Arc Stage Research Articles

ISOTOPE COMPOSITION OF Pb AND Nd OF THE LATE NEOPROTEROZOIC–EARLY PALEOZOIC GRANITOIDS IN EASTERN TUVA (Case study of the Kaa-Khem batholith)

We present the results of Nd and Pb isotope studies of late Neoproterozoic–early Paleozoic granitoids of the Kaa-Khem batholith, which formed in different geodynamic settings within the early Caledonian structures of Eastern Tuva (Altai–Sayan sialic area). Based on the obtained isotope data, we assess the role of different source materials in the formation of melts for these granitoids and compare these rocks with coeval granitoids from the Lake Zone of Mongolia. The plagiogranitoid associations of the island arc stage of the Kaa-Khem batholith formation (572–562 Ma) are characterized by mantle Nd isotope values. In contrast, the Pb isotope characteristics of these intrusive associations correspond to an upper-crust source formed predominantly by terrigenous sediments. The granitoid associations of the accretion–collision stage of the Kaa-Khem batholith formation (512–450 Ma) have a less radiogenic primary Pb isotope composition as compared with the island arc granitoids. Evidently, during the magma formation at this stage, the relative role of terrigenous material decreased, while the role of depleted mantle material increased. At the same time, the decrease in ɛNd values in the granitoids as compared with the depleted mantle indicates that, along with the upper-mantle component, the role of the lower-crust component increased. These Nd and Pb isotope characteristics of the accretionary granitoids of the Kaa-Khem batholith indicate a significant contribution of the lower-crust source to their formation. This source is the crystalline basement of the Tuva–Mongolian microcontinent. The Pb isotope characteristics of the accretion–collision granitoids of the Kaa-Khem batholith are shifted to the upper-crust compositions relative to those of the Lake Zone granitoids. The reason is that the latter formed predominantly through the transformation of early Caledonian island arc complexes, which were generated with a higher portion of mantle component.

Configuration of the early-mid Cenozoic extensional arc volcanism of the North Patagonian and the Southern Central Andes (33–44°S)

The late Eocene to earliest Miocene volcanic arc of the Southern Central Andes (33–40°S) and North Patagonian Andes (40–44°S) exhibits distinctive features that shed light on the dynamics of subduction-related volcanic activity in extensional tectonic settings. This period is particularly significant in the Andes, as it records the opening of forearc, intra-arc, and retroarc extensional basins, accompanied by high-volume volcanism, preceding the middle-late Miocene final major uplift phase of the Andes. This extensional event occurred in two phases, associated with changes in the geometry and dynamics of the subduction system: from oblique convergence at low rates during the late Eocene-early Oligocene to an orthogonal convergence at high rates during the late Oligocene-early Miocene. This study presents a compilation of field, petrographic, geochemical, and isotopic data from volcanic sequences in the North Patagonian Andes and the Southern Central Andes, aiming to analyze petrogenetic processes at different stages of arc evolution.During the late Eocene to early Oligocene, subduction-related volcanism occurred in a wide area from 100 to 300 km from the trench. In this period, magmatic arc rocks exhibit intermediate composition and a relatively homogeneous geochemical signature, transitional between tholeiitic and calc-alkaline series, with a marked subduction-related signature. These magmas originated from depleted mantle sources with high slab-derived fluid and sediment contributions and limited crustal interactions. In contrast, the late Oligocene to earliest Miocene volcanic activity, at the peak of the extensional regime, displays significant geochemical differences along the Andes. In the Southern Central Andes (33–40°S), volcanism displays compositions similar to those of the previous stage with varying influence from the subducting slab. In some sectors, magmas register the climax of the extensional conditions, whereas the youngest volcanic rocks towards the north (33–34°S) present evidence of the onset of contractional tectonics, demonstrated by the increased interaction of the magmas with a progressively thickened crust. In the North Patagonian Andes (40–44°S), subaerial and subaqueous arc-related lava flows are interbedded with marine deposits, indicating a rapid subsidence regime. These rocks exhibit geochemical features transitional from arc to E-MORB and even OIB compositions, which suggest a genesis primarily dominated by decompression melting of a heterogeneous mantle source. Above all, the contrasting nature of late Eocene-early Miocene arc products in the Southern Central and Patagonian Andes highlights that no single process can explain the changes in composition, distribution, and evolution of arc magmatism; rather, it is a combination of factors, including variations in subduction zone configuration and mantle dynamics and composition.

