M.A.Z. Vasconcellos

Paleoproterozoic Magmatic Provenance of Detrital Zircons, Porongos Complex Quartzites, Southern Brazilian Shield

Provenance investigation using detrital zircon U-Pb SHRIMP (sensitive high-resolution ion microprobe) age dating for six quartzite samples (167 spot determinations on 166 grains) indicates that metasedimentary rocks of the Porongos Complex from the southern Brazilian shield were derived almost entirely from Paleoproterozoic sources. Intensive study of igneous and metamorphic rocks in this portion of southwestern Gondwana employing SHRIMP geochronology over the past five years provides important evidence for provenance investigation of zircon grains. Ages of magmatic sources of the zircon detritus in the deformed quartz sandstones show an eight-peak distribution (known equivalent rocks in parenthesis): 2470 Ma (Neto Rodrigues Gneiss), 2350 Ma (Santa Maria Chico Complex Granodiorite), 2200 Ma (Encantadas Complex Tonalite), 2140 Ma (Paso Severino Dacite, Miséria Mylonite), 2100 Ma (Sierra Azul Granite), 2080 Ma (Chacofy Tonalite, Villa Monica Monzogranite), and 2040 and 2020 Ma (Itapema Monzogranite). Ages between 2260 and 2000 Ma correspond to the Trans-Amazonian Cycle, and were already known in the basement of the region, but the 2470 Ma and 2350 Ma ages are a new contribution regarding the tectonic evolution of the southern Brazilian shield. Tectonic scenarios for the Porongos basin fill are restricted to two different environments: (1) cratonic cover, if the fill is near 1500 Ma; or (2) passive margin if the age of fill is near 1000 Ma (this explanation requires additional studies). We have thus elucidated a major problem in the provenance of detrital zircon in the southern Brazilian shield, because we now know that the protolith sediments of the Porongos Complex were deposited under stable tectonic conditions and derived from Paleoproterozoic sources. This occurred after cratonization of the crust and formation of supercontinent Columbia following the end of the Trans-Amazonian Cycle. Nearly all zircon analyses have Th/U ratios higher than 0.2. Thus, only magmatic crystals survived the sedimentary processes. We show that detailed study of detrital zircon is a powerful tool for the understanding of the provenance of sandstones.

Cyclic Fatigue Resistance of Rotary Nickel-Titanium Instruments Submitted to Nitrogen Ion Implantation

INTRODUCTION The aim of this study was to assess cyclic fatigue resistance in rotary nickel-titanium instruments submitted to nitrogen ion implantation by using a custom-made cyclic fatigue testing apparatus. METHODS Thirty K3 files, size #25, taper 0.04, were divided into 3 experimental groups as follows: group A, 12 files exposed to nitrogen ion implantation at a dose of 2.5 x 10(17) ions/cm(2), accelerating voltage of 200 kV, currents of 1 microA/cm(2), 130 degrees C temperature, and vacuum conditions of 10 x 10(-6) torr for 6 hours; group B, 12 nonimplanted files; and group C, 6 files submitted to thermal annealing for 6 hours at 130 degrees C. One extra file was used for process control. All files were submitted to a cyclic fatigue test that was performed with an apparatus that allowed the instruments to rotate freely, simulating rotary instrumentation of a curved canal (40-degree, 5-mm radius curve). An electric motor handpiece was used with a contra-angle of 16:1 at an operating speed of 300 rpm and a torque of 2 N-cm. Time to failure was recorded with a stopwatch in seconds and subsequently converted to number of cycles to fracture. Data were analyzed with the Student t test (P < .05). RESULTS Ion-implanted instruments reached significantly higher cycle numbers before fracture (mean, 510 cycles) when compared with annealed (mean, 428 cycles) and nonimplanted files (mean, 381 cycles). CONCLUSIONS Our results showed that nitrogen ion implantation improves cyclic fatigue resistance in rotary nickel-titanium instruments. Industrial implementation of this surface modification technique would produce rotary nickel-titanium instruments with a longer working life.

Synthesis and characterization of Fe-Al2O3 composites

Abstract We have synthesized composites of the type Fex(Al2O3)100−x by arc-melting compacted pellets of iron and alumina powders with starting compositions of x=0.4, 2, 3, 5, 10, 20, 40 and 60. Aluminas of two different nominal grades (99.7% and 99.99%) were used in the preparation of the composites. The samples were characterized by X-ray diffractometry, magnetization and Mossbauer spectroscopy. The results revealed the formation of the spinel FeAl2O3+z (hercynite phase) and of metallic iron partially nanostructured by this melting process. The Mossbauer subspectral areas of the formed phases were plotted against x, showing that increasing the starting iron content of the mixed powders favors the hercynite reaction. It was also observed that when the more pure alumina is used for cast, more hercynite is formed or, alternatively, less iron nanocrystallites precipitate, when compared to the less pure oxide. An arc-melted sample (x=2) was annealed at 1200°C in a hydrogen atmosphere, revealing hercynite reduction and phase separation of metallic iron.

FRACTURE SEALING IN ZIRCON AS EVALUATED BY ELECTRON MICROPROBE ANALYSES AND BACK-SCATTERED ELECTRON IMAGING

Abstract The internal structure of zircon is commonly revealed by back-scattered electron (BSE) images. Grains from a 750-700 Ma old talc-alkaline mafic metatonalite from southernmost Brazil show relict igneous faces, irregular and diffuse metamorphic overgrowths, and fracture sealing. Sealing of fractures is observed as light grey patches in BSE images. EPMA profiles show strong but differential increases in Hf and U in the sealed fractures, unlike the common assertion in the literature that fractures are only pathways of leaching. Sealing of fractures occurs near borders of grains as well as in their cores and is not visible during standard petrographic examination; thus careful BSE image evaluation is needed before either in situ or whole grain isotopic determinations are made.

