Strong Nb Research Articles

Geochemistry of Paleoproterozoic metavolcanic rocks from the southern Ashanti volcanic belt, Ghana: Petrogenetic and tectonic setting implications

Geochemical data are presented for Paleoproterozoic metavolcanic rocks from the southern Ashanti volcanic belt with the aim of inferring their petrogenesis and tectonic setting in which they were formed. The metavolcanic rocks, which are predominantly basalts/basaltic andesites and andesites, have high Cr and Ni contents, indicating that they have not undergone significant fractional crystallization from mantle-derived melts. Two types of basalts/basaltic andesites, Type I and Type II, were identified. The Type I basalts show flat to slightly LREE-depleted patterns with (La/Sm) N = 0.69–1.03, (La/Yb) N = 0.57–0.71 and minor negative and positive Eu anomalies (Eu/Eu* = 0.92–1.24). The Type II basalts/basaltic andesites show fractionated REE patterns with (La/Sm) N = 1.34–2.31, (La/Yb) N = 2.08–4.25 and minor positive Eu anomalies (Eu/Eu* = 1.09–1.13). The andesites also show fractionated REE patterns, (La/Sm) N = 1.97–2.78 and (La/Yb) N = 4.11–8.48 with minor positive to non-existent Eu anomalies (Eu/Eu* = 0.99–1.15). N-MORB-normalized, trace element patterns show that the Type II basalts/basaltic andesites and the andesites have geochemical patterns characterized by enrichment in LILE relative to HFSE and in LREE relative to HREE. The andesites and the Type II basalts/basaltic andesites exhibit characteristics of subduction zone-related magmas; i.e., the former displays strong Nb and Ti anomalies and relatively high Th/Nb ratios (0.69–95) whereas the latter displays strong negative Nb anomalies but relatively smaller Ti anomalies and lower Th/Nb ratios (0.35–0.37). The Type I basalts/basaltic andesites are generally moderately enriched in LILEs and depleted in HFSEs and HREEs relative to N-MORB. Like the andesites and the Type II basalts/basaltic andesites, the Type I basalts/basaltic andesites display subduction-related trace element characteristics of positive Ba and Sr anomalies together with slightly negative Nb–Ta anomalies. The low Th/Nb ratios of 0.06–0.11 coupled with their MORB-like REE patterns indicate no or negligible amount of sedimentary component in the Type I basalts/basaltic andesites. Crustal contamination may be ruled out as the cause of the negative Th and HFSE anomalies observed in the basaltic and andesitic rocks. Rather the anomalies are interpreted, on the basis of Th–Nb–La–Ce, to reflect a recycled slab-derived lithosphere component. The LREE-depleted tholeiitic Type I basalts/basaltic andesites exhibit back-arc basin geochemical signatures. The calc-alkaline andesites and the Type II basalts/basaltic andesites show intra-oceanic island arc signatures. The high Mg basaltic/andesitic rocks may, however, have evolved in a forearc setting. We, therefore, infer an intra-oceanic island arc-forearc-backarc setting for the Paleoproterozoic Birimian metavolcanic rocks from the southern Ashanti volcanic belt. REE modeling indicates that the magma from which the Type I basalts were formed was generated by about 17–30% partial melting of a depleted mantle (DMM) composed of spinel lherzolite in an extensional marginal basin. The parental magmas of the high Mg calc-alkaline basaltic/andesitic rocks were produced in the forearc by about 20–30% partial melting of the metasomatized mantle, and the Type II basalts/basaltic andesites and the andesites are related by fractional crystallization.

(Nb2W4O19), TMA2, Na4(OH2)14(SO4): A New Layered Structure with Lindqvist Heteropolyanions, XAS Characterization of the HPAs.

