Average composition of basic magmas and mantle sources of island arcs and active continental margins estimated from the data on melt inclusions and quenched glasses of rocks

Based on the generalization of data on melt inclusions and quenched glasses, the average compositions of subduction (island arc and active continental margin settings) basic magmas were estimated. The main geochemical features of the average composition of these magmas are significant depletion in Nb and Ta, less significant depletion in Ti, Zr, and Sm, and enrichment in Cl, H₂O, F, and P in the primitive mantlenormalized patterns. The average normalized contents of moderately incompatible HREE in these magmas are close to those in the basic magmas of other geodynamic settings. Subduction basic magmas exhibit negative correlation of Li, Y, Dy, Er, Yb, Lu, and Ti contents with MgO content. Most of incompatible elements (Nb, Ta, U, Th, LREE) do not correlate with MgO, but correlate with each other and K₂O. Variations in element contents are related to crystallization differentiation, magma mixing, and possibly, participation of several sources. The water content in the island arc basic magmas varies from almost zero value to more than 6 wt %. Most compositions are characterized by weak negative correlation between H₂O and MgO contents, but some compositions define a negative correlation close to that in magmas of mid-ocean ridges (MOR). Considered magmas demonstrate distinct positive correlation between MgO content and homogenization temperature, practically coinciding with that of MOR magmas. Modeling of phase equilibria revealed widening of crystallization field of olivine in the magmas of subduction zones compared to MOR magmas. This can be related to the high water content in subduction magmas. Simultaneous liquidus crystallization of olivine and clinopyroxene in subduction magmas occurs at pressure approximately 5 kbar higher than that of MOR magmas. Based on the average ratios of trace element to K2O content, we determined the average compositions for subduction magma sources. Relative to depleted mantle, they are enriched in all incompatible elements, with positive anomalies of U, Rb, Ba, B, Pb, Cl, H₂O, F, and S, and negative anomalies of Th, K, Be, Nb, Ta, Li, Nd, Pb, and Ti. A general elevated content of incompatible elements indicates a reworking of the rocks of mantle wedge by fluids and melts that were released from the upper layers of subducted plate.     

Direct data on the ore potential of acid magmas of the Uzel’ginskoe ore field (Southern Urals,Russia)

Doklady Earth Sciences -     

Principal physicochemical parameters of natural mineral-forming fluids

The authors’ database (which includes data from more than 17500 publications on fluid and melt inclusions in minerals) was used to generalize information on the principal physicochemical parameters of natural mineral-forming fluids (temperature, pressure, density, salinity of aqueous solutions, and the gas composition of the fluids). For 21 minerals, data are reported on the frequency of occurrence of the homogenization temperatures of fluid inclusions in various temperature ranges, which make it possible to reveal temperature ranges most favorable for the crystallization of these minerals. Data on 5260 determinations were used to evaluate the frequency of occurrence of certain temperature and pressure ranges of natural fluids within the temperature intervals of 20–1200°C and 1–12000 bar. Within these intervals, frequencies of occurrence were evaluated for water-dominated and water-poor or water-free fluid inclusions in minerals. The former are predominant at temperatures below 600°C and pressures below 4000 bar, whereas the latter dominate at temperatures of 600–1200°C and pressures of 4000-12000 bar. Illustrative examples are presented for visually discernible magmatic water that exists as an individual high-density phase in melt inclusions in minerals from various rocks sampled worldwide (in the Caucasus, Italy, Slovakia, United States, Uzbekistan, New Zealand, Chile, and others). Attention is drawn to the fact that extensive data testify to fairly high (>1000–1500 bar) pressures during hydrothermal mineral-forming processes. These pressures are much higher not only than the hydrostatic but also the lithostatic pressures of the overlying rocks. Data on more than 18000 determinations are used to evaluate the frequency of occurrence of certain temperature and salinity ranges of mineral-forming fluids within the intervals of 20–1000°C and 0–80 wt % equiv. NaCl and certain temperature and density ranges of these fluids at 20–1000°C and 0.01–1.90 g/cm³. Information is presented on the gas analysis methods most commonly applied to natural fluids in studying fluid inclusions in minerals in 1965–2007. The average composition of the gaseous phase of natural inclusions is calculated based on more than 3000 Raman spectroscopic analyses (the most frequently used method for analyzing individual inclusions).     

Dynamic model of accumulation of total biomass of agricultural crops

Issues concerning the growth and biological time of agricultural crops are under consideration. A closed system of equations is derived for calculating total dry biomass and biological time of plants. The model parameters are given, and the model is verified based on the experimental data of observations of the sunflower in the southern conditions of the Ukraine.     

