A Sequence of Pan-African and Hercynian Events Recorded in Zircons from an Orthogneiss from the Hercynian Belt of Western Central Iberia--an Ion Microprobe U-Pb Study

Sensitive high-resolution ion microprobe U–Pb dating showsthat a biotite orthogneiss from the Hercynian belt of westerncentral Iberia contains 1000–300 Ma zircon. Older, 1000–570Ma ages within this range represent inherited, detrital materialamong which four age components may be recognized:     

Indicator Simulation Accounting for Multiple-Point Statistics

Geostatistical simulation aims at reproducing the variability of the real underlying phenomena. When nonlinear features or large-range connectivity is present, the traditional variogram-based simulation approaches do not provide good reproduction of those features. Connectivity of high and low values is often critical for grades in a mineral deposit. Multiple-point statistics can help to characterize these features. The use of multiple-point statistics in geostatistical simulation was proposed more than 10 years ago, on the basis of the use of training images to extract the statistics. This paper proposes the use of multiple-point statistics extracted from actual data. A method is developed to simulate continuous variables. The indicator kriging probabilities used in sequential indicator simulation are modified by probabilities extracted from multiple-point configurations. The correction is done under the assumption of conditional independence. The practical implementation of the method is illustrated with data from a porphyry copper mine.     

The 1.80–1.74-Ga gabbro–anorthosite–rapakivi Korosten Pluton in the Ukrainian Shield: a 3-D geophysical reconstruction of deep structure

The deep structure of the gabbro–anorthosite–rapakivi granite (“AMCG-type”) Korosten Pluton (KP) in the northwestern Ukrainian Shield was studied by 3-D modelling of the gravity and magnetic fields together with previous seismic data. The KP occupies an area of ca. 12,500 km² and comprises several layered gabbro-anorthositic intrusions enveloped by large volumes of rapakivi-type granitoids. Between 1.80 and 1.74 Ga, the emplacement of mafic and associated granitoid melts took place in several pulses. The 3-D geophysical reconstruction included: (a) modelling of the density distribution in the crust using the observed Bouguer anomaly field constrained by seismic data on Moho depth, and (b) modelling of the magnetic anomaly field in order to outline rock domains of various magnetisation, size and shape in the upper and lower crust. The density modelling was referred to three depth levels of 0 to 5, 5 to 18, and 18 km to Moho, respectively. The 3-D reconstruction demonstrates close links between the subsurface geology of the KP and the structure of the lower crust. The existence of a non-magnetic body with anomalously high seismic velocity and density is documented. Most plausibly, it represents a gabbroic stock (a parent magma chamber) with a vertical extent of ca. 20 km, penetrating the entire lower crust. This stock has a half-cylindrical shape and a diameter of ca. 90 km. It appears to be connected with a crust–mantle transitional lens previously discovered by EUROBRIDGE seismic profiling. The position of the stock relative to the subsurface outlines of the KP is somewhat asymmetric. This may be due to a connection between the magmatism and sets of opposite-dipping faults initially developed during late Palaeoproterozoic collisional deformation in the Sarmatian crustal segment. Continuing movements and disturbances of the upper mantle and the lower crust during post-collisional tectonic events between 1.80 and 1.74 Ga may account for the long-lived, recurrent AMCG magmatism.     

Simulation of Orissa Super Cyclone (1999) using PSU/NCAR Mesoscale Model

In this study a non-hydrostatic version of Penn State University (PSU) -- NationalCenter for Atmospheric Research (NCAR) mesoscale model is used to simulate thesuper cyclonic storm that crossed Orissa coast on 29 October 1999. The model isintegrated up to 123 h for producing 5-day forecast of the storm. Several importantfields including sea level pressure, horizontal wind and rainfall are compared with theverification analysis/observation to examine the performance of the model. The modelsimulated track of the cyclone is compared with the best-fit track obtained from IndiaMeteorological Department (IMD) and the track obtained from NCEP/NCAR reanalysis. The model is found to perform reasonably well in simulating the track and in particular, the intensity of the storm.     

Iodine-xenon analysis of ordinary chondrite halide: implications for early solar system water

We report the results of iodine-xenon analyses of irradiated halide grains extracted from the H-chondrite Monahans (1998) and compare them with those from Zag (Whitby et al., 2000) to address the timing of aqueous processing on the H-chondrite parent body. Xe isotopic analyses were carried out using the RELAX mass spectrometer with laser stepped heating. The initial ¹²⁹I/¹²⁷I ratio in the Monahans halide was determined to be (9.37 ± 0.06) × 10⁻⁵ with an iodine concentration of ∼400 ppb. Significant scatter, especially in the Zag data, indicates that a simple interpretation as a formation age is unreliable. Instead we propose a model whereby halide minerals in both meteorites formed ∼5 Ma after the enstatite achondrite Shallowater (at an absolute age of 4559 Ma). This age is in agreement with the timing of aqueous alteration on the carbonaceous chondrite parent bodies and ordinary chondrite metamorphism and is consistent with the decay of ²⁶Al as a heat source for heating and mobilisation of brines on the H-chondrite parent body. Post accretion surface impact events may have also contributed to the heat source.     

