Sediment hosted lead–zinc deposits of the Neoproterozoic Bambuí Group and correlative sequences, São Francisco Craton, Brazil: A review and a possible metallogenic evolution model

The Proterozoic sediment-hosted Zn–(Pb) sulfide and non-sulfide deposits of the São Francisco Craton, Brazil, are partially syn-diagenetic and epigenetic and were probably formed during extensional events. The majority of the deposits occur within shallow water dolomites. The Pb isotopic data of sulfides are relatively homogeneous for individual deposits and plot above the upper crust evolution curve of the Plumbotectonic model. Some of the deposits are characterized by highly radiogenic lead (²⁰⁶Pb/²⁰⁴Pb ≥ 21) originating from the highly radioactive crust of the São Francisco Craton. Pb and S isotopic data suggest the sources of metal and sulfur for the deposits to be the basement rocks and seawater sulfates in the sediments, respectively. The relatively high temperatures of formation (100 to 250 °C) and moderate salinity (3% to 20% NaCl equiv.) of the primary fluid inclusions in the sphalerite crystals suggest the participation of basinal mineralizing fluids in ore formation. The steep paleo-geothermal gradient generated by the radioactively enriched basement rocks probably assisted in heating up the circulating mineralizing fluids.     

On the azimuthal brightness variations of Saturn's rings

We present a simple, semiquantitative explanation that accounts both for the presence of the azimuthal brightness variations in Saturn's ring A and for their absence in ring B. Our explanation avoids any ad hoc reliance on albedo variations and/or synchronous rotation of ring particles. Instead, it requires only some degree of self-gravitation between nearby orbiting bodies. A bias in the particle distribution and corresponding photometric effects are thereby produced—the latter corresponding very closely to the variations observed in ring A. Their absence in ring B is primarily a consequence of the higher optical thickness and decreasing importance of self-gravitation in that ring.     

Attenuation tomography: An application to gas-hydrate and free-gas detection

We estimate the quality factor (Q) from seismic reflections by using a tomographic inversion algorithm based on the frequency‐shift method. The algorithm is verified with a synthetic case and is applied to offshore data, acquired at western Svalbard, to detect the presence of bottom‐simulating reflectors (BSR) and gas hydrates. An array of 20 ocean‐bottom seismographs has been used. The combined use of traveltime and attenuation tomography provides a 3D velocity–Q cube, which can be used to map the spatial distribution of the gas‐hydrate concentration and free‐gas saturation. In general, high P‐wave velocity and quality factor indicate the presence of solid hydrates and low P‐wave velocity and quality factor correspond to free‐gas bearing sediments. The Q‐values vary between 200 and 25, with higher values (150–200) above the BSR and lower values below the BSR (25–40). These results seem to confirm that hydrates cement the grains, and attenuation decreases with increasing hydrate concentration.     

Imposex levels and concentrations of organotin compounds (TBT and its metabolites) in Nassarius nitidus from the Lagoon of Venice

Specimens of Nassarius nitidus were collected in seven stations of the Venice Lagoon to assess the levels of tributyltin (TBT) and its metabolites monobutyltin and dibutyltin in the tissues and monitor their effect on organisms, in particular the phenomenon of imposex (superimposition of male sexual characteristics on females). The following values of population indices were found: vas deferens sequence: 1.2 ± 0.7–4.0 ± 0.5; relative penis length: 6–47%. The least impacted station was situated in the northern part of the Lagoon, where females without imposex were found and Butyltin (BuTs) concentrations in the organisms (average sum of BuTs = 43 ± 14 ng Sn g⁻¹ w.) were significantly lower than in the other stations (range of average sum of BuTs: 101 ± 22–217 ± 27 ng Sn g⁻¹ d.w.). Population indices were found to be related to the TBT content in the tissues. In particular VDSI had a significant logarithmic correlation: r = 0.95, n = 8, p < 0.05.     

Melt inclusions in detrital spinel from the SE Alps (Italy–Slovenia): a new approach to provenance studies of sedimentary basins

Detrital spinel is a widespread heavy mineral in sandstones from the Maastrichtian–Middle Eocene sedimentary basins in the SE Alps. Chemistry of detrital spinels from the Claut/Clauzetto and Julian Basins (N Italy and NW Slovenia) is used to constrain petrological and geochemical affinities and tectonic provenance of the source rocks. In addition, we have analysed melt inclusion compositions in the detrital volcanic spinels to better constrain the nature of their parental magmas. This is the first study of melt inclusions in detrital spinels. Two principal compositional groups of detrital spinels are recognised based on their TiO₂ and Fe²⁺/Fe³⁺; one derived from peridotites, the other from basaltic volcanics. Peridotitic spinels are more abundant and have TiO₂ < 0.2 wt% and high Cr/Cr + Al (40–90), characteristic of suprasubduction zone harzburgites. Significant chemical variations among volcanic spinels (TiO₂ up to 3 wt%, Al₂O₃ 12–44 wt%) suggest multiple sources, with geochemically distinct characteristics, including MORB-type and backarc basin basalts, subduction-related magmas and tholeiites produced during early continental rifting. Compositions of homogenised melt inclusions in spinels with TiO₂ > 0.2 better distinguish the differences between the compositions of their host spinels and help to further clarify the geodynamic provenance of extrusive source rocks. Several compositional groups of melt inclusions have been recognised and represent diverse magmatism of marginal basins, including MORB- and subduction-related geochemical types, as well as magmas characteristic of early continental rifting. These results, combined with the data on regional ophiolitic complexes and tectonic reconstructions favour the Internal Dinarides of Yugoslavia as a possible source area for the SE Alps sediments. Received: 20 January 2000 / Accepted: 25 April 2000     