Dynamic climbing behavior and self-extinction mechanism of DC arc in a gradually changed air gap

DC arc behavior is normally a complicated and rapid time-dependent evolution process. To investigate the climbing behavior, dynamic properties, as well as the self-extinction mechanism, an experimental platform in collaboration with a pair of Jacob’s ladder electrodes was established. The overall evolution process including arc generation, arc combustion, and arc self-extinction was well accomplished. The dynamic climbing behavior and dynamic emission spectrum were captured. The dynamic volt-ampere characteristic, excitation temperature and electron density were all analyzed during the overall evolution process. Furthermore, the self-extinction mechanism under the function of Jacob’s ladder electrodes was explored. The experimental observations show that a two-stage climbing behavior is performed in succession. The dynamic resistance is fluctuated significantly in purple arc stage, while it is stable in yellow arc stage. The excitation temperature is positively correlated with arc current, but not strictly linear. The co-action results of two dynamic features make it more favorable for quick separation from the external ability of power supply. As a result, the arc is transitioned from self-sustaining state to non-self-sustaining state. This paper is regarded as a preliminary research aiming to provide some references for dynamic properties and self-extinction mechanism of the climbing arc in arcing horn.

Measurement of medium-voltage AC air arc temperature and particle number density based on dual-wavelength Moiré deflection technology

AC air arcs are generated in medium-voltage (MV) power systems under the effect of harsh weather conditions, equipment aging, and high penetration of distributed generation, threatening equipment and public safety. The arc current and temperature are low due to the wide application of arc suppression devices. In this scenario, the MV AC air arc does not satisfy the local thermodynamic equilibrium (LTE) condition. In addition, the repeated arcing and extinguishing processes further complicate the arc discharge mechanism, which bring challenges in the modeling and detection of MV AC air arcs. Experimental methods are a direct and efficient approach to determine the properties of arc plasmas. In this study, a dual-wavelength Moiré deflection diagnostic system was established to determine the time evolution of the particle density and radial distribution of the temperature in an MV AC air arc without relying on the LTE assumption. The electron number density and heavy particle number density change transiently during the arc discharge process and change gradient along the radial direction. The heavy particle temperature and electron temperature were then calculated based on the measured particle number density. During the arcing stage, the temperature of the electrons exceeded that of the heavy particles significantly, and the arc deviated from LTE. Finally, the limitations of the traditional single-wavelength Moiré deflection method are analyzed. The classic single-wavelength Moiré deflection method, while capable of estimating heavy particle temperature in plasma, exhibits a significant error in electron density estimation compared to the dual-wavelength Moiré deflection method.

Particle simulation on the ion acceleration in vacuum arc discharge

The supersonic ion jet generated in vacuum arc plasma has numerous applications, but the ion acceleration mechanism is incompletely understood. In this paper, a one-dimensional spherical vacuum arc discharge model is established, and the ion acceleration from vacuum breakdown to steady arc stage is investigated using particle-in-cell direct simulation Monte Carlo method. The results show that a potential hump exists and locates about far away from the cathode, but the potential drop provides a small contribution to the ion acceleration. A potential valley forms near the plasma–vacuum boundary and propagates towards the anode as the plasma expands during the vacuum breakdown stage. The ion mounting this valley experiences continuous acceleration by the dynamic electric field of the potential valley. As the plasma reaches the anode, a steady arc stage is gradually build-up, and the potential valley disappears. Consequently, the maximum mean ion velocity decreases. The source dominating the ion acceleration in this stage is turned into the electron–ion Coulomb collision.

