The role of shoshonitic and calc-alkaline suites in the tectonic evolution of the Carajás District, Brazil

In the Carajás District, the Bahia Prospect and Grão Pará volcanosedimentary sequences, both of Archean age, have been the object of extensive petrochemical studies for major, trace, and rare earth elements. In spite of the widespread spilitization that affected the volcanic mafic rocks, the petrochemical nature of the mafic volcanism may still be assessed on the basis of the less mobile elements like P, Cr, Zr, Nb, Ti, and REE. The Bahia Prospect basalts have a petrochemical affinity with Phanerozoic island-arc calc-alkaline basalts. The Grão Pará basalts and basaltic andesites, on the other hand, have a petrochemical affinity with Phanerozoic basalts of a mature island arc environment. The petrochemical data presented here lead to a new geotectonic interpretation of the area, where the volcanic sequences are genetically linked to Archean subduction processes.     

Experimental hydrogen isotope studies III: Diffusion of hydrogen in hydrous minerals,and stable isotope exchange in metamorphic rocks

Diffusion parameters for hydrogen diffusion in epidote-group minerals and micas have been measured under hydrothermal conditions, or calculated from existing experimental data, for bulk hydrogen isotope exchange experiments between hydrous minerals and water. Activation energies in the range 14 to 31 kcals/g-atom H are comparable to those derived by application of kinetic theory to experimental hydrogen isotope exchange data, and to those for oxygen diffusion in minerals under hydrothermal conditions. Diffusion of hydrogen in epidote is about four orders of magnitude faster than in muscovite, and about two orders of magnitude faster than in zoisite. Hydrogen diffusion in micas is about five orders of magnitude faster than oxygen diffusion, and hydrogen transport occurs dominantly parallel to the layers rather than parallel to the c-axis as for oxygen.Rapid hydrogen transport in minerals may proceed by hydrolysis of Si-O and Al-O bonds, followed by exchange of hydrolyzed oxygens with slower-diffusing (OH) or H₂O. Water appears to be essential for stable isotope exchange between minerals in slowly cooling metamorphic rocks.Stable isotope data for regional metamorphic mineral assemblages suggests that water is usually present in small amounts during cooling of prograde regional metamorphic systems, and estimated closure temperatures for cessation of stable isotope exchange are often more comparable to those calculated from diffusion data than to likely temperatures of metamorphism.Alpine deformation of the Hercynian Monte Rose Granite (Frey et al. 1976) permitted access of water and initiated stable isotope exchange amongst coexisting minerals. The diffusional behaviour of species in relict Hercynian muscovites is consistent with available experimental diffusion data.     

Extreme relativistic model for neutron stars and pulsars

Exact solutions have been obtained for a massive fluid sphere under the extreme causality condition (dP/dρ)=1. Radial pulsational stability of these structures has been discussed. It is found that for pulsationally stable configurations the surface to central density ratio is greater than 0.30, the maximum values for surface and central redshifts are 0.85 and 3.40 respectively in the extreme case, and the maximum mass and size are respectively 4.8M _⊙ and 20.1 km. It has also been shown that these structures are gravitationally bound, with a maximum binding energy per unit rest mass equal to 0.25 for a surface to central density ratio ?0.40. Slow rotation of these configurations has also been considered, and the relative drag and moment of inertia have been calculated. These results have been applied to the Crab pulsar and the mass of the pulsar has also been calculated based upon this model.     

Assessing the relative packing behaviour of Chandarpur and Khairagarh sandstones of Proterozoic Chhattisgarh Basin

The paper concerns mainly with properties of packing of the Proterozoic Chandarpur and Khairagarh sandstones of Chhattisgarh Basin. An attempt has been made to decipher the nature of boundary between Chandarpur Group and overlying Raipur Group on the basis of grain packing. For the purpose of study based on eight representative thin sections of Chandarpur and Khairagarh sandstones, measurements of grain size distribution, types of grain contacts, packing density (Pd), packing proximity (Pp) and linear regression analysis has been carried out. The relation of packing density with packing proximity of Khairagarh sandstone shows positive correlation coefficient (r) with linear regression equation (y) and no clear trend in packing proximity (Pp) vs grain size (Is) is observed whereas Chandarpur sandstone shows packing density increases and packing proximity decreases with linear regression equation (y) and correlation coefficient (r). Chandarpur grain size tends to increase with packing density (Pd) whereas packing proximity (Pp) increases but Khairagarh sandstone packing density (Pd) decrease with increase of grain size (Is) and packing proximity (Pp) tends to increase with decrease in grain size. Above study indicates that packing behaviour of Chandarpur sandstones and Khairagarh sandstones are entirely different.     