Compton dragged gamma-ray bursts: the spectrum

Recently several studies have jointly analysed data from different cosmological probes with the motivation of estimating cosmological parameters. Here we generalize this procedure to allow freedom in the relative weights of various probes. This is done by including in the joint χ ² function a set of 'hyper-parameters', which are dealt with using Bayesian considerations. The resulting algorithm, which assumes uniform priors on the log of the hyper-parameters, is very simple: instead of minimizing     (where     is per data set j ) we propose to minimize     (where N _j is the number of data points per data set j ). We illustrate the method by estimating the Hubble constant H ₀ from different sets of recent cosmic microwave background (CMB) experiments (including Saskatoon, Python V, MSAM1, TOCO and Boomerang ). The approach can be generalized for combinations of cosmic probes, and for other priors on the hyper-parameters.     

On the origin and tectonic significance of the intra-plate events of Grenvillian-type age in South America: A discussion

The objective of this article is to examine the available evidence of intra-plate tectonic episodes of “Grenvillian-type age”, affecting the South-American continent, assessing their possible causal correlation with the tectonic processes occurring within the orogenic belts active at their margins. For the Amazonian Craton, the active margin is represented by the Rondonian-San Ignacio and Sunsas belts. However, active margins of similar age are not recognized for the São Francisco and the Rio de La Plata Cratons, and the intra-plate events over them could be reflections of the Kibaran, Irumide or Namaqua orogenic collisions in Africa. Grenvillian-type age events over the Amazonian Craton can be described in four different aspects: shearing and tectonic reactivation along zones of weakness, cratogenic granitic magmatism, alkaline ring complexes, and pervasive regional heating in some localized regions. The first of them may reflect the compressional stresses at active margins, however the others may have different origins. Within the type-region of the K’Mudku tectono thermal episode, mylonites and pseudotachylites cut across the regional granitoid and metamorphic rocks. These shear belts developed under low-to-moderate temperature conditions, that induced resetting of K–Ar and Rb–Sr mineral ages. In the São Francisco Craton, extensional and compressional events of Grenvillian-type age are well registered by the structural features exhibited by the sedimentary rocks of the Espinhaço Supergroup. For example, in Bahia state, an Appalachian-style structure is observed, with large synclines and anticlines extending along hundreds of kilometers. The major difference between the Amazonian and the Congo-São Francisco Cratons is related to heat originated from the Earth’s interior. Amazonia exhibits very large areas heated up to 350–400 °C, where the K’Mudku thermo-tectonic episodes were detected. In addition, Amazonia comprises a large amount of cratogenic granitic intrusions, and some alkalic complexes of Mesoproterozoic age, whose origin could be attributed, at least partially, to deeper sources of heat. This is not reported for the São Francisco Craton, and also for its African counterpart, the Congo Craton. Moreover, the Grenvillian-type age intra-plate features over South America demonstrate that while many cratonic fragments were colliding to build Rodinia, rifting was already occurring in parts of the Amazonian and the Congo-São Francisco Cratons.     

The link between partial melting,granitization and granulite development in central Ribeira Fold Belt,SE Brazil: New evidence from elemental and Sr–Nd isotopic geochemistry

Elemental and Sr–Nd isotopic data on metatexites, diatexites, orthogneisses and charnockites from the central Ribeira Fold Belt indicate that they are LILE-enriched weakly peraluminous granodiorites. Harker and Th–Hf–La correlation trends suggest that these rocks represent a co-genetic sequence, whereas variations on CaO, MnO, Y and HREE for charnockites can be explained by garnet consumption during granulitic metamorphism.Similar REE patterns and isotopic results of ?_Nd⁵⁶⁵ = ?5.4 to ?7.3 and ⁸⁷Sr/⁸⁶Sr₅₆₅ = 0.706–0.711 for metatexites, diatexites, orthogneisses and charnockites, as well as similar T_DM ages between 2.0 and 1.5 Ga are consistent with evolution from a relatively homogeneous and enriched common crustal (metasedimentary) protolith.Results suggest a genetic link between metatexites, diatexites, orthogneisses and charnockites and a two-step process for charnockite development: (a) generation of the hydrated igneous protoliths by anatexis of metasedimentary rocks; (b) continuous high-grade metamorphism that transformed the “S-type granitoids” (leucosomes and diatexites) into orthogneisses and, as metamorphism and dehydration progressed, into charnockites.     

Turbulence in the Stable Boundary Layer at Higher Richardson Numbers

We present some algebraic and numerical simulations of the stable boundary layer. We also discuss the problem of the existence of a critical Richardson number (Ri), beyond which the turbulence is suppressed. We compare the results of a second-order algebraic model with those of a third-order numerical model and, to this purpose, numerical simulations of a wind-tunnel flow, which is characterized by various Richardson numbers, were performed. As far as the second-order model is concerned, solutions, for the Richardson number greater than any critical value, can be obtained by modifying the time scales of the second-order equation pressure correlation terms in order to account for a buoyancy damping factor. We show that using a third-order model allows the same results (no critical Richardson number) to be obtained without modifications to the time scales. It is suggested that the non-locality, accounted for by the third-order moments, could allow the turbulence to persist also for Ri > 1.