Home – Home – Jornals – Russian Geology and Geophysics 2011 number 5

Experimental data on phase transformations and melting in peridotite and eclogite systems with a C-O-H fluid at 6-30 GPa have been analyzed with special attention to the influence of redox conditions. It has been found that melting in systems with H₂O depends heavily on its total content and considerably on its solubility in nominally anhydrous rock-forming minerals. Partial melting occurs when the total H₂O content of the system exceeds the H₂O storage capacity in the rock under given physicochemical conditions. Melting in CO₂-containing systems is determined by carbonate stability and the chemical composition of the system, mainly its Na₂O and K₂O contents, and, to a smaller extent, the content of CO₂ itself. Studies of peridotite and eclogite systems containing H₂O, CO₂, H₂O + CO₂, and a reduced C-O-H fluid show that most solidi flatten out at pressures above 6-8 GPa when intersecting the geotherms of subduction and average mantle. Mantle melting at constant pressure in the presence of a C-O-H fluid depends not only on temperature but also on redox conditions. The oxidation of the system causes redox melting. The stability boundary of a Fe-Ni alloy (it may coincide with the lithosphere-asthenosphere boundary under cratons, 200-250 km) and the 410 km discontinuity are paramount to redox and decarbonatization-dehydration melting. Also, the paper provides evidence that subducted carbonates play the leading role in the "big" mantle wedge model for stagnant slabs. Volatile-containing eclogite systems melt at lower temperatures than peridotite ones (the difference is up to 100-200 °C). This suggests that eclogites are of global importance in mantle melting, which agrees with modern geochemical models.

We studied the isotopic composition of lead in feldspars from the intrusive rocks of enclosing, ore-bearing porphyry, and post-ore complexes and in sulfides from the Sora magmatic center including the Sora porphyry Cu-Mo deposit. The arrangement of the isotopic composition points below the orogen evolution curve on the uranogenic lead diagram evidences a predominance of lead of mantle genesis in the igneous rocks and a minor content of crustal lead.
On the ²⁰⁷Pb/²⁰⁴Pb - ²⁰⁶Pb/²⁰⁴Pb diagram, the lead of magmatites forms a linear sequence (trend), which is interpreted as mixing of lead from different sources. According to the geochemical parameters, weakly depleted mantle metasomatized by subduction fluids was a source of magmatism. The linear correlation between the ratios of Pb isotopes in the igneous rocks of the Sora magmatic center is explained by the mixing of the mantle and subduction fluids. Mantle lead is predominant in the basites of the enclosing, porphyry, and post-ore complexes and in the leucogranites of the enclosing complex. According to lead isotopy data, the differentiated magmatites (enclosing monzonitoids, ore-bearing porphyry, and post-ore syenite-porphyry) contain a subduction component, whose content increases in passing from early to late complex.
The Pb-isotopic composition of sulfides suggests the mantle source of Pb in molybdenite of the breccia ores and the heterogeneous source (mantle and subduction fluids) of Pb in chalcopyrite of the veinlet ores.

In the Tanuu-Ola zone, there are island-arc (Late Vendian) and subduction-accretionary (first half of the Early Cambrian) complexes whose mantle sources were generated fr om melts produced in suprasubduction conditions as a result of melting of mantle wedge peridotites. The island arc, whose fragment is present in the Tannu-Ola zone, formed tentatively at 570 Ma and ceased as an individual structure at the Vendian-Cambrian boundary, though the subduction continued till the middle Early Cambrian. From the Vendian-Cambrian boundary to the middle Early Cambrian, the accretion of island-arc, back-arc, oceanic, and metamorphic structure-lithologic complexes took place at this segment of the Central Asian Fold Belt. From middle to late Early Cambrian, the Tannu-Ola zone was an active continental margin, wh ere basalts with island-arc characteristics, quartz microdiorites of adakite composition, basalts, dolerites, and quartz microdiorites of intraplate type, and diorite-tonalite-plagiogranite association of island-arc type formed. To explain the spatial and temporal conjugation of different magmatism manifestations on the active margin, we applied the slab-window model.

The paper presents geochronological data on silicate and endogenic carbonate rocks from the Oshurkovo apatite-bearing pluton, namely, the U-Pb and Rb-Sr isochron age of gabbros and carbonatites. The U-Pb age of the carbonatites from this pluton (126.55 ± 0.85 Ma) is close to that of other carbonatite occurrences in southwestern Transbaikalia. Also, its similarity to the age of the gabbro (125.4 ± 2 Ma), which makes up the bulk of the pluton, suggests that these rocks are comagmatic.