Determination of the critical contact separation length for DC interruption in a vacuum subject to an external transverse magnetic field

The objective of this work is to experimentally determine the critical contact separation length Lcritical for DC interruption in vacuum under an external transverse magnetic field (TMF), according the interruption behaviors and vacuum arc characteristics. L is defined as the separation length at the instant of TMF application. Lcritical is the critical and the minimal necessary separation length of the contacts for a successful DC interruption in vacuum. A pair of Helmholtz coils was used to generate the external TMF. A high-speed camera was used to record the vacuum arc appearance. The experimental results quantitatively determine Lcritical for DC vacuum interruption under a certain DC and the external TMF. When the DC ranged from 100 to 500 A under an external TMF of 30 mT, Lcritical ranged from 2.1 to 3.7 mm and did not show linear variation. Second, the extinguishing time of DC vacuum interruption te at various currents significantly increased in a similar pattern under decreasing L in the range of 0.3–1.6 ms. The decrease in L mainly increased the duration of the stable stage and had little apparent influence on the duration of the unstable stage of the DC vacuum arc. Third, the physical mechanism determining Lcritical in DC vacuum interruption under a TMF could be explained from two aspects: the expansion of the vacuum arc column and the diffusion of the metal plasma. In successful DC vacuum interruption, Lcritical should be large enough for the formation and expansion of the “C”-shaped arc column, and enough space should be available for the diffusion of the metal plasma.

Detrital zircon U–Pb-Hf isotopes-based reconstruction of the depositional history of (ultra)high-temperature metamorphic rocks from the Socorro-Guaxupé Nappe (southern Brasília orogen, Brazil)

Deciphering the provenance of paraderived rocks that underwent high-to ultrahigh (HT-UHT) temperature metamorphism is a challenging task. Such metamorphic conditions can generate protracted zircon U–Pb age records of unclear meaning. However, the combination of U–Pb and Lu–Hf in zircon geochronology might be used as a powerful tool in order to retrieve the geological significance of geochronological data and track the signature of possible protosources. Six samples of ortho- and paraderived rocks from the São João da Mata complex in the Socorro-Guaxupé Nappe (SE Brazil), a section mainly containing a continental magmatic arc built at the margins of the Paranapanema paleocontinent affected by HT-UHT metamorphism have been investigated. U–Pb and Lu–Hf isotopic analyses were carried out to evaluate disturbances in the U–Pb system, identify minimum crystallization ages and the Lu–Hf signatures of protosources and protoliths. Five protosources were identified, being four showing crustal Hf signatures (εHf(804-776) = −16 to −2) and minimum U–Pb ages between ca. 804 Ma and ca. 776 Ma, and one with a moderately juvenile Hf signature (εHf(816) = +3 to +9) and a minimum U–Pb age of ca. 816 Ma. The moderately juvenile signature suggests contribution of an island or continental arc-related source. The remaining protosources exhibit similar Lu–Hf signatures to those of orthoderived rocks found within the Socorro-Guaxupé Nappe. We propose that the analyzed paraderived rocks were deposited in a restricted, arc-related setting, that evolved through the following stages: i) an island to early-continental arc stage, with a maximum depositional age of ca. 816 Ma; ii) a continental arc setting, recorded by the protosources with crustal signatures and exhibiting a maximum depositional age of ca. 776 Ma; iii) HT-UHT metamorphism between ca. 690–630 Ma, with high temperature conditions lasting until the end collisional stage of the Orogen (ca. 630–610).