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(Nb2W4O19), TMA2, Na4(OH2)14(SO4): a new layered structure with Lindqvist heteropolyanions, XAS characterization of the HPAs

The new (Nb2W4O19),TMA2, Na4(OH2)14(SO4) has been evidenced as a minor phase during the Nb2W4O19TMA (tetramethylammonium) salt synthesis. Its crystal structure has been refined from single crystal X-ray diffraction data, system monoclinic, a=10.166(5) Å, b=17.93(1) Å, c=24.81(1) Å, β=93.057(7)°, space group (S.G.) C2/c, Z=4, R1=3.96%, wR1=4.50%. It shows the stacking of cationic and anionic bidimensional layers. The anionic layer of formula [(Nb2W4O19), TMA2 ]2− is formed of isolated Lindqvist HPAs surrounded by TMA groups. The isolated layers adopt a trigonal symmetry that is lost in the crystal by the association of the cationic sheets. These later, of formula [Na4(OH2)14(SO4)]2+ form porous net-like sheets with nearly circular cavities of diameter 7.5 Å. SO43− groups host the available cavities in a disordered manner. The cohesion between the sheets is performed by both electrostatic interactions and a set of hydrogen bonds. In the cationic layers, the highly symmetrical surrounding of HPAs by TMA groups yields a homogeneous electrostatic field at their external surface leading to a statistic Nb/W disorder over the three available independent metallic positions. Then, XAS experiments at the L1/L3-W edge complementarily helped to highlight the preferential cis configuration of (Nb2W4O19)4− anions, help to the strong Nb vs W contrast in their contribution to the backscattering paths. Previously to these experiments, it was of course checked that both the two phases present in the prepared sample contain Nb2W4O19 anions with nearly unchanged geometry.

Cu Ion Plating as a Technique for Enhancing the Mechanical, Electrical and Thermal Bonding Between Cu Stabilizer and the RHQT-Processed<tex>$rm Nb_3rm Al$</tex>Conductors

The surface of RHQT-processed Nb/sub 3/Al wires with pure Nb matrix are covered by a strong stable Nb oxide surface layer. In order to obtain good mechanical, electrical and thermal bonding between the Cu stabilizer and RHQT-processed Nb/sub 3/Al wires, it is required to destroy the Nb oxide layer on the surface of the wire. We tried to fabricate a thin Cu layer on the surface of the RHQT-processed Nb/sub 3/Al wires through the Cu ion-plating technique. Before electroplating of thick Cu stabilizers, Cu was ion-plated to a thickness of about 1 /spl mu/m. The Cu ion-plated wire showed no folded projections, cracks, or exfoliation of the Cu stabilizer even when the wire was bent through 180 degrees, showing that Cu and the wire were tightly bonded mechanically. This tight bonding between Cu and the wires should be due to the removal of stable Nb oxide layers from the surface of the wire. The V-I characteristic of wires that were ion-plated and then applied with an appropriate amount of Cu by an electroplating showed no quenching, and the wires were able to carry current up to the normal critical current transition. The measured values of recovery current were almost equivalent to the calculated values, showing that the ion-plated Cu/Nb interface had a very tight bond, allowing good electrical and thermal conductivity. In addition, a reel to reel Cu-ion plating apparatus for the long length wires is demonstrated, and Cu ion plating has been successfully carried out on 100 m long RHQT-processed Nb/sub 3/Al wire at present.

Geochemical and petrological evidence for subduction–accretion processes in the Archean Eastern Indian Craton

The composition of sandstones often provides key evidence about the tectonic, weathering and transport processes operating on the surface at the time of deposition. Petrologic and geochemical analyses of little-metamorphosed middle–late Archean sandstones from the Eastern Indian Craton show that the sedimentary rocks were derived from dominantly 3.3-Ga-old amphibolites of the Older Metamorphic Group (OMG) and tonalites of the Older Metamorphic Tonalite Gneisses (OMTG), the two oldest lithologic units of this craton. Chondrite-normalized rare earth element (REE) patterns of the sandstones show a light REE-enriched signature with (La/Sm)N varying from 5.2 to 6.7 with no Eu anomaly, while the heavy REEs display flat patterns with (Gd/Lu)N values of 0.9–1.6. Primitive mantle-normalized incompatible and compatible trace element plots of these sandstones demonstrate an overall similarity with global Proterozoic–Archean sandstones, including strong Nb–Ta negative anomalies. In an fSm/Nd vs. ϵNd(0) diagram, the sandstones plot precisely between the regional amphibolites and tonalites. We infer from the REE abundances and the fSm/Nd vs. ϵNd(0) plot that the sandstones represent a bimodal mechanical mixture of OMG and OMTG. The low Ce/Pb ratios of these rocks of 1–4 indicate a variably Pb-enriched Archean crust and that the Ce/Pb ratio acquired the continental crustal signature, distinctly different from those of the bulk silicate earth and mantle values at least as early as mid-Archean. Strong Nb–Ta depletion relative to the primitive mantle suggests the sandstones were derived from subduction-related magmatic arc sources. This latter suggestion is strongly supported by the low Nb/Ta and high Zr/Sm ratios of these sandstones, identical to Archean tonalite–trondhjemites that require, based on recent trace-element partitioning results, their protoliths to have formed by subduction melting of low-magnesium amphibolites or metamorphosed hydrous basalts [1,2]. The average Nd model ages of the sandstones are greater than the Sm–Nd crystallization ages of the OMG and OMTG at ∼3.3 Ga. The geochemical data presented here can be collectively interpreted to suggest the presence of subduction–accretion processes operational in the mid-Archean Eastern Indian Craton. Because the OMG and OMTG, the source rocks of the sandstones, formed in a subduction-related arc setting, the basement rocks upon which this arc was constructed must have been older. This observation and the depleted mantle Nd model ages (TDM) of the sandstones, ranging from 3.6 to 4.0 Ga, strongly indicate the presence of continental crust in this Eastern Indian craton older than 3.3 Ga and possibly as old as 4.0 Ga.