Tidal currents,energy flux and bottom boundary layer thickness in the Clyde Sea and North Channel of the Irish Sea

A high-resolution three-dimensional model of the Clyde Sea and the adjacent North Channel of the Irish Sea is used to compute the major diurnal and semidiurnal tides in the region, the associated energy fluxes and thickness of the bottom boundary layer. Initially, the accuracy of the model is assessed by performing a detailed comparison of computed tidal elevations and currents in the region, against an extensive database that exists for the M₂, S₂, N₂, K₁ and O₁ tides. Subsequently, the model is used to compute the tidal energy flux vectors in the region. These show that the major energy flux is confined to the North Channel region, with little energy flux into the Clyde Sea. Comparison with the observed energy flux in the North Channel shows that its across-channel distribution and its magnitude are particularly sensitive to the phase difference between elevation and current. Consequently, small changes in the computed values of these parameters due to slight changes of the order of the uncertainty in the open-boundary values to the model, can significantly influence the computed energy flux. The thickness of the bottom boundary layer in the region is computed using a number of formulations. Depending upon the definition adopted, the empirical coefficient C used to determine its thickness varies over the range 0.1 to 0.3, in good agreement with values found in the literature. In the North Channel, the boundary layer thickness occupies the whole water depth, and hence tidal turbulence produced at the sea bed keeps the region well mixed. In the Clyde Sea, the boundary layer thickness is a small fraction of the depth, and hence the region stratifies.Responsible Editor: Phil Dyke     

Assessing the use of saltmarsh flats by fish in a temperate Australian embayment

Little is known about the importance of salmarshes to juvenile and adult fishes in temperate Australia. We assessed diel and feeding patterns of fish inhabiting saltmarsh in a sheltered embayment along the coast of Victoria, Australia, between October 2002 and May 2003. The saltmarsh flat was generally only inundated during low-pressure weather systems (barometric pressure <1,013 hP). Fish were sampled over the saltmarsh flats using fyke and seine nets. A total of 2,047 individuals (10 species, including juveniles and adults) were caught.Atherinasoma microstoma was most abundant (fyke [F], μ=1.6 fish h⁻¹; seine [S], μ=28.2 fish shot⁻¹), followed byFavonigobius, lateralis (F: 0.5; S: 0.6),Galaxias maculatus (F: 0.1),Heteroclinus adelaide (F:<0.1),Kestratherina esox (F: <0.1; S: 1.6),Leptatherina presbyteroides (F: <0.1; S: 7.1) andTetractenos glaber (F: 1). Commercial species, includingAldrichetta forsteri (F: <0.1; S: 3.2),Sillaginodes punctata (F: <0.1; S: 0.9), andRhombosolea tapirina (F: 0.4), were commonly sampled. Variability in species richness or fish abundance was not explained by water temperature, salinity, depth, or barometric pressure. Significantly more species were sampled with the seine during nocturnal periods (p=0.002); fish abundance did not vary between diel periods, nor did fish abundance and species richness in fyke net samples. Diets of the most abundant species (A. microstoma, A. forsteri, andF. lateralis) were primarily composed of gammaridean amphipods and hemipteran insects. There was no correlation between fish diets and the composition of benthic invertebrates as sampled at 3 different regions of the saltmarsh flat. The saltmarsh flats in our study region are inhabited by several species normally associated with alternative habitat types such as seagrass, and the patterns of habitat use observed appear to be partially attributed to foraging behavior.     

A numerical model for porosity modification at a sandstone-mudstone boundary by quartz pressure dissolution and diffusive mass transfer

In many sandstones quartz cements significantly reduce porosity. The origin of these cements is often unclear. This paper investigates a possible mechanism for the generation of silica for quartz cement by pressure dissolution in interbedded mudstones. Theoretical models of quartz pressure dissolution, including the effects of silica precipitation kinetics, show that the concentration of dissolved silica in the pore fluids of a compacting sediment increases with decreasing grain size and silica precipitation constant. Quartz precipitation is strongly inhibited by the presence of small amounts of clay within a sediment, suggesting that siltstones and quartz-rich mudstones which are undergoing pressure dissolution may act as a source of dissolved silica for export to nearby, coarser sediments. A computational model for the diagenetic modification of a sandstone-mudstone interface due to pressure dissolution is described. Both sandstone and mudstone layers are assumed to be actively compacting by pressure dissolution, and mass transport by molecular diffusion is considered. As quartz precipitation in the mudstone layer is relatively slow compared to that in the sandstone, significant amounts of dissolved silica become available in the mudstone, and may be exported into the adjacent sandstone. In the absence of pore-fluid advection, this may result in the formation of extensive secondary quartz within the sandstone, close to the interface. The volume of silica exported from the mudstone is limited by the length scale over which diffusion through the mudstone is effective. This is typically 3–5 m. The volume of silica available therefore suggests that extensive porosity modification within the adjacent sandstone can only occur close to the mudstone. Thus it is possible that thin sandstones could become cemented by slow diffusive transfer of silica, but that in thicker sandstones the silica may become dispersed by pore-fluid advection.     

Mercury partition in the interface between a contaminated lagoon and the ocean: The role of particulate load and composition

After having estimated the patterns of flow to the ocean and found some seasonal and tidal differences, mainly with regard to the relative importance of dissolved and particulate fractions, mercury partitioning at the interface between a contaminated lagoon and the Atlantic Ocean was investigated during four tidal cycles in contrasting season and tidal regimes. Mercury was found to be located predominantely in the particulate fraction throughout the year, contributing to its retention within the system. Seasonal conditions, variations in marine and fluvial signals and processes affecting bed sediment resuspension influenced the character and concentration of suspended particulate matter in the water column. Variation in the nature, levels and partitioning of organic carbon in the particulate fraction affected levels of particulate mercury as well as mercury partitioning. These results highlight the dominant role of suspended particulate matter in the distribution of anthropogenic mercury and reinforce the importance of competitive behavior related to organic carbon in mercury scavenging.