Variations in the elemental composition of ferromanganese structures from Lake Baikal

The elemental composition of ferromanganese sedimentary structures from the bottom of Lake Baikal represented by nodules and crusts, as well as the enclosing sediments, have been studied by the atomic absorption, chemical, and ICP-MS methods. It is established that the contents of the rock-forming and accompanying elements in them are highly variable. In this connection, the examined samples are divided in two groups differing by their Mn/Fe values. In most of the samples, they range from 0.01 to 0.1, although some of the ferromanganese structures or their parts are substantially enriched in manganese. The contents of most of the rare and dispersed elements in the ferromanganese structures are usually at the background level, although the samples maximally enriched in iron or manganese are characterized by relatively high copper, nickel, cobalt, vanadium, zinc, and molybdenum concentrations.     

Smooth models of overshooting at the base of the solar convective zone

A set of smoothed temperature gradient profiles around overshooting layers at the solar convective zone bottom is considered. In classical local theories of convection the one point defined according to the Schwarzschild criterion is enough to describe a convective boundary. To get a sophisticated picture of the overshooting we use four points to compute the transition overshooting functions. Analyzing the transition gradient profiles we found that the overshooting convective flux may be either positive or negative. A negative overshooting flux appears in nonlocal convective theories and causes a steep temperature gradient profile. But we propose an evenly smoothed gradient which corresponds to a convective flux positive everywhere. To outline the effect of the temperature gradient on the solar oscillations the squared Brunt–Väisälä frequency N ² is calculated. In local convective theories the N ² profile shows the discontinuity of the first derivative at the convective boundary, while all smoothed profiles eliminate the break.     

Elemental diffusion and segregation processes in partially ionized solar plasma

Although diffusion is usually associated with equalizing of the chemical composition, the pressure and temperature gradients inside the Sun cause elemental diffusion segregation. While light hydrogen is flowing up to the solar envelope, helium and heavier elements are settling down to the core. The target of our simulation is an accurate estimation of the settling rate in solar plasma during the course of solar evolution. The rate of helium depletion in the envelope is a key parameter of the solar evolution and depends on position and conditions around the base of the convective mixing zone. The rate of heavy element settling is sensitive to the degree of ionization and interaction with the radiation flux. We estimate the effect of ion ionization on the settling rate for several heavy elements up to iron in the framework of the LTE assumption and the thermodynamic calculation according to SAHA-S EOS.     

Hydrothermal manganese mineralization in the Peterbourgskoye ore field (North Atlantic)

The manganese crust covered by pelagic sediment was recovered from the 3 km depth from the Peterbourgskoye ore field located on the eastern flank of the Mid-Atlantic Ridge. The crust comprises a platy brittle aggregate 1–5 cm thick made of black heterogeneous and partly porous material. The inner structure consists of aggregated parallel microplaits several micrometers to 0.2 mm thick consisting of well-crystallized bisnessite with a minor admixture of colloform vernadite. The chemical composition of the crust is dominated by manganese (more than 60% MnO) with minor iron (1.7% Fe₂O₃) and somewhat notable sodium and sulfur. The trace element composition is characterized by very high molybdenum, moderate gallium and uranium, and very low values of 40 other trace elements. Compared to previous publications, the composition of this crust is fairly different from the average values previously defined for hydrothermal crusts. On the other hand, it is rather close to some crusts recovered from subsea volcanoes in the Sea of Japan.     

Solar-Type Stars with the Suppression of Convection at an Early Stage of Evolution

The evolution of a solar-mass star before and on the main sequence is analyzed in light of the diminished efficiency of convection in the first 500 Myr. A numerical simulation has been performed with the CESAM2k code. It is shown that the suppression of convection in the early stages of evolution leads to a somewhat higher lithium content than that predicted by the classical solar model. In addition, the star’s effective temperature decreases. Ignoring this phenomenon may lead to errors in age and mass determinations for young stars (before the main sequence) from standard evolutionary tracks in the temperature–luminosity diagram. At a later stage of evolution, after 500 Myr, the efficiency of convection tends to the solar value. At this stage, the star’s inner structure becomes classical; it does not depend on the previous history. On the contrary, the photospheric lithium abundance contains information about the star’s past. In other words, there may exist main-sequence solar-mass stars of the same age (above 500 Myr), radius, and luminosity, yet with different photospheric lithium contents. The main results of this work add considerably to the popular method for determining the age of solar-type stars from lithium abundances.

     

Modes of iron-manganese mineralization on the bottom of Lake Baikal

The investigation of the bottom of Lake Baikal carried out during the 2008 summer season by means of Mir manned deep-submergence vehicles resulted in the recovery of a series of sediments, ferruginous crusts, and peculiar mineralized tubes several centimeters high and up to 2–6 cm in diameter. According to the scanning electron investigation, these formations consist mainly of the enclosing sediment particles and biogcnie silica cemented by iron and minor manganese hydroxides. The chemical composition of the tubes is similar to both that of the enclosing sediments and slightly ferruginous crusts and nodules, but the tubes and crusts are somewhat richer relative to the sediments in some microelements, namely, arsenic, cadmium, and uranium. In general, the structure and composition of these tubes reminds one of the worm tubes common in the sediments of a number of seas. The investigation of the rare earth elements in some samples or ferruginous formations and sediments revealed a positive europium anomaly, which might be related to either the composition of the surrounding continental magmatic rocks or to the influence of hypothetical hydrothermal solutions.     

The composition of ferromanganese nodules of the Chukchi and East Siberian seas

Ferromanganese nodules from the floors of the Chukchi and East Siberian seas were examined by means of analytical electron microscopy and plasmic and chemical analysis. It was found that the nodules consisted of aggregates of colloform and poorly crystallized matter of globular and irregular shapes, in which the minerals of iron and manganese were mainly presented by hematite, vernadite, and asbolane. The samples from the Chukchi Sea contained single micrograins of native gold. The tested nodules were comparable to oceanic ones in iron content. The contents of manganese, ore elements, and most of microelements are well below those of the ocean. The comparison of the composition of bottom sediments and considered nodules showed that the latter were enriched during their formation by factors of 100–200 in manganese, of 10–18 in cobalt, and of 4–8 in iron, nickel, and zinc. However, the process of diagenetic accumulation of metals by nodules in the marine environment is less effective than in the open ocean with the sediments of higher con-tent of mobile element forms and of higher exposure time compared to marine sediments.