Geochemistry of boninite-type volcanic rocks in the Frotet-Evans greenstone belt, Opatica subprovince, Quebec: implications for the evolution of Archaean greenstone belts

Low titanium (TiO 2=0.35–0.67 wt.%), high silica (SiO 2=49.0–59.8 wt.%) and magnesium-rich (MgO=4.5–13.1 wt.%) volcanic rocks in the Frotet-Evans greenstone belt (FEGB) of the Archaean Superior Province display U-shaped REE profiles, based on La/Sm N (1.4–2.1) and Gd/Lu N (0.67–1.1) ratios, that are inherent to Cenozoic boninites. These Archaean volcanic rocks, although not boninites in sensu stricto, show strong Nb and Ta depletions, enrichment in LREE and Th as well as distinct positive Zr and Hf anomalies in primordial mantle-normalised spidergrams. The Zr and Hf anomalies are characteristic of many Western Pacific boninite suites. The stratigraphic position of the Frotet-Evans boninite-type rocks is of significance because it differs from other known greenstone belts. The boninite-type rocks are contained within a predominantly tholeiitic basalt assemblage composed of transitional tholeiites, magnesian tholeiites, and ferrotholeiites. The studied sequence may either overlie adakitic-type pyroclastic rocks or be interstratified with andesitic to rhyodacitic calc-alkaline volcanic formations that compare favourably with oceanic arcs. This stratigraphic association of the FEGB differs from the Abitibi greenstone belt in which boninite-type volcanic rocks are interstratified with komatiites that formed a proto-arc sequence and were controlled by a combination of subduction- and plume-generated processes. In contrast, the proposed setting for the subduction-generated boninitic rocks of the FEGB is a marginal oceanic basin following forearc splitting and extension caused by hinge rollback.

Middle Cretaceous back-arc formation and arc evolution along the Asian margin: the Shyok Suture Zone in northern Ladakh (NW Himalaya)

The Shyok Suture Zone of the Ladakh palaeovolcanic arc is made of Cretaceous volcanosedimentary formations intruded by granodioritic plutons. Field observations show a tectonic stacking of litho-units from different volcanic arc and back-arc environments. In the western part (Skardu area), the Shyok Suture Zone can be subdivided into two groups: (1) The Northern Group, which consists of olistolith basaltic blocks and tuffs. The basalts are LREE depleted with a LILE enrichment and a slight Nb depletion suggesting a back-arc basin origin. (2) The Southern Group, which consists predominantly of andesites that show LREE enrichment, a flat HREE pattern, strong Nb–Ta depletion, and LILE enrichment. The volcanic rocks of the Southern Group appear to have island-arc tholeiite (IAT) to calc-alkaline affinities. In the eastern part of the suture zone (Nubra–Shyok area), Albian to Cenomanian age silicoclastic sediments predominate. These sediments correspond to a large detrital platform built on the northern part of the Ladakh Arc. At the top, these sediments interlayer with basaltic to andesitic lavas. These lavas appear to be more differentiated and calc-alkaline in nature than the Skardu Southern Group lavas, but show similar volcanic arc affinities. No evidence of a back-arc basin was found in this area. Our data from these two areas shows a northwest–southeast evolution, from back-arc to arc formations in northern Ladakh. Opening of this back-arc basin occurred on the northwestern side of the Ladakh Arc. This back-arc was progressively closing eastward. The arc itself was more mature in the east, resembling the southern Tibetan continental arc, and was characterised by more continental sedimentation. Subsequent Himalayan tectonometamorphic evolution, closure of the back-arc basin and deformation along the Shyok Suture, reflects an early asymmetrical geometry along the Asian margin and Kohistan–Ladakh Arc.