The paper gives major-element and extended trace-element descriptions of plutons from the Shakhtama complex in the Aga tectonic zone (Transbaikalia). They are composed of hornblende-biotite gabbrodiorites, monzonites, diorites, and granodiorites. It has been found that they were initially enriched in granitophile (including ore-forming), trace elements. This suggests consanguinity between the intrusions of the Shakhtama complex (J₂-J₃) and the ore-bearing Kukul'bei rare-metal complex of granites and leucogranites (J₃) (Aga zone, Transbaikalia). Since granitophile elements are incompatible, their content in granitoids with an elevated mafic content from the Shakhtama complex is substantially less above the clarke than that in the rare-metal granites and leucogranites from the Kukul'bei complex, which ended postcollisional magmatism in the Aga zone. Analysis of normalized REE patterns for the granitoids from both complexes has revealed their magmatic heterogeneity. It has been demonstrated that the Kukul'bei granite and leucogranite intrusions in the western Aga zone are granitic differentiates of granitoid magma chambers associated with the Shakhtama complex, which were initially enriched in granitophile trace elements. The granites from the rare-metal intrusions of the Kukul'bei complex (on the eastern periphery of the Aga zone) are not directly related to the magma chambers of the Shakhtama complex. They are magmatic melts, which might result from the deep-level selective melting of metamorphosed rocks enriched in volatile trace elements. It has been confirmed that there is no direct genetic relation between the major- and trace-element compositions of the granitoids and their metallogeny and ore content.
The granitophile rare-metal specialization of the intrusions in Mesozoic Transbaikalian gabbrodiorite-monzonite-granodiorite and leucogranite complexes is the major evidence supporting the theory of the formation of late- and postcollisional magma chambers associated with deep faults, which were initially enriched in volatile and incompatible trace elements.

The paper is devoted to the experimental study of the leak-tightness of water-containing melt inclusions heated in the autoclave at high temperature and under the pressure of D₂O. Quartz-hosted silicate melt inclusions from the rhyolite tuffs of the Taupo volcanic zone (New Zealand) and a tourmaline-pegmatite vein from the southwestern Pamirs were heated at 650?C and 3 kbar. The penetration of heavy water into the inclusions was controlled by IR spectroscopy. The studies have demonstrated that inclusions can remain leak-tight under these conditions and not exchange water with the environment even if the confining pressure is 1.5-3 times above their internal pressure. The influence of water diffusion through the quartz lattice and dislocations on the leak-tightness of inclusions can be neglected in the thermometry of water-containing melt inclusions. The crucial factors determining water exchange between the inclusions and the environment in the experiments performed are mechanical defects (open and healed cracks). Using D₂O as a pressurizing medium makes it possible to control the leak-tightness of the heated inclusions. To do this, it is recommended that melt inclusions be homogenized under the pressure of D₂O above the expected entrapment pressure and studied by IR spectroscopy after the heating.

Using data on the chemical composition of lake waters in the Ol'khon region, we calculated the under- and oversaturation of modern chloride solutions of Lake Tsagan-Tyrm. These data are a basis for the determination of the composition of suspension and bottom sediments. The thermodynamically calculated solubility of carbonates (inverse problem) discovered in the evaporite lacustrine sediments gave an insight into the main regularities of the chemical evolution of ancient brines and the formation conditions of various magnesian calcites. The calculations confirmed that an increase in solution mineralization leads to the deposition of high-Mg calcites, which is also observed in the bottom sediments of other lakes under climate aridization and temperature decrease.

The seismic quality factor ( Q_с), the frequency parameter ( n ), and the attenuation coefficient (δ) of seismic waves in the lithosphere of the southwestern flank of the Baikal rift system are obtained from records of S coda waves from regional earthquakes. There is a strong dependence of the quality factor on the frequency and coda window length: for central frequencies of 0.3 and 12.0 Hz and a coda window length W = 20 s, Q_с varies from 47 ± 51 to 1036 ± 190, and for the same frequencies and W = 60 s, it varies from 83 ± 46 to 1596 ± 573. The values of the quality factor and the attenuation coefficient are calculated for the whole southwestern flank of the Baikal rift system and for individual tectonic blocks: the stable Siberian Platform, Khamar-Daban and East Sayan uplifts, and South Baikal and Tunka basins. A comparison of the obtained values of Q_с with data on the age, tectonics, and seismic activity level of the structures shows that the quality factor depends on the recent tectonic activity of the structures, and to a lesser extent, on the age of the crust.

This paper presents the results of experiments on the effect of variable amplitude and pressure on the P -wave velocity and attenuation in cemented sandstone sample. The measurements were performed in transmitted waves at a predominant pulse frequency of 6.8 kHz in the range of strain amplitudes A ^ε ≈ (0.2-2.4) ·10^-6 at an axial static pressure of 0.25 MPa to 1.0 MPa. The wave velocity depends weakly on the amplitude and decreases only slightly with its increase. With increasing pressure the velocity v_e nonlinearly increases (to 6 %) at all amplitudes. The attenuation Q_e increases linearly (to 2.5 %) with increasing amplitude and decreases nonlinearly with increasing pressure. There is an anomalous change in the attenuation in the pressure range (0.5-0.75) MPa, where it decreases to 30 % at all amplitudes. These results may be useful for improving acoustic and seismic methods of diagnostics.