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Volume 69, Nº 2 (2024)
Articles
Sources and evolution of miocene-pleistocene alkaline magmatism in the north-east part of the arabian plate (data from Sr–Nd–Pb isotopic systematics and K–Ar geochronometry)
Resumo
A geochronological and isotope-geochemical study of alkaline basalts from three areas of young magmatism within the northeastern part of the Arabian Plate (Southeastern Turkey), Batman, Kurtalan and Alemdağ, was carried out. The obtained isotope data have indicated that the volcanism in the studied region developed over 5 mln years from the end of Miocene to the mid of Pleistocene during four pulses separated by breaks in magmatic activity: 6.1-4.9 Ma (Batman area, hawaiites), ~3.0 Ma (Alemdağ plateau, phase I, basalts), 2.0-1.9 Ma (Alemdağ plateau, phase II, tephrites), and 1.5-1.3 Ma (Alemdağ plateau, phase III, basalts; Kurtalan area, basalts). A comparison of spatial-temporal regularities of magmatic activity evolution in the studied part of the Arabian Plate and within the largest basalt plateau of Arabian foreland, Karacadağ plateau, located to the west, was carried out. The results of Sr-Nd-Pb isotope-geochemical studies show that the development of young basalt volcanism within the northern part of the Arabian Plate was characterized at different time by participation of various mantle sources in magma generation under this region. Initial pulses of magmatic activity are associated with melting of Arabian subcontinental lithospheric mantle (SCLM). The processes of fractional crystallization combined with crustal assimilation (AFC) have played an important role in the petrogenesis of lavas as well. In the subsequent time, a deep mantle source (PREMA) with a depleted isotopic composition of Sr and Pb played a leading role in the formation of basaltic magmas of increased alkalinity. The melts generated by this source were mixed with the SCLM material in various proportions at different stages of magmatism with a limited participation of AFC processes in the petrogenesis of the rocks. It was concluded that young basalt volcanism of elevated alkalinity in the northeast of the Arabian Plate is not related to the collision of the Eurasian and Arabian plates genetically, but presumably manifested here as a result of the migration of the initial rift geodynamic set from the Red Sea basin to the north along Levantine and East Anatolian transform faults with associated convective wave flows in the lower part of mantle under this part of the Earth.
3-31
Stishovite – the variability of gehesis under earth’s conditions: physico-geochemical aspects
Resumo
Experimental data of studying the stishovite (SiO2) genesis under the Earth’s conditions are systemized. The PT-diagram of SiO2 state up to 350 GPa and 3200 °C was drawn. The phase fields of quartz, coesite, stishvite, phase SiO2 of CaCl2-type, seifertite (α-PbO2-type) and phase SiO2 of pyrite-type were designated. The early accumulation of the Earth’s mass in the period of meteorite accretion was accompanied with impact-in-waves genesis and underground disposal of stishovite which was completely assimilated with the global magma ocean of pyrolite composition. Crystallization of the magma ocean has been completed with a stratification of the mantle with the seismic boundaries between upper mantle, transition zone and lower mantle. The fractional ultrabasic-basic evolution of the mantle magmas was accompanied by peritectic reactions of ringwoodite+akimotoite and bridgmanite with stishovite formation. These reactions are revealed in physical-chemical experiments at 20 and 26 GPa. These reactions in the diamond-parental carbonate-silicate-carbon melts provided formation of stishovite with its capturing into diamonds as paragenic inclusions. The stishovite genesis is also connected with the global mantle convection as at subduction of the lithospheric plates with silica to the layer D”, so at uplift of the superplumes from the layer D” up to the Earth’s crust. The impact-meteoritic genesis of stishovite is restarted with formation of the Earth’s crust.
32-48
Experimental study of pargasite NaCa2 (Mg4Al)[Si6Al2O22](OH)2 stability at T = 1000–1100°C and the pressure is up to PH2O = 5 kbar
Resumo
The paper presents materials on experimental study of pargasite stability. On the example of calcic amphibole, experimental modeling of the processes occurring in the conditions of volcanic hearth at pressures up to 5 kbar was carried out. The phase diagram of pargasite was clarified. The occurring reactions and their parameters are revealed. Based on the experimental data obtained, the stability of pargasite is controlled by three reactions. The first reaction takes place in the area of low water pressure less than 1 kbar – dehydration reaction: Prg = Fo + Sp + Di + Ne + An + H2O. The second reaction takes place in the area of water pressure more than 1.2-1.5 kbar and temperature around 1100°C. Pargasite decomposition is controlled by incongruent melting: Prg = Fo + Sp + { Di+Ne +An }L + H2O. The third reaction takes place in the same pressure range as the previous one, but at lower temperatures ~1050 °C. This reaction determines the pargasite liquidus in the melt and is related to the interaction between the amphibole and the coexisting melt: Prg + L = Fo + Sp + Di + {Ne +Pl}L + H2O. Presumably, the activity of the melt silica aSiO2 has the greatest impact on the pargasite liquidus.
