Home – Home – Jornals – Advanced Search

The study develops methods for approximate simulation of high-frequency relative parameters in models of 2D conducting media using the pseudogeometrical factor approach. Linear representation of magnetic field is applied to obtain equations of pseudogeometrical factors for relative amplitude and phase difference which define conductivity and permittivity dependences of signals. Modeling the contributions of separate zones of the subsurface to the signals allowed inferences on the down-well resolution of sounding. The new algorithms for the integral contributions and pseudogeometrical factors proved to be efficient tools in instrument design and data processing.

The presented overview traces the development of the strain wave theory over the past 35 years. The study includes classification of strain waves reported in the literature and analysis of the relevant terminology. Twenty five basic terms actually denote various features of the deformation process expressed at different scales. The terms are synthesized in tables that display the original definitions and interpretations, the forms of manifestation of strain waves in nature and recording methods, as well as velocities, sources, and hypothetical mechanisms of generation of these waves. The strain wave theory can provide a physical background for explaining the driving mechanisms of seismic migration.

It is now generally agreed that many of the eclogite xenoliths from kimberlites have protoliths that originated as ancient oceanic crust that was subducted beneath the major cratons worldwide. Similar types of evidence, based mainly upon oxygen isotopes, are used to question the paradigm that peridotite xenoliths in kimberlites are of mantle origin. The same general distribution of δ¹⁸O values about the mean mantle value is seen for the peridotites as for the eclogites, both with significant numbers of values distinctly outside of mantle range. It is concluded that at least some of these ultramafic xenoliths, including some diamondiferous ones, have their ultimate origin in the ancient oceanic crust. Indeed, some of the diamonds may have had their carbon derived from crustal sources as well.

One coated and two cubic fibrous diamonds from Brazil carry microinclusions that contain fluids with wide range of composition. Fluid chemistry is similar to that found in diamonds from Botswana and varies between a carbonatitic end-member rich in carbonate, CaO, FeO, MgO, and K₂O and a silicic end-member rich in water, SiO₂, Al₂O₃, and K₂O. The main difference from the Botswanan set is the wider range of compositions sampled by individual diamonds. One diamond, BR-5, is unique and records growth from two contrasting compositions. The inner part grew from silicic fluid, and the outer part, from a carbonate-rich one. Carbon isotopic compositions vary between diamonds and radially within individual diamonds. Silicic fluids are associated with heavier isotopic compositions (most analyses >-5‰); carbonate-rich fluids with lighter values (most analyses <-5‰). Radial evolution in different diamonds is contrasting but is mostly towards the median value of -5‰. Nitrogen isotopes show more scatter but correlate positively with carbon isotopic composition. It is suggested that fluid chemistry and diamond isotopic composition are affected mainly by fractionation of carbonates and diamonds (and possibly silicates). Separation of CO₂ and interaction of the fluid with host-rock carbon may also be important in controlling the isotopic composition.

Experimental data on melting of carbonatized mantle peridotites, phase equilibria during the high-pressure crystallization of kimberlite melts, and solubility of CO₂ in kimberlite-like melts are analyzed. Melting of lherzolite at ~5 GPa in the presence of CO₂ yields a wide spectrum of compositions depending on the content of the latter: from high-magnesium picritic magmas in CO₂-free systems to dolomitic melts containing <5 wt.% CO₂. Low-calcium melts of kimberlite composition from this series contain ~20 wt.% CO₂. Experimental studies showed that the solubility of CO₂ in kimberlite melts drastically increases at pressures of >4.5 GPa and reaches 20 wt.% at 5 GPa. At ~6 GPa, there is a stable garnet + orthopyroxene + magnesite association on the liquidus of CO₂-saturated kimberlite melt. Experimental results indicate that the melting of magnesite-bearing harzburgite at ~6 GPa produces CO₂-saturated kimberlite-like melts. Analysis of geochemical data showed that the ratios of CO₂ to elements of a close degree of incompatibility (e.g., Th) in such hypothetic melts are nearly the same as in the primitive and depleted mantle. Thus, deep metasomatism of the mantle source is not necessary for the formation of kimberlite magmas, and high contents of incompatible elements might be the result of the extremely low degrees of rock melting. The proposed model for the formation of kimberlite magmas implies the interaction of melts from the asthenospheric mantle with garnet harzburgite in the lower continental lithosphere. This process can lead to CO₂ saturation of the melts at a depth of ~200 km, which will favor a rapid magma ascent and transport of deep-seated minerals.