An Analysis of Sheath Development of Cathode During the Instability Stage of Vacuum Arc

The purpose of this article was to investigate the dynamic evolution laws of the plasma between the gap during the instability phenomena in low-current vacuum arc using the simulation date comparison with the experimental data. The instability phenomena are characterized by high-frequency peaks of the arc voltage which are prior to the actual current chopping. Those peaks, which reach a level of more than ten times the normal arc voltage, have typical correspondence with the fluctuations of arc current and will result in a sudden chopping of the current through interaction with the elements of circuit. The fluctuations of arc voltage were classified into the types of long pulses and short pulses because of different characteristics on the duration time and oscillation morphology, which can summarize the peaks on the instability phenomena. The concepts of prearc cathode sheath and the cathode-emission intensity are proposed to reasonably explain the difference and connection between these two spikes. The distribution and motion of particles in the gap at different cathode-emission intensity, which demonstrating the development direction of the prearc cathode sheath and the relationship with the cathode-emission intensity, are simulated by the particle in cell-Monte Carlo collision (PIC-MCC). As a result, the relationship in the development of the prearc cathode sheath, the voltage spikes, and the intensity of cathode emission is summarized.

Effects of K2TiF6 and Electrolyte Temperatures on Energy Consumption and Properties of MAO Coatings on 6063 Aluminum Alloy

To decrease energy consumption and improve the performance of micro-arc oxidation (MAO) films on 6063 Al alloy, a policy of K2TiF6 additive and electrolyte temperature control was adapted. The specific energy consumption relied on the K2TiF6 additive and more particularly on the electrolyte temperatures. Scanning electron microscopy demonstrates that electrolytes with 5 g/L K2TiF6 can effectively seal the surface pores and increase the thickness of the compact inner layer. Spectral analysis shows that the surface oxide coating consists of γ-Al2O3 phase. Following 336 h of the total immersion process, the impedance modulus of the oxidation film, prepared at 25 °C (Ti5-25), remained 1.08 × 106 Ω·cm2. Moreover, Ti5-25 has the best performance/energy-consumption ratio with a compact inner layer (2.5 ± 0.3 μm). This research found that the time of the big arc stage increased with the temperature, resulting in producing more internal defects in the film. In this work, we employ a dual-track strategy of additive and temperature providing an avenue to reduce the energy consumption of MAO on alloys.

Effect of Surface Dispersion of Fe Nanoparticles on the Room-Temperature Flash Sintering Behavior of 3YSZ

Arc floating in surface flashover can be controlled by reducing the interfacial charge-transfer resistance of ceramics. However, thus far, only a few studies have been conducted on methods of treating ceramic surfaces directly to reduce the interfacial charge-transfer resistance. Herein, we explore the flash sintering behavior of a ceramic surface (3 mol% yttria-stabilized zirconia (3YSZ)) onto which loose metal (iron) powder was spread prior to flash sintering at room temperature (25 °C). The iron powder acts as a conductive phase that accelerates the start of flash sintering while also doping the ceramic phase during the sintering process. Notably, the iron powder substantially reduces the transition time from the arc stage to the flash stage from 13.50 to 8.22 s. The surface temperature (~1600 °C) of the ceramic substrate is sufficiently high to melt the iron powder. The molten metal then reacts with the ceramic surface, causing iron ions to substitute Zr4+ ions and promoting rapid densification. The YSZ grains in the metal-infiltrated area grow exceptionally fast. The results demonstrate that spreading metal powder onto a ceramic surface prior to flash sintering can enable the metal to enter the ceramic pores, which will be of significance in developing and enhancing ceramic-metal powder processing techniques.

