Two-stage evolution of mantle peridotites from the Stalemate Fracture Zone, northwestern Pacific

This paper reports the results of a mineralogical study of 14 mantle peridotite samples dredged in 2009 from the eastern slope of the northwestern segment of the Stalemate Ridge in the northwestern Pacific during cruise SO201-KALMAR Leg 1b of the R/V Sonne. The sample collection included four serpentinized and silicified dunites and ten variably serpentinized lherzolites. The compositions of primary minerals (clinopyroxene, orthopyroxene, and spinel) change systematically from the lherzolites to dunites. Spinel from the lherzolites shows higher Mg# and lower Cr# values (0.65-0.68 and 0.26-0.33, respectively) compared with spinel from the dunites (Mg# = 0.56-0.64 and Cr# = 0.38-0.43). Clinopyroxene from the lherzolites is less magnesian (Mg# = 91.7-92.4) than clinopyroxene from dunite sample DR37-3 (Mg# = 93.7). Based on the obtained data, it was concluded that the lherzolites of the Stalemate Fracture Zone were derived by 10-12% near-fractional melting of a DMM-type depleted mantle reservoir beneath the Kula-Pacific spreading center. The dunites were produced by interaction of residual lherzolites with sodium- and titaniumrich melt and are probably fragments of a network of dunite channels in the shallow mantle. The moderately depleted composition of minerals clearly distinguishes the lherzolites from the strongly depleted peridotites of the East Pacific Rise and indicates the existence of slow-spreading mid-ocean ridges in the Pacific Ocean during the Cretaceous-Paleogene

The influence of the dechanneling process on the photon emission by an ultra-relativistc positron channeling in a periodically bent crystal

We investigate, both analytically and numerically, the influence of the dechanneling process on the parameters of undulator radiation generated by ultra-relativistic positron channelling along a crystal plane, which is periodically bent. The bending might be due either to the propagation of a transverse acoustic wave through the crystal, or due to the static strain as it occurs in superlattices. In either case the periodically bent crystal serves as an undulator which allows to generate X-ray and gamma-radiation. We propose the scheme for accurate quantitative treatment of the radiation in presence of the dechanneling. The scheme includes (i) the analytic expression for spectral-angular distribution which contains, as a parameter, the dechanneling length, (ii) the simulation procedure of the dechanneling process of a positron in periodically bent crystals. Using these we calculate the dechanneling lengths of 5 GeV positrons channeling in Si, Ge and W crystals, and the spectral-angular and spectral distributions of the undulator over broad ranges of the photons. The calculations are performed for various parameters of the channel bending.Comment: published in J. Phys. G: Nucl. Part. Phys. 27 (2001) 95-125, http://www.iop.or

Triple GEM Tracking Detectors for the BM@N Experiment

BM@N (Baryonic Matter at the Nuclotron) is the fixed target experiment aimed to study nuclear matter in the relativistic heavy ion collisions at the Nuclotron accelerator in JINR. The BM@N tracking system is based on Gas Electron Multipliers (GEM) detectors, mounted inside the BM@N analyzing magnet. The structure of the GEM detectors and the results of study of their characteristics are presented. The GEM detectors are integrated into the BM@N experimental setup and data acquisition system. The results of the first test of the GEM tracking system in the technical run with the deuteron beam are shortly reviewed

Comparisons between Tethyan Anorthosite-bearing Ophiolites and Archean Anorthosite-bearing Layered Intrusions: Implications for Archean Geodynamic Processes

Elucidating the petrogenesis and geodynamic setting(s) of anorthosites in Archean layered intrusions and Tethyan ophiolites has significant implications for crustal evolution and growth throughout Earth history. Archean anorthosite-bearing layered intrusions occur on every continent. Tethyan ophiolites occur in Europe, Africa, and Asia. In this contribution, the field, petrographic, petrological, and geochemical characteristics of 100 Tethyan anorthosite-bearing ophiolites and 155 Archean anorthosite-bearing layered intrusions are compared. Tethyan anorthosite-bearing ophiolites range from Devonian to Paleocene in age, are variably composite, contain anorthosites with highly calcic (An44-100) plagioclase and magmatic amphibole. These ophiolites formed predominantly at convergent plate margins, with some forming in mid-ocean ridge, continental rift, and mantle plume settings. The predominantly convergent plate margin tectonic setting of Tethyan anorthosite-bearing ophiolites is indicated by negative Nb and Ti anomalies and magmatic amphibole. Archean anorthosite-bearing layered intrusions are Eoarchean to Neoarchean in age, have megacrystic anorthosites with highly calcic (An20-100) plagioclase and magmatic amphibole and are interlayered with gabbros and leucogabbros and intrude pillow basalts. These Archean layered intrusions are interpreted to have predominantly formed at convergent plate margins, with the remainder forming in mantle plume, continental rift, oceanic plateau, post-orogenic, anorogenic, mid-ocean ridge, and passive continental margin settings. These layered intrusions predominantly crystallized from hydrous Ca- and Al-rich tholeiitic magmas. The field, petrographic and geochemical similarities between Archean and Tethyan anorthosites indicate that they were produced by similar geodynamic processes mainly in suprasubduction zone settings. We suggest that Archean anorthosite-bearing layered intrusions and spatially associated greenstone belts represent dismembered subduction-related Archean ophiolites

Model for screening of resonant magnetic perturbations by plasma in a realistic tokamak geometry and its impact on divertor strike points

This work addresses the question of the relation between strike-point splitting and magnetic stochasticity at the edge of a poloidally diverted tokamak in the presence of externally imposed magnetic perturbations. More specifically, ad-hoc helical current sheets are introduced in order to mimic a hypothetical screening of the external resonant magnetic perturbations by the plasma. These current sheets, which suppress magnetic islands, are found to reduce the amount of splitting expected at the target, which suggests that screening effects should be observable experimentally. Multiple screening current sheets reinforce each other, i.e. less current relative to the case of only one current sheet is required to screen the perturbation.Comment: Accepted in the Proceedings of the 19th International Conference on Plasma Surface Interactions, to be published in Journal of Nuclear Materials. Version 2: minor formatting and text improvements, more results mentioned in the conclusion and abstrac

Modelling of the effect of ELMs on fuel retention at the bulk W divertor of JET

Effect of ELMs on fuel retention at the bulk W target of JET ITER-Like Wall was studied with multi-scale calculations. Plasma input parameters were taken from ELMy H-mode plasma experiment. The energetic intra-ELM fuel particles get implanted and create near-surface defects up to depths of few tens of nm, which act as the main fuel trapping sites during ELMs. Clustering of implantation-induced vacancies were found to take place. The incoming flux of inter-ELM plasma particles increases the different filling levels of trapped fuel in defects. The temperature increase of the W target during the pulse increases the fuel detrapping rate. The inter-ELM fuel particle flux refills the partially emptied trapping sites and fills new sites. This leads to a competing effect on the retention and release rates of the implanted particles. At high temperatures the main retention appeared in larger vacancy clusters due to increased clustering rate