Physicochemical formation conditions of banded iron formations and high-grade iron ores in the region of the Kursk Magnetic Anomaly: Evidence from isotopic data

The oxygen and carbon isotopic compositions of minerals from banded iron formations (BIFs) and high-grade ore in the region of the Kursk Magnetic Anomaly (KMA) were determined in order to estimate the temperature of regional metamorphism and the nature of rock-and ore-forming solutions. Magnetite and hematite of primary sedimentary or diagenetic origin have δ¹⁸O within the range from +2 to 6‰. During metamorphism, primary iron oxides, silicates, and carbonates were involved in thermal dissociation and other reactions to form magnetite with δ¹⁸O = +6 to +11‰. As follows from a low δ¹⁸O_av = ?3.5‰ of mushketovite (magnetite pseudomorphs after hematite) in high-grade ore, this mineral was formed as a product of hematite reduction by organic matter. The comparison of δ¹⁸O of iron oxides, siderite, and quartz from BIFs formed at different stages of the evolution of the Kursk protogeosyncline revealed specific sedimentation (diagenesis) conditions and metamorphism of the BIFs belonging to the Kursk and Oskol groups. BIF of the Oskol Group is distinguished by a high δ¹⁸O of magnetite compared to other Proterozoic BIFs. Martite ore differs from host BIF by a low δ¹⁸O = ?0.2 to ?5.9‰. This implies that oxygen from infiltration water was incorporated into the magnetite lattice during the martite formation. Surface water penetrated to a significant depth through tectonic faults and fractures.     

Integration prospects of the Hungarian transport network into the more advanced European networks

In spite of the advantageous transport-geographic situation of Hungary, the four decades of the so called planned socialist economy, neglecting the transportational infrastructure, made the country lag behind the rest of Europe. The two main points in the current development of the Hungarian transportation system are the establishment of proper links with the more developed western European transportational network, and starting a substantial technical development in all respects of domestic transportation in order to approach the standards of the more developed countries. The paper reviews the main characteristics of the project of the Hungarian development of transport by road and rail.     

Transformation of enstatite — diopside — jadeite pyroxenes to garnet

The high-pressure stability of enstatite(En)-diopside(Di)-jadeite(Jd) pyroxenes has been investigated experimentally with a split-sphere anvil apparatus (USSA-2000). On the enstatite-pyrope join, the compositions of garnet coexisting with enstatite were determined at 100–165 kbar and 1450–1850° C. The results indicate complete solubility between enstatite and pyrope. In the system CaO-MgO-Al₂O₃-SiO₂ (CMAS), the compositions of coexisting pyroxenes and garnet were determined at 100–165 kbar and 1250–1750° C. At 157 kbar, 1650° C, garnet with the composition En₇₉Di₂₁ (mol%) forms on the En-Di join. In the system Na₂O-MgO-Al₂O₃-SiO₂ (NMAS), the compositions of coexisting pyroxenes and garnet were determined at 60–160 kbar and 1200–1850° C. On the En-Jd join, the first garnet has the composition En₄₈Jd₅₂ at 135 kbar, 1650° C, and En₅₃Jd₄₇ at 140 kbar, 1500° C. On the Di-Jd join, the first garnet with the composition Di₆₃Jd₃₇ forms around 170 kbar, 1650° C. In the En-Di-Jd system, the first appearance of garnet with the composition En₄₂Di₉Jd₄₉ is estimated at 133 kbar, 1650° C. The new pyroxene with the composition NaMg_0.5Si_2.5O₆ (NaPx) transforms to garnet at 154 kbar, 1650° C. The experimental results indicate that the transformation of a twopyroxene assemblage to garnet and residual pyroxene in the En-Di-Jd system could occur at pressures consistent with the 400 km seismic discontinuity and in a pressure interval of 0–3 kbar.     

Experimental study of subsolidus phase relations and mixing properties of pyroxene and plagioclase in the system Na2O-CaO-Al2O3-SiO2

In the system Na₂O-CaO-Al₂O₃-SiO₂ (NCAS), the equilibrium compositions of pyroxene coexisting with grossular and corundum were experimentally determined at 40 different P-T conditions (1,100–1,400° C and 20.5–38 kbar). Mixing properties of the Ca-Tschermak — Jadeite pyroxene inferred from the data are (J, K): $$\begin{gathered} G_{Px}^{xs} = X_{{\text{CaTs}}} X_{{\text{Jd}}} [14,810 - 7.15T - 5,070(X_{{\text{CaTs}}} - X_{{\text{Jd}}} ) \hfill \\ {\text{ }} - 3,350(X_{{\text{CaTs}}} - X_{{\text{Jd}}} )^2 ] \hfill \\ \end{gathered} $$ The excess entropy is consistent with a complete disorder of cations in the M2 and the T site. Compositions of coexisting pyroxene and plagioclase were obtained in 11 experiments at 1,190–1,300° C/25 kbar. The data were used to infer an entropy difference between low and high anorthite at 1,200° C, corresponding to the enthalpy difference of 9.6 kJ/mol associated with the C \(\bar 1\) =I \(\bar 1\) transition in anorthite as given by Carpenter and McConnell (1984). The resulting entropy difference of 5.0 J/ mol · K places the transition at 1,647° C. Plagioclase is modeled as ideal solutions, C \(\bar 1\) and I \(\bar 1\) , with a non-first order transition between them approximated by an empirical expression (J, bar, K): $$\Delta G_T = \Delta G_{1,473} \left[ {1 - 3X_{Ab} \tfrac{{T^4 - 1,473^4 }}{{\left( {1,920 - 0.004P} \right)^4 - 1,473^4 }}} \right],$$ where $$\Delta G_{1,473} = 9,600 - 5.0T - 0.02P$$ The derived mixing properties of the pyroxene and plagioclase solutions, combined with the thermodynamic properties of other phases, were used to calculate phase relations in the NCAS system. Equilibria involving pyroxene+plagioclase +grossular+corundum and pyroxene+plagioclase +grossular+kyani te are suitable for thermobarometry. Albite is the most stable plagioclase.     

Die Schwerewirkungen eines elliptischen Zylindersegmentes

Summary A method has been derived for computing the gravity effect of a segment of an infinite homogeneous elliptical cylinder. The initial data was represented by formulae expressing the components of the field of gravity of a homogeneous two-dimensional body by means of line integrals. The method is based on the integration of theln R function over the boundary of the cross-section of the attracting body, R being the distance from a fixed point in which the gravity effect is determined. The problem was solved in confocal co-ordinates.     

A model for the layered upper mantle

Numerical modeling of mantle convection by Liu (1994, Science, 264: 1904–1907) favors a two-layer convection, if the results are reinterpreted for the correct phase relations in (Mg,Fe)₂SiO₄. The resulting chemical isolation of the upper and lower mantle suggests a highly differentiated and layered upper mantle to account for the discrepancy between the observed compositions of mantle xenoliths and the cosmic abundances of elements. It is shown that a layered upper mantle with a hidden reservoir can have a structure consistent with the observed seismic velocity profiles and an average bulk composition corresponding to the cosmic abundances. The evolution of the upper mantle and the origin of komatiites are discussed in the context of the proposed model.