


Vol 54, No 7 (2016)
- Year: 2016
- Articles: 9
- URL: https://journals.rcsi.science/0016-7029/issue/view/9389
Article
Tectonic setting discrimination diagrams for terrigenous rocks: a comparison
Abstract
An attempt is made to compare discrimination diagrams of the first (mid-1980s) and second (early 2010s) generations compiled using data for sedimentary successions of different ages. Our results suggest that the diagrams of different generations allow more or less correct discrimination only between the platform, rift, passive margin, and island arc settings. The data for collision sediments do not form separate fields in these diagrams.



The influence of low aluminum concentrations on the composition and conditions of crystallization of majorite–knorringite garnets: Experiment at 7.0 GPa and 1500–1700°C
Abstract
Crystallization of garnet in high-chromium restite formed under the conditions of partial melting in the spinel facies and subsequently subducted into the garnet depth facies was studied experimentally in the MgO–Al2O3–Cr2O3–SiO2 system. The crystallization of garnet and the dependence of its composition on the temperature and bulk composition of the system with low Al concentration were studied as well. Experiments in the knorringite–majorite–pyrope system with 5, 10, and 20 mol % Prp were carried out at 7 GPa. The phase associations for the starting composition of pure knorringite Mg3Cr2Si3O12 included chromiumbearing enstatite MgSiO3 (up to 3.2 wt % Cr2O3) and eskolaite Cr2O3. Addition of Al resulted in crystallization of high-chromium majoritic garnet. The portion of garnet in the samples always exceeded the concentration of pyrope in the starting composition owing to the formation of the complex majorite–knorringite–pyrope series of solid solutions. With increasing content of pyrope (from 5 to 20 mol %) and increasing temperature, the modal concentration of garnet increased significantly (from 6–12 to 22–37%). The garnet was characterized by high concentrations of the pyrope (23–80 mol %) and knorringite (22–70 mol %) components. The excess of Si (>3 f.u.) with decreasing Cr concentration provided evidence for the contribution of the majorite–knorringite trend to the variation in garnet composition. On the basis of the natural data, most of the garnets composing xenoliths of ultrabasic rocks in kimberlites and occurring as inclusions in diamonds are low-chromium; i.e., their protolith was not subjected to partial melting, at least in the spinel depth facies.



Geochemistry and isotopic ages of granitoids of the Bashkirian Mega-Anticlinorium: Evidence for several pulses of tectono–magmatic activity at the junction zone between the Uralian orogen and East European Platform
Abstract
The study provides the first evidence for post-Riphean phases of granite emplacement in the Bashkirian Mega-Anticlinorium (BMA) at the boundary between the East European Platform and Uralian orogen. The tectono-thermal activity in the BMA is well-constrained by emplacement of the Kusa–Kopan plagiogranitoid intrusion (660 Ma) and late gneiss–granites of the Yurma complex (540 Ma). The geochemical features of these rocks are transitional between within-plate rift and orogenic suites. It was shown that the Paleozoic stage of the BMA was marked by emplacement of granites of the Kialim massif (314 Ma) and Semibratka complex (300 Ma). The age and geochemical features of these rocks are similar to those of Carboniferous granites of the Uralian orogen, which are interpreted to mark the end of subduction and beginning of collision. This similarity suggests that the BMA was adjoined to the Uralian orogen in the Carboniferous and Paleozoic granite emplacement in both structures was the result of their common geological evolution and protoliths of a similar geochemical composition.