The structural role of Ti in aluminosilicate liquids in the glass transition range: insights from heat capacity and shear viscosity measurements

The shear viscosities and 1 bar heat capacities of glasses and melts along the 67mol% silica isopleth in the system SiO₂-Al₂O₃-Na₂O-TiO₂ have been determined in the temperature ranges 780-1140 K and 305-1090 K respectively. Anomalous behaviour of both these properties is observed for compositions rich in TiO₂ and/or Al₂O₃, an observation attributed to liquid-liquid phase separation followed by anatase crystallization. For samples which do not show anomalous behaviour, it is found that the partial molar heat capacity of the TiO₂ component previously determined in Al-free compositions reproduces our heat capacities to within 1.3%. Viscosity data show that addition of TiO₂ tends to increase viscosity and melt fragility at constant temperature. Furthermore, heat capacity and viscosity data may be combined within the framework of the Adam-Gibbs theory to extract values of the configurational entropy of the liquids and qualitative estimates of the variation of the average energy barrier to viscous flow. Configurational entropy at 900K is inferred to decrease upon addition of TiO₂, in contrast to previous results from Al-free systems. The compositional limit separating normal from anomalous behaviour, as well as the data for homogenous melts have been used to constrain the structural role of Ti in these samples. Our data are consistent with a majority of Ti in five-fold coordination associated with a titanyl bond, in agreement with previous spectroscopic studies. Furthermore, we find no evidence for a Ti-Al interaction in our samples, and we are led to the conclusion that Al and Ti are incompletely mixed, a hypothesis consistent with the observed reduction of configurational entropy upon addition of TiO₂, suggesting an important role of medium range order in controlling the variations in thermodynamic properties.     

Barite deposition resulting from phototrophic sulfide-oxidizing bacterial activity

Barite (BaSO₄) deposits generally arise from mixing of soluble barium-containing fluids with sulfate-rich fluids. While the role of biological processes in modulating barium solubility has been shown, no studies have shown that the biological oxidation of sulfide to sulfate leads to barite deposition. Here we present an example of microbially mediated barite deposition in a continental setting. A spring in the Anadarko Basin of southwestern Oklahoma produces water containing abundant barium and sulfide. As emergent water travels down a stream to a nearby creek, sulfate concentration increases from 0.06 mM to 2.2 mM while Ba²⁺ concentration drops from 0.4 mM to less than 7 μM. Stable isotope analysis, microbial activity studies, and in situ experiments provide evidence that as sulfide-rich water flows down the stream, anaerobic, anoxygenic, phototrophic bacteria play a dominant role in oxidizing sulfide to sulfate. Sulfate then precipitates with Ba²⁺ producing barite as travertine, cements, crusts, and accumulations on microbial mats. Our studies suggest that phototrophic sulfide oxidation and concomitant sulfur cycling could prove to be important processes regulating the cycling of barium in continental sulfur-containing systems.     

Pollution history and recovery of a boreal lake exposed to a heavy bleached pulping effluent load

We examined the effects of heavy pulp mill discharges on the Lake Lievestuoreenjärvi ecosystem and the later recovery of diatom and chironomid communities from age-dated short core samples. Beginning in 1927 the lake received a heavy effluent load from a sulphite pulp mill. Except for the recession during the Second World War and the temporary closure of the mill from 1967 to 1971, the industrial load, containing large quantities of nutrients, organic matter and toxic compounds, increased continuously. In the early 1980s, laboratory documents were falsified by the directors of the mill and the systematic illegal effluent overload led to a collapse of the whole lake ecosystem. In 1985, the outdated plant was finally closed down. Based on the assessment of chemical properties and biological remains of the sediment, we distinguished five developmental phases in the ecological state of the lake. In the pre-industrial phase, the pelagic and profundal benthic communities were dominated by species preferring ultraoligotrophic or oligotrophic lakes. Concomitant with the increasing discharge and deposition of chlorine compounds, resin acids, and mercury, as well as strong acidity and hypolimnetic and epilimnetic anoxia, the ecological status changed in a short period from excellent to bad. Finally, in the early 1960s, the majority of the lake was virtually dead and the aquatic life survived only in the uppermost littoral zone. Since 1985, a fast recovery in the water quality has led to a strong, but temporary eutrophy in pelagic communities. The main peak of eutrophication was caused by the invasion of a species new to the lake,Aulacoseira granulata var.angustissima. Later, the pelagic communities shifted towards oligotrophy, but the original, pre-industrial status has not been re-established. The profundal benthic communities have not achieved the pre-industrial structure, but at present indicate mesotrophy.     

Apparent and real bias in numerical transfer functions in palaeolimnology

We present an evaluation of the procedure by which model prediction bias is examined in palaeolimnological transfer function inference models. We argue that most of the prediction biases commonly reported in the literature are, in fact, fallacious, and are the artificial consequence of the inappropriate manner in which residuals are traditionally examined. We show that the extent of the specious model bias is entirely predictable from first principles and is essentially determined by the strength of the predictive model. We suggest that the analysis of residuals should always be examined as a function of the model's predictions and we discuss the implications of the old and new approaches.