Geochronology and Petrogenesis of Ahetala Granodiorite in South Tianshan Orogenic Belt, Xinjiang: New Constraints on the Tectonic Evolution of the South Tianshan Ocean

The Ahetala granodiorite is located in the western section of the South Tianshan Orogenic Belt (STOB), which is of great significance regarding the dispute on the closing date of the South Tianshan Ocean (STO) and the tectonic evolution of STOB. To determine the tectonic setting and petrogenesis, the study of petrography, electron probe microanalysis (EPMA), LA-ICP-MS zircon U–Pb geochronology, and major and trace elements analyses are carried out for Ahetala granodiorite. Based on LA-ICP-MS U–Pb zircon dating, the granodiorite was emplaced at 282.1 ± 1.3 Ma (MSWD = 1.11). Geochemically, Ahetala granodiorite is characterized by metaluminous (A/CNK = 0.86–0.87), rich alkali (K2O + Na2O = 6.80–7.13), which belongs to high-K calc-alkaline I-type granite. They are enriched in LREE and depleted in HREE (LREE/HREE = 9.02–13.89) and exhibit insignificant Eu anomalies (δEu = 0.94–0.97). Ahetala granodiorite is enriched in large ion lithophile elements (e.g., K, Sr, Ba) and depleted in high field-strength elements (e.g., Ta, Ti, Nb, P). The Nb/Ta values (10.97–18.10), Zr/Hf values (39.41–40.19), and Mg# (54.87–56.02) of the granodiorite and the MgO content of biotites (13.42–14.16), the M value (M = Mg/(Mg + Fe2+)) of amphiboles (0.68–0.75), suggest that granodiorite originates from the crustal contamination of the mantle-derived magmas. Combined with regional geological background, previous research, and the nature of the Ahetala granodiorite, we suggest that Ahetala granodiorite was emplaced at a transitional stage of the volcanic arc (syn-collision) to post-collision setting and the South Tianshan Ocean was closed in the Early Permian.

Editorial: Advances in the spatial and temporal evolution of oceanic arc-backarc systems

Oceanic arcs and their associated backarc basins are highly dynamic systems as their evolution is 12 strongly linked with far-field plate tectonic stresses, the regional geodynamic setting, local crustal 13 structure and mantle dynamics. The individual evolutionary stages of arc and backarc systems are 14 associated with significant changes in the physical and chemical conditions that control the 15 compositions of melts during generation, transport and storage, and eventually volcanic eruption. The 16 magmas are generated under variable conditions that range from fluxed melting due to the 17 devolatilization of the slab underneath the forearc and arc to dry adiabatic decompressional melting of 18 the mantle, especially underneath the backarc (Figure). The structure, composition and geometry of 19 the slab from the forearc to the backarc thus exerts an important control on metamorphic and 20 metasomatic processes and melting in the overlying mantle wedge. This research topic covers the 21 spatial evolution of arcs from the mantle (Bénard et al., 2022;Bénard, 2022) common in these settings and their location is controlled by regional geodynamics, magmatism and 72 local tectonics.

Dynamic characteristics of cathode spots during expansion process in a vacuum arc

Cathode spot is the source of plasma and metal vapor which affects the breaking capacity of vacuum circuit breakers. In this paper, the cup-shaped axial magnetic field contacts were used and the images of cathode spots were obtained by an ultra high-speed camera in the experiment. The dynamic characteristics of cathode spots during expansion process in vacuum arc were investigated. Under the conditions of different current amplitudes, different distributions of cathode spots were observed. During the expansion process of cathode spot, there are group spots and individual spots. The group spot only occurs in the cold electrode. The electrons emission characteristics of cathode spot as a function of temperature and electric field was calculated. The formation of group spot is related to the current and electron emission of cathode spot. The cathode spots were identified by Matlab. It was found that the variation of number of cathode spots is consistent with the current. However, at different stages, the reasons for the increase in the number of cathode spots are different, such as the increase in the area of cathode spots in the initial arcing stage and the splitting of cathode spot. It was observed that the cathode spots are not completely evenly distributed on the cathode surface. The farther away from the center of the contact, the longer the distance between the cathode spots. It is mainly effected by the transverse magnetic field (TMF) which is generated by the arc column. The closer to the contact edge, the higher the TMF at the cathode spot and the higher the expansion velocity of cathode spot.