Continuum predictions of deformation in composites with two creeping phases—II. [formula omitted] composites

The primary and steady-state creep behavior of ductile-phase toughened Nb 5Si 3/Nb in situ composites has been simulated using the analytical and finite element (FE) continuum techniques described in Part I of this paper. The material constants in the creep constitutive law have been fitted to experimentally measured creep properties of the bulk Nb 5Si 3 and Nb solid-solution phases. The composite microstructure has been idealized to facilitate the modeling, with a variety of microstructural idealizations resulting in very similar predicted composite creep rates. The models somewhat underpredict the steady-state strain rates and over predict the primary creep strains for the Nb-10 at.% Si composite studied experimentally. Several potential causes for these discrepancies have been identified, including internal damage effects and a possible increase in the Nb 5Si 3 stress exponent at high stress. However, the models correctly make the important prediction that the creep strength of the Nb-rich composite is dominated by the strong Nb 5Si 3 phase. FE predictions of large internal tensile stresses in the matrix are consistent with experimental observations of creep damage.

Concentration dependence of the local order in amorphous NiNb alloys

Abstract The atomic structure of amorphous NiNb alloys has been studied via molecular-dynamic simulations on the basis of effective interatomic potentials calculated using the hybridized nearly-free-electron-tight-binding-bond technique. The partial static structure factors and pair correlation functions at four different compositions are compared with results from neutron diffraction with isotopic substitution. Theory and experiment agree in finding a certain chemical order (similar to that in the crystalline NiNb compounds) and strong NiNb bonds that are contracted compared with the average NiNi and NbNb bond lengths. The chemical short-range order depends only weakly on composition. In addition to the laboratory experiment, the computer simulation also yields information on the partial triplet correlation functions. The bond-angle distributions indicate that the local atomic arrangement is essentially of a polytetrahedral type, with broad peaks in the total bond-angle distribution close to the icosahedral bond-angles. In the partial bond-angle distributions, the peaks are slightly shifted as a consequence of size differences and chemical order. This shift is again similar to the bond angles in the coordination polyhedra of the crystalline Frank-Kasper phase. Compared with NiZr and NiY and Ni-metalloid glasses, NiNb glasses are found to be more random and show no sign of the trigonal prismatic order characteristic of these systems. The physical origin of these structural trends is discussed.

Structural transformation in Nb 2O 5-promoted Rh catalysts during calcination and reduction treatments

The extent of RhNb 2O 5 interaction in the niobia-promoted Rh SiO 2 catalyst depends on parameters such as calcination temperature. For the Nb 2O 5-promoted Rh SiO 2 catalyst with the Nb Rh atomic ratio of unity, no significant SMSI effects were observed after air calcination at 773 K. However, this catalyst exhibits significant SMSI behaviors after calcination at higher temperatures (973 and 1173 K). After high-temperature reduction at 773 K, the capacity of H 2 chemisorption typically diminished almost to zero and the catalytic activity for ethane hydrogenolysis was suppressed by ca. 3.5 orders of magnitude compared with that after low-temperature reduction at 373 K. X-ray diffraction showed that the RhNbO 4 compound was formed, with the amount increasing with increasing calcination temperature. After air calcination at 1173 K, the RhNbO 4 compound was formed almost exclusively on the SiO 2 surface. The RhNbO 4 compound was reduced to Rh and NbO 2 during HTR treatment at 773 K, accompanied by splitting of the large RhNbO 4 particle into a number of smaller Rh particles. The mechanism of RhNb 2O 5 interaction is discussed. It is proposed that the NbO 2 species formed during the HTR treatment block surface Rh atoms to induce strong RhNb 2O 5 interaction (SMSI).