49-65
Quartz diorites of the volkovskiy ore-bearing massif (middle Urals, Russia): U–PB age, Nd–Sr–Pb isotope systematics, geochemical features, petrogenetic and geodynamic implications
Resumo
The results of U–Pb age determination, Nd–Sr–Pb isotope systematics and geochemical study of quartz diorites associated with ore-bearing (Cu–Pd–Au–Ag) gabbro of the Volkovskiy massif localized within the Ural Platinum Belt, at its eastern border with Silurian-Devonian volcanogenic complexes of the Tagil megazone are presented. The age of quartz diorites by U–Pb (TIMS) age determination is 429±9 mln years, MSWD = 0.009. Sr–Nd–Pb isotope characteristics (ƐNd(T) = +5.5 ÷ +6.7; (87Sr/86Sr)t = 0.70382−0.70392; 206Pb/204Pb = 18.38−18.57; 207Pb/204Pb = 15.56−15.58; 208Pb/204Pb = 38.14−38.30) indicate a juvenile source with a model age of 570−760 mln years. The obtained data do not go beyond the values typical for basalts of enzymatic island arcs. The geochemical features of the granitoids of the Volkovskiy massif (low REE concentration, differentiation of their spectrum (La/Yb = 8−14) with weak positive Eu-anomaly (Eu/Eu* = 0.9−1.4)) are consistent with the characteristics of melts obtained in water melting experiments for mafic rocks in equilibrium with amphibole-pyroxene restite. The anomalous strontium concentration (more than 1000 g/t) in quartz diorites is due to the high content of this element in the source. Such source could be the earliest rocks of the Ural Platinum Belt – olivine gabbro and surrounding metamorphic rocks. The same age of quartz diorites of the Volkovskiy massif and monzonitoids of the Kushvinskiy massif allows us to consider these rocks as a result of the mantle and crust melting converged in time at the final stage of the Tagil island-arc system formation.
66-85
Atacamite from paleofumarole cone height 1004 (Tolbachik volcano, Kamchatka): thermodynamic properties
Resumo
Atakamite with the empirical formula (Cu1.97Zn0.01)Cl0.94(OH)3.02, originating from the paleofumaroles of the monogenic volcano Vysota 1004 (Tolbachik, Kamchatka, Russia), has been studied by thermal and electron microprobe analyses, X-ray powder diffraction, IR and Raman spectroscopy, Calve microcalorimetry. Using X-ray diffraction and IR spectroscopy, the process of thermal decomposition of atacamite was studied. The enthalpy of formation from the elements for atacamite of the theoretical composition Cu2Cl(OH)3 (−810.2 ± 7.7 kJ/mol) was determined by melt dissolution calorimetry and the Gibbs energy of formation (−657.0 ± 7.7 kJ/mol) was calculated. Based on the data obtained, thermodynamic modeling of the stability of atacamite in the Cu–O–Cl–H system was carried out, and the boundaries of its stability were calculated under conditions of high alkalinity and high acidity of the mineral-forming medium.
86–97
Variability of the chemical composition of pine forests components in the forest-steppe of the Chelyabinsk region under the influence of anthropogenic load
Resumo
The work provides a comprehensive assessment of changes in the intensity of pollution of soils and plants of pine forests with distance from the main sources of pollution. Within the forest-steppe zone of the Chelyabinsk region, zones of environmental tension have been identified based on the summary index of soil contamination with mobile forms of chemical elements and on the level of pine bark contamination. The main polluting elements were Cr, Pb, Zn, Cd, the content of which in soils was tens of times higher than the background values. The same pollutants and Cu were noted for pine bark, but their concentrations were significantly lower and exceeded the background only several times. High values of correlation coefficients between pollution indicators for soils and bark were noted for Pb, Cu, Zn and the total pollution indicator. The content of most of the studied chemical elements in pine needles does not exceed background values, but the content of Al and Ni in some plots near industrial enterprises can be 2-4 times higher than background values.
98–112