Alkaline magmatism originated at 2.5-2.7 Ga and continuously developed throughout the Earth's history. The appearance of alkaline rocks coincided in time with change in geodynamic regime: The plume tectonics was supplemented by plate tectonics. Global plate-tectonic events at the Archean-Proterozoic boundary caused the subduction of the strongly oxidized volatile-enriched oceanic crust and gave rise to a large-scale mantle metasomatism, which then led to the formation of volatile-enriched reservoirs, the sources of alkaline-carbonatite magmatism. Ion microprobe studies of the metasomatized mantle material revealed impurities of primary carbonate melts highly enriched in trace elements. Based on the results obtained, a new model of the two-stage genesis of Ca-rich carbonatites is proposed: (1) metasomatic wehrlitization and carbonatization of the mantle material, (2) partial melting of the wehrlitized mantle, resulting in either carbonate-rich melts or immiscible liquids (in the excess of alkalies) - silicate, carbonatite, and sulfide (at the high activity of sulfur).
Metasomatic fluids were supplied, most likely, with plumes ascending from the core-mantle boundary.

This study was given to specific features and compositions of mineral inclusions in diamonds from the Botuobinskaya pipe. The initial collection of diamonds (91 specimens taken of the three hundred diamonds inspected) was represented by colorless and transparent crystals, chiefly, of octahedral shape, 3-4 mm in size. The imprisoned minerals were exposed to the day by polishing of diamonds and were instrumentally studied in situ.
The suite of revealed mineral inclusions combines a group of silicates and oxides, which were established in 28 crystals, and sulfides present in 65% of the crystals from the studied lot of diamonds. The silicate minerals are dominated by Cr-spinels with more than 61.0 wt.% Cr₂O₃, with subordinate amounts of garnet and olivine. Rutile, phlogopite, and sanidine are present as solitary inclusions. Sulfides are represented by pyrrhotite, pentlandite, and monosulfide solid solutions with scarce blocks and thin rims of chalcopyrite.
Analysis of compositions of mineral inclusions in diamonds of the Botuobinskaya pipe, with the sulfide phases taken into account, shows that the eclogite paragenesis makes up more than 50% of the bulk of inclusions in diamonds of this kimberlite body. The ultrabasic paragenesis makes up 45%, and about 3.5% of the crystals belong to the pyroxenite association. The high percentage of eclogite paragenesis among inclusions in diamonds is anomalous in the diamond populations in kimberlite pipes of the Yakutian Diamond Province. This suggests a specific composition of the medium of diamond formation and, correspondingly, upper mantle in the region of the new Nakyn kimberlite field.

The Paleoproterozoic collisional system of the northeastern Siberian craton is compared with the underlying lithospheric mantle. This system appeared at about 1.9-1.8 Ga, by accreting microcontinents with an age of 3.1-2.5 Ga. Evidence comes from isotope dating of the formation of ancient terranes, their thermal transformation and melting of collisional granitoids. The crustal structure inferred from geological and geophysical data bear a relict signature of collisional systems, including deformations, up to 58 km thickened crust, and even slope of seismic surfaces along the predicted directions of collisional overriding.
The crustal structures are underlain by thick, up to 260-300 km, diamondiferous lithospheric mantle with higher seismic velocities, which thins out to ≤200 km toward the margins of the region. This local bulge may be identified as a lithospheric keel (root). The spatial relationship between this mantle keel and crustal collisional system of Proterozoic age is geometrically evident, and magmatic events are obviously coeval. But proportions of relevant processes are not clear. The simplest supposition is that the keel formed as a result of accretion of fragments of the Archean lithospheric mantle together with the crustal terranes attached on top. This supposition contradicts the existing ideas that only the crust participated in continental collision, whereas the underlying mantle slipped free and far away. It should be the subject of future studies.

Study of xenoliths from the Udachnaya and Leningradskaya kimberlite pipes has shown that, among the rocks of the bottom of the Earth's crust in the Daldyn-Alakit district, the garnet granulites make up no less than 50%. Geochemical features of garnet granulites (high percentage of potassium and incompatible elements) show that they cannot be considered restites but are fragments of deep-seated intrusions crystallized in the lower crust. According to estimated pressure and temperature of equilibrium, the lower crust is dominated by garnet granulites (9-13 kbar), which upsection grade into two-pyroxene granulites (8.5-10 kbar).
The Sm-Nd dating of xenoliths in garnet granulites shows no isotope equilibrium. An inner isochron was obtained for none of the specimens. Model ages of the xenoliths evidence that most of the lower crust in the Yakutian Diamond Province was formed in the Archean (2.97-2.75 Ga). At the same time, the inner Pb-Pb isochron (1424±21 Ma) for a specimen of garnet granulites from the Leningradskaya kimberlite pipe as well as the model age (1.24 Ga) of granulite from the Udachnaya kimberlite pipe indicate the Neoproterozoic stage of thermal activity.
On the basis of data on velocities of travel of compressional waves in the lower crust (6.8-7.0 km/s) and obtained estimates of xenoliths of garnet granulites in kimberlite pipes, a conclusion is made that in the lower crust of the Daldyn-Alakit district garnet granulites form separate bodies in gneisses rather than a separate bed.