An equivalent model of discharge instability in the discharge chamber of Kaufman ion thruster

The industrial application of the Kaufman ion thruster in its arc stage is limited owing to the instability of the discharge pulse. Presently, a complete prediction model that can predict the discharge pulse in the high-current stage does not exist. In this study, a complete prediction model for the pulse in the ion thruster is established using the zero-dimensional plasma discharge model and equivalent circuit model. The zero-dimensional plasma discharge model is used to obtain the corresponding plasma parameters by calculating the beam current, discharge current, voltage, and gas flow under actual working conditions. The input parameters of the equivalent circuit model are calculated using empirical formulae to acquire the estimated discharge waveforms. The pulse waveforms obtained using the model are found to be consistent with the experimental results. The model is used to evaluate the process of rapid changes in plasma density. Additionally, this model is employed to predict changes in the pulse waveforms when the volume of the discharge chamber and grid plate transmittance are changed.

Metallogeny of Endogenous Gold in Tuva

Abstract —Analysis shows that the occurrences of ore gold of different mineragenic types and ages widespread in the Republic of Tyva are related to the intricate and long geologic history of this region. The Tyva region is of polycyclic development; in the period from the Precambrian to the Cenozoic, there were stages of subduction, accretion–collision, and rifting processes accompanied by volcanism of different types and a wide manifestation of granitoid magmatism. Gold metallogeny was discretely developed throughout the geologic history of the region and was most widely manifested at the island arc stage (570–510 Ma), in connection with the formation of gold-bearing pyrite–polymetallic deposits. The late Cambrian–Ordovician period of granitoid magmatism was also productive for gold mineralization: It was marked by the formation of porphyry Au–Cu–Mo and gold deposits and gold–skarn, gold–quartz, and gold–beresite ore occurrences. Several gold-bearing clusters confined to different structure-formational zones are distinguished in the Tuva area. By now, commercial gold contents have been established in the Tardan gold deposit with Au reserves of 7371.8 kg, in the gold-bearing Kyzyl-Tashtyg pyrite–polymetallic deposit with estimated gold reserves of 15.08 tons and silver reserves of ~700 tons, and in the gold-bearing Ak-Sug porphyry Cu–Mo deposit with C2 gold reserves of 83 tons and silver reserves of 368 tons. In the course of prospecting and revision performed at many gold ore clusters in recent years, additional information about the high endogenous-gold potential of the Republic of Tyva has been obtained. There are a number of objects promising for commercial reserves of endogenous gold, such as the Kara-Bel’dir gold ore cluster with C1 and C2 gold reserves of 22.8 tons, the Aldan–Maadyr gold ore cluster with predicted P2 gold resources of 38 tons and Р3 gold resources of 80 tons, the Eilig-Khem ore cluster with predicted P3 gold resources of 36 tons, the Kyzyk-Chadr ore cluster with predicted Р1 + Р2 gold resources of 39.7 tons and silver resources of 198 tons in the Kyzyk-Chadr deposit, and the East Tannu-Ola ore district with gold resources of 68.3 tons. Thus, the intricate geologic development of the Tyva area and various geologic events, including large-scale magmatic processes, led to the formation of both gold and gold-bearing deposits and ore occurrences, which give reason to anticipate revealing of large commercial reserves of endogenous gold and the development of gold mining industry in the republic.

Petrogenesis of arc-related serpentinized peridotites (Egypt): Insights into Neoproterozoic mantle evolution beneath the Arabian-Nubian Shield