Ion microprobe (SHRIMP) dates complex granulite from Santa Catarina, southern Brazil

Complex polymetamorphic granulites have been dated in the Santa Catarina granulite complex of southern Brazil through SHRIMP study of zircon. This complex is dominated by intermediate-acid plutonic rocks and contains small volumes of mafic and ultramafic rocks, and minor quartzite and banded iron formation. Porphyroblasts of orthopyroxene, clinopyroxene and plagioclase in mafic and acid rocks are interpreted as magmatic remnants in a volumetrically dominant granoblastic aggregate (M1) of the same minerals and hornblende. Hornblende formed during a later M2 metamorphic event constitutes rims around pyroxene, but the hornblende is also rimmed by granoblastic simplectites of orthopyroxene, clinopyroxene, hornblende and plagioclase in a second granulite facies event (M3). Chlorite and epidote occur in shear zones (M4). This granulite terrain is part of a Neoproterozoic craton, because it was little affected by the Brasiliano Cycle. The two granulite-facies events (M1 and M3) are dated by U/Pb zircon SHRIMP at about 2.68 and 2.17 Ga, while the magmatic protoliths formed at about 2.72 Ga. The amphibolite facies event (M2) probably occurred close to the 2.17 Ga granulitic metamorphism.

Structural and magnetic behavior of Ar+‐implanted Co/Pd multilayers: Interfacial mixing

The magnetization behavior of Co/Pd multilayers has been analyzed as a function of the degree of interfacial mixing among the Co and Pd layers. Controlled atomic mixing was induced by low‐dose and low‐flux ion implantation and a follow‐up of the structural status of the samples was made by simulation of the high‐angle x‐ray‐diffraction data. Values of the saturation magnetization as a function of the broadness of Co concentration profile are presented and explained by a simple model based on the parameters obtained from the x‐ray simulations.

The Effect of Argon and Nitrogen Ion Implantation on Nickel-Titanium Rotary Instruments

INTRODUCTION This qualitative study investigated the effect of N(2)(+) and Ar(+) ion implantation on morphologic alterations and fatigue resistance in Pro Taper S1 NiTi (Dentsply-Maillefer, Ballaigues, Switzerland) rotary instruments. METHODS Instruments were divided into three groups: N(2)(+) implanted, Ar(+) implanted, and unmodified control group. All instruments were used to prepare five curved canals in epoxy resin blocks with brushing motion. The instruments were examined in a scanning electron microscope (SEM) before use, after first use, and after the fifth use. A more demanding cyclic fatigue test was undertaken, submitting the instruments to 15-second periods of continuous rotation inside the curved canals without a brushing motion. Crack formation was analyzed with the SEM, and the number of 15-second periods required to fracture each instrument was recorded. RESULTS No significant morphologic alterations were observed in the instruments after the preparation of five canals. Crack density was similar in all groups. In the subsequent cyclic fatigue test, instruments implanted with nitrogen performed worse than those implanted with argon and the control group. Fracture faces show differences in the fracture modes. CONCLUSIONS Ar(+) implantation improved the performance of S1 files moderately, whereas nitrogen ion-implanted files performed worse in the fatigue test. A reduction in file performance seems to be caused by nitrogen diffusion in the grain boundaries, instead of the desired improvement caused by titanium nitride formation.

Near-surface composition and microhardness profile of plasma nitrided H-12 tool steel

Samples of AISI H-12 tool steel were plasma nitrided at 500°C in a mixture of H2-20% N2 under a total pressure of 6 mbar, by using DC and pulsed glow discharges. The treatment time varied from 1 to 6 h. X-ray diffraction (glancing angle and u-2u geometry), conversion electron Mossbauer spectroscopy, conversion X-ray Mossbauer spectroscopy, electron probe microanalysis, optical micrograph and Vickers microhardness were used as analytical techniques. The obtained results suggest that, under the present experimental conditions: (i) The near-surface compound layer consists of a mixture of g%-Fe4N and o-Fex(N, C); (ii) the near-diffusion zone compound layer consists of a mixture of g%-Fe4N, o-Fex(N, C), aƒ-Fe16N2 and g-austenite; (iii) the dependence of compound layer thickness on nitriding time violates the parabolic behavior and emphasizes effects from cathode sputtering and radiation-enhanced diffusion.

57Fe CEMS characterization of iron-aluminum thin-film alloys☆

Abstract Conversion electron Mossbauer spectroscopy is presented here as a useful technique to investigate interdiffusion and reaction in thin solid films as induced by thermal annealing, ion mixing or a simultaneous combination of these two processes. As an illustrative example, the experimental results of the particular iron-aluminum bilayer system are reviewed. Some X-ray diffraction patterns are also shown for the sake of comparison.

Interdiffusion and reaction in the FeAl bilayer IV: Phase characterization of ion-beam-mixed samples

Abstract This is the fourth in a series of papers concerning the experimental investigation of the interdiffusion processes and solid phase reactions occuring in FeAl bilayers subjected to furnace annealing, ion beam mixing and simultaneous combination of these two types of metallurgical treatment. Conversion electron Mossbauer spectroscopy and X-ray diffraction allowed us to identify as Fe2Al5 and the metastable phase FeAl6 the phases obtained under the combined effect of ion bombardment and controlled substrate temperature. The bilayer surface and interface were also characterized by scanning electron microscopy and the dependence of the sheet resistivity on the bombardment parameters was determined. The results are compared with those obtained in the FeAl bilayer system subjected to furnace annealing only.