Generation of mantle-derived basaltic andesites in volcanic arcs

Primary magmas in volcanic arcs exhibit wide compositional diversity on both local and global scales. Processes responsible for this diversity are generally ascribed to some combination of mantle melting or crustal differentiation processes. One widespread view is that arc magmagenesis is driven by combination of H2O-fluxed and decompression melting of peridotitic mantle wedge, and that primary, mantle-derived melts are high-MgO basalt. However, a variety of other mantle-derived primitive arc magmas, ranging in composition from high-Mg andesite to picrite, has been recognised and it remains unclear to what extent this diversity can be generated by mantle melting processes modulated, for example, by changes in the thermal state of the mantle wedge or the supply of fluid from the slab. Here we use high pressure and temperature experiments to constrain magma generation conditions of a primitive magnesian (8.8 wt% MgO) basaltic andesite from Klyuchevskoy volcano, Kamchatka arc, Russia. We use an inverse experimental approach to define a multiple saturation point on the liquidus surface of the basaltic andesite. The experimental multiple saturation point defines the pressure and temperature at which an erupted melt could have last been in equilibrium with a polymineralic source rock, such as mantle peridotite, and hence provides a robust estimate of magma source conditions. Equilibrium piston-cylinder experiments were carried out between 0.5 and 1.0 GPa under hydrous conditions (3 to 6 wt% added H2O) at fO2 = DNNO+1. We show that Klyuchevskoy basaltic andesite is multiply saturated with the lherzolite assemblage ol (Fo90) + clinopyroxene + orthopyroxene + Cr-spinel close to its liquidus (≥ 95% melt) in the pressure range of 0.6 to 1 GPa (23 to 36 km depth) and 1220-1240 ºC. Amphibole is present at temperatures just below the multiple saturation point (≤ 1200 °C). Our results show that basaltic andesite was produced by 8 to 11 wt% partial melting of amphibole-lherzolite source and therefore represents a primary, undifferentiated magma extracted from its source at near- Moho depths. These findings are in a good agreement with geophysical studies of Klyuchevskoy volcano that show magmas are supplied directly from a reservoir at near- Moho depths (25-30 km) through a sub-vertical, pipe-like feeder system. Coeval high-MgO basalts from the volcano may correspond to higher degree mantle melts extracted at slightly greater depths. Our results provide a tight constraint on the thermal structure of the mantle wedge beneath Klyuchevskoy. Experimental temperatures are higher than those calculated from steady-state thermal models suggesting that upwelling asthenosphere might directly impinge the Moho in a similar fashion to mid-ocean ridges. Intra-arc rifting promotes asthenospheric decompression beneath the Central Kamchatka Depression. The presence of amphibole in our experiments at temperatures up to 1200 °C indicates that dehydration melting of amphibole peridotite formed by metasomatism of mantle wedge by slab-derived fluids is the primary magma generating process. Amphibole stability exercises an important control on melting conditions. Integrating our results with published multiple-saturation experiments we show that Klyuchevskoy basaltic andesite is one of a family of primary arc magmas whose compositions depend on mantle wedge thermal structure and H2O activity

Experimental evidence for polybaric differentiation of primitive arc basalt beneath St. Vincent, Lesser Antilles

Equilibrium crystallization experiments have been performed on a primitive high-MgO basalt (HMB) from Soufrière, St. Vincent, with three initial H2O contents (0·6, 2·3 and 4·5 wt %), at pressures of 0·4, 0·7, 1·0 and 1·3 GPa and temperatures from 1350 to 950°C. Redox conditions, as determined by µXANES analysis of Fe3+ in experimental glasses, were 1–4 log units above the nickel–nickel oxide (NNO) buffer. The aim of the study was to explore the differentiation conditions that gave rise to the observed geochemical variation in lavas and plutonic (cumulate) xenoliths from St. Vincent. An experiment with 4·5 wt % initial H2O is multiply saturated close to its liquidus (1180°C and 1·3 GPa) with a spinel lherzolite assemblage, which is consistent with a primary origin for HMB in the mantle wedge. Multiple saturation of HMB with 2·3 wt % H2O was not observed, but is inferred to occur at pressures >1·3 GPa. The experimental results show that initial H2O content has significant influence on differentiation paths of primary HMB magma, with different lava varieties generated under discrete, well-constrained P–T–H2O conditions. Low-magnesian basalts (LMB) can be generated from HMB with 2·3–4·5 wt % H2O at pressures of 1·0–1·3 GPa, corresponding to Moho depths beneath St. Vincent. The CaO contents of LMB are sensitive to differentiation pressure: high-CaO LMB are produced at pressures >0·5 GPa. Basaltic andesites (BA) can be generated at 0·7–1·0 GPa from HMB with 0·6–2·3 wt % H2O. High-alumina basalts (HAB) are produced at mid- to upper-crustal conditions (≤0·4 GPa) by differentiation of HMB with high initial H2O (≥4 wt %) through delay of plagioclase crystallization and dominant fractionation of olivine, clinopyroxene and spinel. St. Vincent andesites could be produced from relatively dry (≤0·6 wt % H2O) HMB only at lower-crustal conditions. This is suggestive of a partial melting origin from precursor HMB that had solidified at depth to produce gabbros with ∼30% hornblende (i.e. ∼0·6 wt % structurally bound H2O). The experimentally determined differentiation conditions are consistent with polybaric differentiation within a hot zone that extends from the Moho and uppermost mantle to the mid- or upper crust. Within the hot zone differentiation occurs by a combination of crystallization of HMB with 2–5 wt % H2O and partial melting of ancestral HMB gabbros. Although the experimental melts provide an excellent match to erupted lava compositions, experimental crystal compositions do not match either phenocrysts or cumulate crystals, as preserved in xenoliths. The failure to reproduce natural crystal compositions suggests that these are formed as differentiated magmas ascend and attain their H2O-saturated liquidi at shallower pressures. Thus there is a disconnect between the high-pressure phase compositions and assemblages that generate liquid compositional diversity and the low-pressure composition and assemblages that occur as phenocrysts and in cumulate xenoliths. This finding lends support to the idea of cryptic fractionation in the generation of arc magmas

