Carbonatites from the southern Brazilian Platform: A review. II: Isotopic evidences

Early and Late Cretaceous alkaline and alkaline-carbonatitic complexes from southern Brazil are located along the main tectonic lineaments of the South America Platform. Calcium-, magnesium-, and ferrocarbonatites are well represented and frequently associated even in the same complex. Primary carbonates present significant variations in C-O isotopic compositions, which are mainly due to isotope exchange with H2O-CO2-rich hydrothermal fluids, whereas fractional crystallization or liquid immiscibility probably affects the δ18O and δ13C values by no more than 2δ‰. Our isotope exchange model implies that the most significant isotopic variations took place in a hydrothermal environment, e.g., in the range 400-80°C, involving fluids with the CO2/H2O ratio ranging from 0.8 to 1. Sr-Nd-Pb isotope systematics highlight heterogeneous mixtures between HIMU and EMI mantle components, similar to the associated alkaline rocks and the flood tholeiites from southern Brazil. In spite of the strong variation shown by C-O isotopes, Sr-Nd-Pb-Os isotopic systematics could be related to an isotopically enriched source where the chemical heterogeneities reflect a depleted mantle metasomatized by small-volume melts and fluids rich in incompatible elements. These fluids are expected to have promoted crystallization of K-rich phases in the mantle, which produced a veined network variously enriched in LILE and LREE. The newly formed veins (enriched component) and peridotite matrix (depleted component) underwent a different isotopic evolution with time as reflected by the carbonatites. These conclusions may be extended to the whole Paraná-Etendeka system, where isotopically distinct parent magmas were generated following two main enrichment events of the subcontinental lithospheric mantle at 2.0-1.4 and 1.0-0.5 Ga, respectively, as also supported by Re-Os systematics. The mantle sources preserved the isotopic heterogeneities over a long time, suggesting a nonconvective lithospheric mantle beneath different cratons or intercratonic regions. Overall, the data indicate that the alkaline-carbonatitic magmatism originated from a locally heterogeneous subcontinental mantle

Crystallochemical comparison between Portland cements and mineral trioxide aggregate (MTA)

Purpose: The aims of this study were to compare the crystal chemical properties of some commercial mineral trioxide aggregate (MTA) and Portland cements (PC) and to propose a new white MTA product. Methods: The samples (four MTA and two PC types) were analyzed by 1) optical microscopy; 2) laser granulometry; 3) X-ray diffraction and fluorescence; 4) scanning electron microscopy (SEM) and electron probe microanalysis (EPM) (wavelength-dispersive). Results: MTA and PC specimens yielded similar characteristics in their clinker component. The MTA-Angelus specimens displayed a composition overlapping the classical clinker composition (wt%) i.e. 25 silica, nine alumina and 66 lime. However, the bismite, present in large amounts (~15-19 wt%) in all MTA products, contained considerable and diffused heavy (toxic) metals as Pb and Mo, other than Bi. In the MTA clinkers the formation of Portlandite, at water-clinker interface, is favored by the smaller grain size of the MTA particles. However, this may also favor the diffusion of the toxic elements linked to Bi. Significance: In terms of bulk physico-chemical properties, the MTA products can be easily substituted by a fine-grained Portland clinker by adding a non-toxic radio-opaque component, for example, Ba-carbonat

Niobium and rare earth minerals from the Virulundo carbonatite, Namibe, Angola

The Virulundo carbonatite in Angola, one of the biggest in the world, contains pyrochlore as an accessory mineral in all of the carbonatite units (calciocarbonatites, ferrocarbonatites, carbonatite breccias, trachytoids). The composition of the primary pyrochlore crystals is very close to fluornatrocalciopyrochlore in all these units. High-temperature hydrothermal processes caused the pseudomorphic replacement of the above crystals by a second generation of pyrochlore, characterized by lower F and Na contents. Low-temperature hydrothermal replacement of the above pyrochlores, associated with production of quartz-carbonates-fluorite veins, controled the development of a third generation of pyrochlore, characterized by high Sr contents. Finally, supergene processes produced the development of a secondary paragenesis in the carbonatite, consisting in late carbonates, goethite, hollandite and REE minerals (mainly synchysite-(Ce), britholite-(Ce), britholite-(La), cerite-(Ce)). Separation of Ce from the other REE was allowed by oxidizing conditions. Therefore, Ce4+ was also incorporated into a late generation of pyrochlore, which is also strongly enriched in Ba and strongly depleted in Ca and N

Two typologies of endodontic instrument handle are here investigated for tactility modifications.

AIM: The purpose of this comparative study was to evaluate possible modifications of tactility (deep and discriminative tactile sensitivity) using manual endodontic instruments equipped with traditional handles compared with innovative silicone handle versions. METHODS: Sixteen subjects were selected and supplied with resin Endo-Training-Block simulators, latex gloves and manual endodontic instruments with diameters ranging from 10 to 40 mm. All instruments had traditional and silicon handles. The investigation of deep and discriminative tactile sensitivity was performed at 0, 15, 30, and 45 minutes of the manual instrumentation with an esthesiometer. We obtain each of our data from the median of three consecutive measurements. The results were then statistically analyzed thanks to the ANOVA test: P=0.003 for subjects, P<0.001 for time factor, and P=0.010 for sensitivity-time interaction. RESULTS: We observed a better tactility with silicone handle instruments, comparing the two kinds of handle (regardless of the finger tested) as well as singly analyzing the thumb and index finger. CONCLUSION: Despite the sample limited extension, the loss of deep cutaneous and discrimination sensitivity seems to be reduced using silicone handles, most likely favoring an improvement of the working performance