• Two ophiolitic peridotites in the SE Desert of Egypt provide a window into the Neoproterzoic mantle. • Geochemistry of peridotite and its mineral chemistry suggest an origin in the arc-related setting. • SED peridotites are refractory mantle residues (25–40% melting), like modern fore-arc peridotites. • Geochemical aspects of peridotite indicate changing from extension during subduction initiation. Korab Kansi and Abu Dahr Neoproterozoic ophiolitic peridotites in the Southern Eastern Desert (SED) of Egypt, are parts of the largest ophiolitic nappes in the Arabian-Nubian Shield. They mainly comprise refractory harzburgites with minor dunites that are commonly altered to serpentinites and talc carbonates. Olivine relics in serpentinized peridotites of the two complexes have olivines with high forsterite (Fo 90.6-92.2 ) and NiO (0.39 wt%) and low MnO (0. 13 wt%) contents, similar to those of mantle olivines. High Mg# (0.91–0.95) of orthopyroxene is consistent with that of depleted harzburgites. Primary spinel Cr# (0.55–0.75) and TiO 2 contents (<0.04 wt%) of the two peridotite complexes are similar to forearc peridotite spinels. The estimated degree of partial melting of Korab Kansi peridotites (25–40% melting) is slightly lower than that of Abu Dahr (30–40% melting), consistent with modern forearc peridotites. Korab Kansi peridotites exhibit lower oxygen fugacity (Δlog ƒO 2 , FMQ + 0.7 on average) and equilibrium temperature (610–710 °C) than those of Abu Dahr ( T , 750–900 °C at Δlog ƒO 2 , FMQ + 2.1), both suggesting subduction-modified peridotites. The low concentrations of incompatible elements, U-shaped REE patterns and high spinel Cr# of both peridotites indicate highly depleted mantle protoliths, reflecting multiple metasomatic and melting episodes in the mantle wedge. Enrichment of fluid mobile elements (e.g. B, Li, Th, U, Pb) relative to high-field strength elements (e.g. Nb, Ta, Ti, Zr) may reflect interaction between slab-derived fluids and peridotites. Calculated parental melts in equilibrium with Korab Kansi and Abu Dahr peridotite spinels have tholeiitic (MORB-like) and boninitic affinities, which were generated during proto-forearc spreading to a mature arc stage. Variations in melt compositions suggest a transitional setting from early proto-forearc spreading of mantle beneath Korab Kansi to the mature arc stage of Abu Dahr peridotites. Subduction initiation possibly started in the W and/or SW with proto-forearc spreading, and progressed to the E and/or NE where a mature arc stage with boninitic melts above an E- and/or SE-dipping subduction zone.

Zircon petrochronology reveals the moderately juvenile signature of a diatexite from the boundary zone between the Brasília and Ribeira orogens (SE Brazil): Relict of a Tonian arc?

Unraveling the tectonic evolution of Precambrian high-grade rocks is often challenging owing to widespread metamorphic overprint under high to ultra-high temperature conditions. In this scenario, isotopic systems-based investigations constitute an important tool to obtain the time constrains of formation and evolution of rocks.We present a petrochronological investigation using U–Pb and Lu–Hf data of zircon grains extracted from a biotite-bearing granitic leucosome from a diatexite found in the boundary zone between the Brasília and Ribeira orogens, SE Brazil. Clusters of 176Hf/177Hf(t) ratio values characterize two distinct zircon data groups, suggesting at least two different crystallization stages. 176Hf/177Hf(t) ratios from group 1 range from 0.28230 to 0.28247 with εHf (t) between +7.54 and −1.55, representing a moderately juvenile to evolved magma. This group constrains a time interval of ca. 890-540 Ma, and the ancient zircon core (890 Ma) could potentially record the minimum age for the magmatic crystallization of a moderately juvenile protolith, with ϵHf (890) ranging from +8.66 to +0.36. Excluding this ancient zircon (since it is the only date older than 800 Ma), the oldest grain provides a minimum crystallization age of ca. 797 Ma, with ϵHf (797) between +6.73 and -1.61. Group 2, exhibiting 176Hf/177Hf(t) ratios from 0.28214 to 0.28225and negative εHf(t) values (−3.35 to −7.53), represents xenocrysts of crustal origin from the protolith or an influx of allochthonous crustal melt during high-grade metamorphism. The age spreading from ca. 890 Ma to ca. 540 Ma along the concordia is consistent with a Pb-loss scenario, with cores locally providing younger ages than rims. This is also consistent with prolonged metamorphism, as newly crystallized domains also present a spread of ages. Our data indicate that the leucosome is the metamorphic product of a moderately juvenile rock. When placed in the regional context, the biotite-bearing granitic leucosome may represent the relic of a magmatic arc, now located in the boundary zone between the Brasília and Ribeira orogens. Its moderately juvenile signature suggests the following tectonic setting interpretations: (i) an intra-oceanic subduction setting similar to modern island arcs or an early stage of continental subduction, formed at ca. 890–800 Ma and, thus could be potentially correlated with juvenile magmatic arcs found in both Brasília (Mara Rosa – Arenópolis arc system) and Ribeira (Serra da Prata – Rio Negro arc system) orogens, suggestive of a large ocean realm to the south of the São Francisco paleocontinental block in Tonian time; and (ii) the most primitive relic of an early stage of the continental magmatic arc installed on the active margin of the Paranapanema paleocontinent.

