Home – Home – Jornals – Russian Geology and Geophysics 2008 number 3

Two samples of Na-substituted leucite with Na/(Na + K) = 0.74 (1) and 0.89 (2) were studied by dilatometry, thermogravimetry, and X-ray powder diffraction method. Sample 1 is a mixture of analcime and leucite, and sample 2 consists of analcime alone. The miscibility of K⁺ and Na⁺ in hydrated analcime is estimated at 15-20 at.% K⁺. The thermal behavior of the samples varies depending on their Na/K ratio. Dehydrated Na,K-analcime undergoes a trigonal distortion; its structural model is proposed. The earlier recognized difference in the thermal behavior of primary and secondary (obtained from leucite through K⁺-Na⁺ exchange) analcimes might be due to the influence of unsubstituted K⁺.

Epidote is traditionally considered a mineral indicating metamorphic and metasomatic genesis of rocks. Recently, the magmatic nature of epidote of many granitoid intrusions has been proved. We present the first data on the magmatic origin of epidote, which is one of the rock-forming minerals of anorthite fassaite and biotite-amphibole gabbros. These rocks form block inclusions in the serpentinite melange plate localized in gneisses of the Proterozoic Selyankino strata of the Il'meny Mountains complex in the South Urals. The holocrystalline structure and cenotypal appearance of the epidote gabbros, their gabbro, porphyritic, and ophitic textures, the induction surfaces of synchronous growth between the hosted minerals, the mineral crystal faceting, and high-temperature paragenesis evidence the magmatic origin of the rocks.

The JXA-8100 microanalyzer software provides precise electron-probe microanalysis of rock-forming minerals, with a relative deviation within 1.0-1.5%, if the matrix effect in the samples is predominantly determined by only one physical factor: either atomic number or absorption. If the matrix effect is of binary character, i.e., is owing to the simultaneous action of two commensurate factors (absorption and atomic number), the analysis error grows up to 4-14%.

Modern hydrotherms of the Baikal Rift Zone (BRZ) are divided according to contents of ore and rare elements and gas composition into: (1) carbonaceous, forming in carbonate rocks, (2) hydrosulfuric and H₂S-free nitric and methane, discharged in Cenozoic deposits, and (3) nitric, produced in granitoids. Carbonaceous hydrotherms are enriched in Sr, Ba, Y, Co, and Cr; nitric ones, in Li, Rb, Cs, Mo, W, and other granitophile elements; and hydrosulfuric hydrotherms are saturated with complex metals. High contents of U and associated elements are specific for oxidized nitric alkaline thermal waters and are atypical of carbonaceous ones, and high contents of Au are observed in methane hydrotherms. Separation of ore elements between organic and clayey matter takes place as early as the initial stage of sediment formation. During sediment accumulation, these elements pass into hydrothermal solution in the same separated associations. In oxidizing (Eh ranges from +50 to +260 mV) alkaline medium, U and, obviously, other elements of uranium association, present not only in granitoids but also in the organic matter of sediments, pass into the solution most actively. Alkaline medium (7 < pH < 10) favors the transition of Mo and W from igneous rocks and sediments into the solution. Methane hydrotherms (8 < pH < 9, Eh < -70 mV) can accumulate gold. In strongly reducing (Eh ranges from -200 to -360 mV) alkaline (pH > 8) hydrosulfuric anoxic medium, chalcophile complex metals of the clayey matter of sediments are the most active to pass into the solution. Thus, as Eh decreases and H₂S concentration increases, hydrothermal solution extracts various element associations from igneous and sedimentary deposits in the sequence U-Mo, W-Au-Sn, Cu, Pb. In the solution the differentiation of elements continues. Hence, the separation of ore-element associations at the initial stage of sediment formation, long before the hydrothermal activity, is the first track in a chain of processes leading to the differentiation of ore deposits according to formational types. Hydrotherms confined to deep-fault zones contain an endogenous fluid with mantle ³He/ ⁴He values close to those in methane and carbonaceous hydrotherms of the Tunka rift depression in the BRZ. According to results of thermodynamic modeling, the fluid consists of CH₄, H₂O, and CO₂ and obviously lacks mantle ore elements, as it was initially of hydrocarbon composition.

The petroleum potential of the Riphean-Vendian Chunya sedimentary basin has been explored by seismic reflection profiling and drilling in recent years. The results of the study have been used to estimate the initial hydrocarbon resources in the basin and separately in four oil and gas areas distinguished in Riphean, Lower Vendian, and Vendian-Lower Cambrian reservoirs.

The task was to detect the groundwater contamination with hydrocarbons (gasoline, kerosene, diesel fuel) without drilling special boreholes, by means of NMR geotomography. Near the town of Abakan (Khakasia, RF), experimental measurements were carried out at a gas station where gasoline leakage is possible and the contamination of groundwaters was monitored through observation wells. In the immediate vicinity of the source of pollution, signals with essentially different times of relaxation were rather distinct: Short signals came from hydrogen protons of water, and much longer signals, from hydrogen protons of gasoline.

Tomographic inversion of ~130,000 Pand ~11,000 Sarrivals from 1045 events recorded by the world seismological network (ISC catalog data) has been applied to image the three-dimensional velocity structure of the upper mantle beneath the Baikal rift and Mongolia. The inversion-derived P and S velocity anomalies show a good agreement. At depths above 200 km, low-velocity zones occur along the contours of the high-velocity Siberian craton and in Mongolia and coincide with fields of Cenozoic volcanism. The deeper mantle appears quite homogeneous, with anomalies no greater than 0.5% and a single low-velocity feature beneath the Siberian craton. The tomographic image is poorly consistent with the hypothesis implying the existence of large mantle plumes under Mongolia which has been checked with synthetic tests. According to the tomography-based geodynamic model, volcanism in the East Sayan mountains may be induced by a hot plume rising from beneath the Siberian craton, but the source of volcanism in the area of Hangayn remains open to discussion.

Ungrounded horizontal loop transient responses of uniform conductive and magnetically viscous earth have been simulated using two different codes. One algorithm employs the relationship between viscous magnetization and the magnetic flux it induces in the receiver loop. In the other algorithm, the Helmholtz equation in a boundary-value problem is solved using the Fourier transform with frequency-dependent magnetic permeability. The two solutions are identical for noncoincident loops but differ when the transmitter and receiver loops are closely spaced (at 1 cm or less). In the latter case correct results are provided by the first code. The magnetic relaxation and eddy current responses appear to be independent at conductivities typical of the real subsurface. Therefore, TEM responses of magnetically viscous conductors can be computed using the superposition principle. Although transients change in an intricate way as a function of loop geometry and earth parameters, these changes exhibit certain patterns which may be useful at the stages of exploration and TEM data processing. In configurations where the receiver loop is laid outside the transmitter, the interaction of magnetic relaxation and eddy current decay causes sign reversal in transients. This reversal occurs at late times after an earlier sign reversal due uniquely to eddy current.