Some Structural and Mineralogical Peculiarities of Quenching Liquids Obtained by Melting of Mantle Ultrabasite Xenoliths and Mafic Volcanics by an Electron Beam

Doklady Earth Sciences - The experimental results presented below shine a light upon the physics of phase transition processes during melting, intensive evaporation, and subsequent explosive...     

The South Ushkoty Eluvial Kaolin Deposit in the Southeastern Orenburg Region

The South Ushkoty eluvial kaolin deposit discovered and explored by the authors is situated 20 km away from the railway in the Dombarov district of the Orenburg region. The deposit is localized in the axial zone of the East Ural Uplift. Leucogranites of the Upper Ushkoty Pluton serve as bedrocks. The deposit comprises five lodes, among which the largest deposit has a reserve of 40 Mt. The lodes are erosion remnants of the well-developed linear-areal weathering zone related to the fragments of Mesozoic peneplain at the hypsometric level not lower than 360 m. Variation of the kaolin zone thickness, the entire weathering zone, and the deposit morphology is largely governed by the nearly meridional and northeastern faults. Two productive zones—the normal kaolin (kaolinite + quartz) zone and the alkaline kaolin (kaolinite + quartz + potassium feldspar and/or muscovite) zones—are recognized in the weathering profile of leucogranites of the Upper Ushkoty Pluton. The alkaline kaolin is of commercial significance as a complex raw material if the potassium oxide content is above 3.5%. The composition and properties of the concentrated kaolin meet the requirements of its traditional consumers (fine ceramic works, paper mills, and others). The discovery of the South Ushkoty deposit confirmed the previous forecast on considerable prospects of the Orsk Transural region for kaolin. We hope that this discovery will stimulate the further investigation and assessment of resources in the Mugodzhary kaolin-bearing subprovince.     

Rhyolite xenolith from the neovolcanic basalts of the rift valley of the Juan de Fuca Ridge, northeastern pacific: Reconstructsen MOR silicic rocks and basic magmas

In this paper, we discuss the formation conditions of rhyolites and results of their interaction with later portions of basic magmas on the basis of the investigation of melt and fluid inclusions in minerals from a rhyolite xenolith and host neovolcanic basalts of the Cleft segment of the Juan de Fuca Ridge. In terms of bulk chemistry and the compositions of melt inclusions in pyroxene and olivine phenocrysts, the basic rocks of the southern part of this segment are typical MOR basalts. Their olivine, clinopyroxene, and plagioclase crystallized at temperatures of 1160–1280°C and a pressure range between 20 and 100 MPa. The xenolith is a leucocratic rock with negligible amounts of mafic minerals, which clearly distinguishes it from the known occurrences of silicic rocks in the rift valleys of MOR. The rhyolite melt crystallized at temperatures of 900–880°C. The final stages of rhyolite melt crystallization at temperatures of 780–800°C were accompanied by the release of a saline aqueous fluid with high chloride contents. Based on the geochemical characteristics of melt inclusions and melting products, it can be suggested that the magmatic melt was produced by melting of metamorphosed oceanic crust within the Cleft segment under the influence sof saline aqueous fluid trapped in the pores and interstices of the rock. The rock represented by the xenolith is a late differentiation product of such melts. The ultimate products of silicic melt fractionation show high volatile contents: H₂O > 3.0 wt %, Cl ～ 2.0 wt %, and F ～ 0.1 wt %. The interaction of the xenolith with the host basaltic melt occurred at temperatures equal or slightly higher than those of ferrobasalt melts (1190–1180°C). During ascent the xenolith occurred for a few tens of hours in high-temperature basic magma, and diffusion exchange between the basaltic and silicic melts was very minor.     

Host galaxy of the dark gamma-ray burst GRB 051008

We have discovered and studied the host galaxy of the “dark” gamma-ray burst GRB051008, a burst for which no optical afterglow has been detected. We studied the properties and determined the redshift of the host galaxy and analyzed its neighborhood. We perform a comparative analysis of the properties of GRB051008 and its host galaxy with the properties of other known dark gamma-ray bursts. We analyze the causes of the lack of optical afterglow in this gamma-ray burst.     

Influence of inhomogeneities in the upper boundary of magmatic bodies beneath volcanoes on the generation of ore-forming fluid systems during retrograde boiling

A two-dimensional mathematical model is proposed to describe the crystallization of magmatic bodies with roof complicated by apophyses, around which fluidogenic ore deposits could be formed during retrograde boiling of cotectic melts. The most favorable environments for the formation of productive ore-forming systems were numerically simulated Among them are magmatic gas flows and fluid systems of mixed type. Magmatic fluid systems related to the retrograde boiling of cotectic melts in presence of localized sink of volatiles into permeable zones from apophyses in the chamber roof would evolve by two ways: (1) at L ₂/h < 1 for intrusion thickness (L ₂) and height of apophysis (h), the systems exist as high-temperature quasistationary fluid systems discharging on the surface of volcanic edifices; (2) at L ₂/h > 1, the central part of the stock is overcooled and magmatic fluid system ceases to be active after all gas dissolved in the upper half of the stock has been exhausted. Such cases are favorable for the formation of fluid systems of mixed type.     

Some questions of analytical simulation of contact metamorphism

The results of revised studies in analytical simulation of processes leading to development of isochemical contact metamorphic zonation by means of conductive heat transfer are reported. The peculiarities in temperature distribution in contact aureoles of intrusives are discussed for cases of convective heat transfer.     

Investigations in isochemical contact metamorphism

Isochemical contact metamorphism was observed in 3 localities based on 184 chemical analyses of rock samples and the significance tested using simple statistical techniques. The intrusions included gabbro, granosyenite, and granitic type rocks being intruded into respective schists, clays and shales, and schists and gneisses. Conductive heat transfer appeared to be the most important heat transfer mechanism in isochemical contact metamorphism. During isochemical metamorphism both H₂O and CO₂ were rather mobile and impoverished in the metamorphic rocks.     

Modeling the mantle-crust ore-magmatic systems of the Siberian traps

We consider the applications of existing mathematical models to heat and mass transfer dynamics in different zones of mantle-crust magmatic systems and discuss problems concerning quantitative modeling of mineralization associated with fractional crystallization of mafic melts in magma chambers, with or without crustal contamination.