Effect of redox conditions on iron metal phase segregation during experimental high-temperature centrifuge modeling of the origin of the Moon’s core
Abstract
The possible origin of the Moon’s metallic core at the precipitation of iron–sulfide phases during the partial melting of ultramafic material under various redox conditions was experimentally modeled by partially melting the model system olivine (85 wt %) + ferrobasalt (10 wt %) + metallic phase Fe95S5 (wt %) in a high-temperature centrifuge at 1430–1450°C. The oxygen fugacity fO2 was determined from the composition of the quenched experimental silicate melts (glasses). A decrease in fO2 is proved to be favorable for the segregation of iron–sulfide melt from the silicate matrix. The metallic phase is most effectively segregated in the form of melt droplets, and these droplets are accumulated in the lower portions of the samples under strongly reduced conditions, at fO2 ∼ 4.5–5.5 orders of magnitude lower than the iron–wüstite buffer.



Formation of adakitic granitoids in the collisional orogens: Evidence from the Early Paleozoic granitoids of the Munku–Sardyk Range, Eastern Sayan
Abstract
The classical models of adakite formation by melting of basaltic layer of oceanic lithosphere in the subduction zone were verified using geochemical and Sr–Nd isotope data on the Early Paleozoic granitoids of Eastern Sayan. The presence of adakites in fold belts is usually regarded as geochemical proxy for paleogeodynamic reconstruction. The formation of felsic derivatives with adakitic signatures in the collisional orogens is inconsistent with these models and requires their revision. It is shown that the composition of the granitoids and their evolution cannot be described with these models. In order to solve this problem, two hypotheses of granitoid formation by mixing of two geochemically contrasting reservoirs were proposed and verified. According to the first hypothesis, the granitoids represent the mixing products between alkaline olivine basalts and partial melts of the gray gneiss basement of this region. The second model relates the formation of the granitoids with melting of geochemically 2700 Ma-old enriched source in the subcontinental lithospheric mantle. In spite of differences, both these hypotheses are based on the remobilization of sources formed at the previous stages of the geological evolution of the region. In both cases, adakitic geochemical characteristics of forming felsic magmas are determined by the composition of protolith rather than by their geodynamic position. Obtained preliminary results place constraints on genetic models and geochemical reservoirs participating in the formation of the granitoids.



Solubility and migration ability of rhodium in natural conditions: model experimental data
Abstract
The contents of dissolved rhodium species in the near-neutral environments have been studied for the first time and data on the interaction of Rh with organic matters of natural waters and its sorption behavior during contact with the components of geochemical barriers were obtained. The solubility method was used to analyze the behavior of rhodium hydroxide in the Rh(OH)x–H2O and Rh(OH)x–H2O–FA (fulvic acids) systems. The possible contents of inorganic species of rhodium and its compounds with humic organic ligands were determined within the pH range typical of surface waters. The solubility of rhodium shows a twoorder- of magnitude increase in the presence of humic matters (FA). The sorption interaction of the soluble rhodium species with the main components of geochemical barriers such as iron oxyhydroxides (III), (including fulvic-acid modified ones), alumosilicates, and precipitates of humic acids in contact with natural waters was studied. It was revealed that rhodium has the high affinity to all studied materials; its species are sorbed by ferrihydrite within several hours. It is suggested that rhodium is mainly transferred as colloid with suspended particulate matters of waters and then is accumulated in bottom sediments. The differences revealed in the sorption behavior of Pt(IV), Pd(II) and Rh(III) may be used to predict the distribution of the considered platinum group elements between the components of ecosystems.



Leaching rare-earth and radioactive elements from alkaline rocks of the Lovozero Massif, Kola Peninsula
Abstract
The paper presents data on the leaching efficiency of rare-earth (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) and radioactive (Th and U) elements by various reagents from alkaline rocks of the Lovozero Massif. Element concentrations were analyzed by ICP-MS and instrumental neutral activation (INAA). A new complex technique is suggested for analyzing elements on the solid phase of polymer hydrogels. This technique makes it possible to enhance the sensitivity and selectivity of INAA when these elements are simultaneously contained in rocks. Data are presented on the selective leaching of trace elements and the application of environmentally safe reagents.



Short Communications
Experimental study of gallium oxide solubility in chloride solutions at 300–400°C



Enthalpy of formation of natural polyhalite


