Home – Home – Jornals – Russian Geology and Geophysics 2006 number 2

A semi-empirical model is presented, which describes the processes of formation, migration, and accumulation of radiogenic lead (Pb_Rn) in opals deposited in open cavities. In contrast to lead that forms in situ through radioactive decay of uranium trapped by opal, Pb_Rn is produced from uranium disseminated in the rock enclosing the cavities. Its incorporation into the opal is described by the following chain of processes: decay of parental uranium to yield radon - emanation and diffusion migration of radon from the rock into the cavity - decay of radon to yield lead - diffusion migration of lead in the cavity - adsorption of lead on colloidal silica particles - coagulation and settling of the colloidal particles - formation of opal. Besides the colloidal adsorption, Pb_Rn can also be incorporated into the growing opal through direct diffusion flow onto its surface. The latter mechanism is also relevant to minerals crystallizing from ionic solutions; it is less efficient than the mechanism of colloidal adsorption.
Distribution of Pb_Rn isotopes throughout a cavity depends on the cavity geometry and the half-life of the parental Rn isotope. In cavity filled with stagnant water, the concentration profiles of Pb_Rn show maxima at some distance from the cavity wall. The movement of water through a cavity leads to a more complex distribution of Pb_Rn isotopes. The model describes accumulation of Pb_Rn on silica micelles during their growth until the critical size of coagulation (5-10 nm) is reached and during the subsequent coagulation. For micelles 5-50 nm in size, the calculated concentrations of Pb_Rn agree with those in young (Miocene and younger) natural opals.
Opals formed in open cavities in acidic volcanic rocks (with U = 4-5 ppm) can trap significant amounts of Pb_Rn. Applying standard U-Pb dating equations, which ignore Pb_Rn, will yield an overestimated age of such opals. This is true for young opals (younger than ≈30 Ma). As the age of opal increases, the share of radiogenic Pb formed in situ also grows, and the PbRn correction becomes negligible.

The paper deals with the petrography and geochemistry of graphite-bearing granitoids (garnet-biotite granites, leucogranites, pegmatoid granites, pegmatites, plagioclasites, and syenites) composing up to 2 m thick veins among blastomylonites and blastocataclasites in the southeast of the Main Sayan Fault, at the boundary between the Sharyzhalgai uplift of the basement of the Siberian Platform and the Caledonian Slyudyanka crystalline complex. The granitoids are the products of crystallization of low-temperature (760-710

The Keping basalts occur on the western margin of the Tarim plate and are confined to the Kupukuziman and Kaipaizileike Formations of Early Permian age. They are enriched in Fe₂O₃, TiO₂, and P₂O₅ and are undersaturated in SiO₂. The overwhelming majority of the explored specimens belong to the alkali basalt series, and only one specimen, to the tholeiite series. Rare-earth and trace elements are geochemical signatures of the basalts that appeared under the conditions of within-plate extension. They have the following characteristics: ε _Nd ( T ) from -1.73 to -3.69, ε _Sr ( T ) from +27.56 to +56.87; ²⁰⁶Pb/²⁰⁴Pb = 17.87-18.02, ²⁰⁷Pb/²⁰⁴Pb = 15.45-15.53, ²⁰⁸Pb/²⁰⁴Pb = 38.22-38.49. Nd, Sr and Pb isotopic compositions suggest that the Keping basalts have been derived from the Precambrian enriched-type continental lithospheric mantle. This is also typical of the mafic igneous rocks, which are the most abundant in the Tarim basin and on its margin. For this reason, the Xinjiang Uigur Autonomous Region is divided into two obviously different geochemical provinces, southern and northern. The southern and northern provinces are characterized by isotopic compositions of enriched and depleted mantle, respectively. The Late Paleozoic igneous rocks in these two provinces seem to be produced by different geologic processes.

Exemplified by one of the diamond deposits (Guaniamo, Venezuela), it has been shown that the complex study of the internal structure of diamond crystals by the Laue-SR method yields new information on diamond formation in deep-seated zones of continental lithosphere. The diamonds initially grown as sufficiently large crystals underwent several stages in their postgrowth history, owing to changes in both pressure and temperature and to mechanical actions. In their ultimate state, the crystals are flat cleavage platelets, slightly dissolved (or etched), or chips of other configurations with obvious traces and figures of plastic deformation. The weakening of brown color usually observed in deformed but nonannealed diamond crystals and typical traces of polygonization revealed by the Laue-SR method suggest long-lasting annealing, which occurred after the chips subparallel to sliding plane {111} had been separated from the crystals. The applied technique is useful to study diamond crystals in situ and to obtain additional genetic information.

The paper addresses the structure and stress field of faulted upper crust on the eastern side of Lake Baikal. The reported structural map of the study area provides details of crustal heterogeneity and images major fault-bounded blocks. The regional stress field was studied along the mapped faults by structural and tectonophysical methods. The obtained stress pattern indicates faulting in shear and extension environments.

We report the method of azimuthal QVO (QVOA) analysis (seismic quality factor Q versus offset and azimuth) of P -wave attenuation variations as a function of wave-propagation direction in an azimuthally anisotropic (HTI) medium, which is an effective model of vertically fractured rocks. The method is intended to determine crack orientation in fractured reservoirs. The derived approximation of attenuation as a function of the wave-normal direction appears to have the same structure as Rüger's approximation for the PP reflection coefficient widely applied in the azimuthal AVO (AVOA) analysis. We introduced two new parameters: QVO gradient, an azimuth-dependent seismic attribute, and the QVO intercept corresponding to attenuation in the isotropy plane. The use of the QVO gradient, suggested by analogy with the AVO gradient, approaches the new QVOA method to the known AVOA analysis. The QVO gradient is maximum when the source-receiver line is directed along the symmetry axis (or normal to crack planes) and minimum when the two are orthogonal. The relative difference in P -wave attenuation in crack-parallel and crack-normal directions was found out to depend on crack parameters and on V_S/V_P ratio in the background rock, the impact of V_S/V_P being greater than that of the crack density and aspect ratios of cracks and their filling. Azimuthal variations of the QVO gradient can be used to determine crack orientation, and the maximum QVO gradient divided by the intercept provides estimates of V_S/V_P ratio.

Systematic monitoring of the geomagnetic field at secular variation and repeat stations was stopped 15 years ago in expectation of satellite measurements. Comparison of secular geomagnetic variations estimated directly from annual observatory data and those inferred from changes of IGRF-modeled components measured at observatories shows that much of information is lost because satellite measurements cannot provide the appropriate monitoring of space and time secular variations and regional observatory networks are scarce in many regions of the world, especially in Siberia. Therefore, systematic monitoring of the geomagnetic field by the system of observatories and secular variation stations should be resumed.

The paper presents a new method of focusing electrical logging (FL) which implies a nontraditional layout of transmitter and receiver electrodes. This configuration provides compensation of the background signal from formation and highlights anomaly signals. We suggest a 1D numerical model to process the synthetic differential signal from a FL ring loop and analyze apparent resistivity curves obtained from a 1D two-layer

Stress dependence of seismic velocities and attenuation has been studied using the available reported data and original laboratory experiments on quartz single crystals and sandstone samples. The experiments show velocity decrease and attenuation increase with stress in quartz crystals.P and S velocities in sandstone decrease also with changing pore pressure at invariable hydrostatic pressure. The velocity and attenuation functions show open hysteresis loops in a full loading-unloading cycle.