硅酸盐-盐（硫酸盐）流体不混溶：蒙古南部Mushugai-Khuduk含火成碳酸岩杂岩体矿物中的熔体包裹体研究

Crystalline and melt inclusions were studied in garnet,diopside,potassium feldspar,and sphene from the garnet syenite porphyry of the carbonatite-bearing complex Mushugai-Khuduk,southern Mongolia.Phlogopite,clinopyroxene,albite,potassium feldspar,spheric,wollastonite,magnetite,Ca and Sr sulfates,fluorite,and apatite were identified among the crystalline inclusions. The melt inclusions were homogenized at 1010～1080℃and analyzed on an electron microprobe.Silicate,salt,and combined silicate- salt melt inclusions were found.Silicate melts show considerable variations in SiO_2 concentration(56 to 66wt% ),high Na_2O K_2O (up to 17wt% ),and elevated Zr,F,and C1 contents.In terms of bulk rock chemistry,the silicate melts are alkali syenites.During thermometric experiments,salt melt inclusions quenched into homogeneous glasses of predominantly sulfate compositions containing no more than 1.3wt% SiO_2.These melts are enriched in alkalis,Ba,Sr,P,F,and C1.The investigation of the silicate and salt melt inclusions in minerals of the garnet syenite porphyries indicate that these rocks were formed under influence of the processes of crystallization differentiation and magma separation into immiscible silicate and salt(sulfate)liquids.     

Immiscibility between silicate magmas and aqueous fluids: a melt inclusion pursuit into the magmatic-hydrothermal transition in the Omsukchan Granite (NE Russia)

Exsolution (unmixing) of the volatile element-rich phases from cooling and crystallising silicate magmas is critical for element transport from the Earth’s interior into the atmosphere, hydrosphere, crustal hydrothermal systems, and the formation of orthomagmatic ore deposits. Unmixing is an inherently fugitive phenomenon and melt inclusions (droplets of melt trapped by minerals) provide robust evidence of this process. In this study, melt inclusions in phenocrystic and miarolitic quartz were studied to better understand immiscibility in the final stages of cooling of, and volatile exsolution from, granitic magmas, using the tin-bearing Omsukchan Granite (NE Russia) as an example.

Primary magmatic inclusions in quartz phenocrysts demonstrate the coexistence of silicate melt and magma-derived Cl-rich fluids (brine and vapour), and emulsions of these, during crystallisation of the granite magma. Microthermometric experiments, in conjunction with PIXE and other analytical techniques, disclose extreme heterogeneity in the composition of the non-silicate phases, even in fluid globules within the same silicate melt inclusion. We suggest that the observed variability is a consequence of strong chemical heterogeneity in the residual silicate-melt/brine/vapour system on a local scale, owing to crystallisation, immiscibility and failure of individual phases to re-equilibrate. The possible evolution of non-silicate volatile magmatic phases into more typical “hydrothermal” chloride solutions was examined using inclusions in quartz from associated miarolitic cavities.     

The Raigorodok stockwork gold-sulfide-quartz deposit in the North Kazakhstan gold ore province

The Raigorodok stockwork gold-sulfide-quartz deposit is located in the contact zone of a monzogabbro-diorite intrusion belonging to the Stepnyak complex (442 Ma). Two main ore formation stages have been established: early, with disseminated gold-bearing pyrite-chalcopyrite mineralization related to skarn, propylite, and K-feldspar formation, and late, with productive mineralization. The late stage was accompanied by the formation of beresite, sericite, and quartz and comprised two substages: (1) quartz-gold-pyrite-chalcopyrite mineralization and (2) quartz-carbonate-gold-polysulfide mineralization. The ⁴⁰Ar/³⁹Ar ages of beresites and sericites are 422.9 ± 4.3 Ma and 416.7 ± 4.3 Ma, respectively, which is 19-22 Myr younger than the age of the host intrusion. The deposit ores formed stepwise, at temperatures of 112-335 °C and fluid salinity of 0.7-21.2 wt.% NaCl equiv. Sulfur for skarns, propylites, beresites, and ores might have been provided by a deep magma chamber, and the carbonaceous shales of the Vendian Sharyk Formation might have been the source of gold. The isotope and geochemical data and the results of fluid inclusion study suggest that Raigorodok is an intrusion-related deposit. At the same time, the deposit has a number of signs of porphyry-epithermal ore-magmatic systems.     

Gallium concentration in natural melts and fluids

We generalize, for the first time, published and original data on the gallium concentrations in natural magmatic melts and fluids obtained by studying quenched glasses in volcanic rocks and inclusions in minerals. Based on 2688 determinations, gallium concentrations in magmatic melts vary between 0.47 and 495 ppm at average content of 18.0 ppm (+4.2/–3.4). Gallium concentrations in magmatic melts generated in different geodynamic settings show different distribution. Minimum concentrations (on average, 16.0 ppm, +3.6/–2.9) are typical of the island-arc melts, while maximum contents were determined in melts of oceanic islands (on average, 29.1 ppm, +13.4/–9.2) and intracontinental rifts and hot spots (26.5 ppm, +25.4/–13.0). Published and new 339 determinations of gallium concentrations in natural fluids indicate the wider range of their variations as compared to those of melts: from 0.02 to 11260 ppm, at average 1.6 ppm (+10.8–1.4). The possible gallium fractionation in fluid—magmatic systems is discussed.     

Platinoan Vysotskite with Inverse Zoning and Skeletal Cooperite in Metamorphosed Sulfide Ores Within the Eastern Flank of the Oktyabrskoe Deposit,Noril’sk ore Field

Geology of Ore Deposits - Platinoan vysotskite metacrystals with inverse zoning are described. Their size is up to 0.5 mm. Association of these metacrystals are characterized by a...