Iron Nanoparticle Production by the Method of Electric Explosion of Wire

Background: The widespread use of iron nanopowders is connected with a wide range of characteristics such as size, magnetic characteristics and high surface area and that is why many researches present its different applications in the literature. Objective: The work studies the influence of the conditions for the iron wire electrical explosion on the course of the explosion process and the dispersed composition of the resulting metal nanopowder. Method: Experiments on the electrical explosion of iron wires were carried out in the laboratory setup with the different initial conditions of the electrical explosion of the iron wire. Results: The influence of the initial wire electrical explosion conditions on the explosion regime, the specific energy input into the conductor, and the specific energy released in the arc stage of discharge are definitely determined. The empirical equations for the calculation of the initial wire electrical explosion conditions for providing the critical explosion in the argon medium at a pressure of 2·105 Pa, were defined. It has been established that for the synthesis of iron nanopowders with a narrow particle size distribution, it is preferable to use modes with a high level of the energy released in the arc stage of the discharge. Conclusion: It was found that disabling the arc stage of the discharge during EEW leads to the decrease of the average surface particle size by 50%.

New approach to breakdown equation in gases based on energy concept from inception stage to arc condition

In this paper, the proposed breakdown equation of gases based on energy concept is introduced from initiating stage to complete breakdown (arc) stage. In compounds, the chemical enthalpy represented in bonding energy may be used as a fundamental principle to determine the electrical breakdown voltage. The bond dissociation enthalpy of gaseous compounds indicates the minimum energy needed to release chemical compounds in the material. The breakdown equation based on energy concept is a governing equation and relates between the field of streamer inception E0 and the field in different discharge stages. These stages can be specified from the breakdown index (n) value. Under arc condition, the temperature inside the discharge trail of arc may be up to 19200K according to the proposed breakdown equation. At this temperature, n equal 64 and the field under arc condition Eb=8E0. At atmospheric pressure and 1cm distance electrode gap, the Eb has been calculated for air, N2, H2, and SF6 (36, 39, 18, 81.9kV/cm respectively). The results comprised in this research compare with experimental and analytical consequences.

Simulation and Analysis of Muzzle Arc of Electromagnetic Rail Launch

Arc ignition technology is one of the most feasible strategies for releasing the residual energy of electromagnetic rail launch (EMRL). The muzzle arc characteristic must be considered in the design of arc ignition devices, because it is a main factor affecting the intermediate ballistic trajectory. Based on the theory of magnetohydrodynamics (MHDs), this work has made numerical simulation of muzzle arc and analyzed the temperature field, electromagnetic (EM) field, and flow field of the muzzle arc at different stages. The results show that the temperature is the highest, the EM field is the most intensive, and the flow is the strongest at the initial stage of the muzzle arc. The optimization of arc ignition device is pointed out to be reducing the current transfer time to improve the temperature, EM, and flow environments of the launcher, projectile, and guidance device.