Technique of realizing academic conditions of reflexive attitude formation for a prospective foreign language teacher in the course of professional training

The article presents the authors’ technology of realization of pedagogical conditions (actualization of reflexive life experience of a future teacher of a foreign language in situations of pedagogical interaction by means of problem-variable construction of lessons; involvement of the future foreign language teacher in educational professionally-oriented activities through the solution of reflexive and professional tasks; the use of interactive forms of training to work out the reflection algorithm as a means of entering into a reflective position of the future foreign language teacher

STATUS AND SIGNIFICANCE OF THE VILLAGE ELDER IN PEASANTS’ SELF-ADMINISTRATION FROM CLOSE OF THE 19THTO THE EARLY 20TH CENTURY

Purpose of the study: The article focuses on the research of the role of village elders in rural self-administration at the turn of the 20th century. The article shows how rural elders executed their duties, who was selected as a rural elder, as well as the attitude of the countrymen towards this authority. Methodology: In the process of the study, the authors relied mainly on documents of the State Archive of the Russian Federation and the State Archive of the Tambov Region. The retrospective research method, first of all, allowed us to study the participation of rural residents in self-government. In addition, describe the role of the elder and the functions assigned to him in the process of managing the settlement. Main Findings: It has been revealed if the country people and elders had conflicts in the performance of their duties. Rural elders were supposed to eliminate arrears of taxes and levies. The article considers the cases when rural elders supported local people in confrontation with superior civil servants. In conclusion, the article estimates the “institution” of rural elders. Applications of this study: The results of the study are primarily useful for the formation of a program for the development of the organization of agriculture in the Tambov region. An understanding of the traditions, historical features of the development of the region should be taken into account by legislative bodies and municipal structures when searching for effective management tools. Understanding the features of rural management is interesting for researchers studying similar practices in different countries of the world. For example, the application of researchers involved in local government issues, when comparing the historical experience of different states and regions. Novelty/Originality of this study: The study shows not only how the functions of the elders in the process of managing rural territories changed, but also the reaction of the population to various actions of the authorities. In particular, the typical causes of discontent among local residents on the actions of the "elders". However, often these reasons were due to the peculiarities of interactions between elders and representatives of state authorities

Contact Joints Shear Strength of Composite Concrete Structures Laced with STACHEMENT-2000M Hyperplasticizer

The urgency of using composite concrete and reinforced concrete structures and the problem of solid adhesion of the element in the zone of the contact joint of composite concrete and reinforced concrete structures are considered in the article. The investigations of contact joints shear strength laced with STACHEMENT-2000M hyperplasticizer are carried out. Conclusions are drawn from the results obtained

Early arc crust formation preserved in the Grenadines archipelago, southern Lesser Antilles arc

Intra-arc diversity in volcanic activity and composition is ubiquitous, but its underlying causes remain largely unresolved in many settings. In this work, we examine such variability in the Grenadines archipelago, southern Lesser Antilles arc. Here, juxtaposed volcanic centres exhibit eruptive longevities and chemistries distinct from northern counterparts in the same arc. Our goal is to explain this deviation by investigating variations in magmatic processes using petrological data from erupted crustal xenoliths and lavas, and interpreting these findings within the context of the archipelago’s tectonic history and geophysical structure. Textural analyses of xenoliths reveal crystallisation over a wide range of pressure temperature-melt composition conditions in the crust. Mineral phases display discrete compositional trends pointing towards significant inter-island variability in underlying plumbing systems. The geochemical variety of erupted magmas is reminiscent of the entire arc. We speculate that the Grenadines represents the early onset of subduction forming the modern day Lesser Antilles arc. Extrusive volcanism initiated as submarine activity. Subsequent uplift eroded the original topography of these volcanic centres following the eventual cessation of volcanism in the Neogene. The positioning of the Grenadines on an elevated platform provides rare modern insight into early arc crust formation not commonly preserved in established active arcs