individual n-alkanes in crude oil

A quantitative kinetic model has been proposed to simulate the large D and ¹³C isotope enrichments observed in individual n-alkanes (C₁₃-C₂₁) during artificial thermal maturation of a North Sea crude oil under anhydrous, closed-system conditions. Under our experimental conditions, average n-alkane δ¹³C values increase by ∼4‰ and δD values increase by ∼50‰ at an equivalent vitrinite reflectance value of 1.5%. While the observed ¹³C-enrichment shows no significant dependence on hydrocarbon chain length, thermally induced D-enrichment increases with increasing n-alkane carbon number. This differential fractionation effect is speculated to be due to the combined effect of the greater extent of thermal cracking of higher molecular weight, n-alkanes compared to lower molecular weight homologues, and the generation of isotopically lighter, lower molecular weight compounds. This carbon-number-linked hydrogen isotopic fractionation behavior could form the basis of a new maturity indicator to quantitatively assess the extent of oil cracking in petroleum reservoirs. Quantum mechanical calculations of the average change in enthalpy (ΔΔH^‡) and entropy (ΔΔS^‡) as a result of isotopic substitution in n-alkanes undergoing homolytic cleavage of C-C bonds lead to predictions of isotopic fractionation that agree quite well with our experimental results. For n-C₂₀ (n-icosane), the changes in enthalpy are calculated to be ∼1340 J mol^-1 (320 cal mol^-1) and 230 J mol^-1 (55 cal mol^-1) for D-H and ¹³C-¹²C, respectively. Because the enthalpy term associated with hydrogen isotope fractionation is approximately six times greater than that for carbon, variations in δD values for individual long-chain hydrocarbons provide a highly sensitive measure of the extent of thermal alteration experienced by the oil. Extrapolation of the kinetic model to typical geological heating conditions predicts significant enrichment in ¹³C and D for n-icosane at equivalent vitrinite reflectance values corresponding to the onset of thermal cracking of normal alkanes. The experimental and theoretical results of this study have significant implications for the use of compound-specific hydrogen isotope data in petroleum geochemical and paleoclimatological studies. However, there are many other geochemical processes that will significantly affect observed hydrogen isotopic compositions (e.g., biodegradation, water washing, isotopic exchange with water and minerals) that must also be taken into consideration.     

Contribution of galaxies to the background hydrogen-ionizing flux

We estimate the evolution of the contribution of galaxies to the cosmic background flux at 912 Å by means of a semi-analytic model of galaxy formation and evolution. Such modelling has been quite successful in reproducing the optical properties of galaxies. We assume that high-redshift damped Lyman α systems are the progenitors of present-day galaxies, and we design a series of models that are consistent with the evolution of cosmic comoving emissivities in the available near-infrared, optical, ultraviolet and far-infrared bands along with the evolution of the neutral hydrogen content and average metallicity of damped Lyman α systems. We use these models to compute the galactic contribution to the Lyman-limit emissivity and background flux for 0 ≃  z  ≤ 4. We take into account the absorption of Lyman-limit photons by H  I and dust in the interstellar medium of the galaxies. We find that the background Lyman-limit flux due to galaxies might dominate (or be comparable to) the contribution from quasars at almost all redshifts if the absorption by H  I in the interstellar medium is neglected. Such H  I absorption would result in a severe diminishing of this flux — by almost three orders of magnitude at high redshifts and by one to two orders at z  ≃ 0. Though the resulting galaxy flux is completely negligible at high redshifts, it is comparable to the quasar flux at z  ≃ 0.