The Ordovician chondrite from Brunflo, central Sweden: III. Geochemistry of terrestrial alteration

The fossil H chondrite Brunflo, found in a slab of Ordovician limestone from central Sweden, is pervasively altered to an assemblage dominated by calcite and barite. The meteorite is surrounded by a 15–20 cm wide zone of lighter colors than the unaffected limestone due to dissolution of hematite. Here we present detailed geochemical analyses of two meteorite samples, 14 limestone samples at distances from 0 to 29 cm along two profiles from the meteorite, and a reference sample of Brunflo limestone. Element concentrations in Brunflo and surrounding bleached limestone have been strongly disturbed during two stages of alteration (early oxygenated and deep burial). In the meteorite, the Ni/Co ratio has changed from an initial value of 20 to 0.8 and redox sensitive elements like V, As, Mo, Re and U are strongly enriched. The sulfur isotope composition of barite from Brunflo (δ³⁴S=+35‰) indicates initial loss of meteoritic sulfide, followed by later accumulation of sea water sulfate as barite. During deep burial under more reducing conditions, reduction processes supported by an externally derived reductant possibly derived from alum shale underlying the limestone, were largely responsible for the observed redox phenomena. In spite of massive redistribution of many elements, concentrations of Pt, Ir and Au remain at chondritic levels. The geochemistry and mineralogy of alteration determined for Brunflo are similar to those in “reduction spots” in red beds, where accumulation of a similar suite of elements (except Mo, Re) occurred as a result of isolated reduction activity.     

Effect of Fe on garnet–melt trace element partitioning: experiments in FCMAS and quantification of crystal-chemical controls in natural systems

Garnet–melt trace element partitioning experiments were performed in the system FeO–CaO–MgO–Al₂O₃–SiO₂ (FCMAS) at 3 GPa and 1540°C, aimed specifically at studying the effect of garnet Fe²⁺ content on partition coefficients (D^Grt/Melt). D^Grt/Melt, measured by SIMS, for trivalent elements entering the garnet X-site show a small but significant dependence on garnet almandine content. This dependence is rationalised using the lattice strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454], which describes partitioning of an element i with radius r_i and valency Z in terms of three parameters: the effective radius of the site r₀(Z), the strain-free partition coefficient D₀(Z) for a cation with radius r₀(Z), and the apparent compressibility of the garnet X-site given by its Young's modulus E_X(Z). Combination of these results with data in Fe-free systems [Van Westrenen, W., Blundy, J.D., Wood, B.J., 1999. Crystal-chemical controls on trace element partitioning between garnet and anhydrous silicate melt. Am. Mineral. 84, 838–847] and crystal structure data for spessartine, andradite, and uvarovite, leads to the following equations for r₀(3+) and E_X(3+) as a function of garnet composition (X) and pressure (P):

r₀(3+) [Å]=0.930X_Py+0.993X_Gr+0.916X_Alm+0.946X_Spes+1.05(X_And+X_Uv)−0.005(P [GPa]−3.0)(±0.005 Å)

E_X(3+) [GPa]=3.5×10¹²(1.38+r₀(3+) [Å])^−26.7(±30 GPa)

Accuracy of these equations is shown by application to the existing garnet–melt partitioning database, covering a wide range of P and T conditions (1.8 GPa<P<5.0 GPa; 975°C<T<1640°C). D^Grt/Melt for all 3+ elements entering the X-site (REE, Sc and Y) are predicted to within 10–40% at given P, T, and X, when D^Grt/Melt for just one of these elements is known. In the absence of such knowledge, relative element fractionation (e.g. D_Sm^Grt/Melt/D_Nd^Grt/Melt) can be predicted. As an example, we predict that during partial melting of garnet peridotite, group A eclogite, and garnet pyroxenite, r₀(3+) for garnets ranges from 0.939±0.005 to 0.953±0.009 Å. These values are consistently smaller than the ionic radius of the heaviest REE, Lu. The above equations quantify the crystal-chemical controls on garnet–melt partitioning for the REE, Y and Sc. As such, they represent a major advance en route to predicting D^Grt/Melt for these elements as a function of P, T and X.     

Variability of East Asian Winter Monsoon in Quaternary Climatic Extremes in North China

In order to examine high-frequency variations of East Asian winter monsoon in Quaternary climatic extremes, two typical loess–paleosol sequences in the Chinese Loess Plateau were investigated. Sandy layers in the loess deposits, the “Upper sand” and “Lower sand” (layers L9 and L15, respectively), which represent a high-resolution record of paleomonsoon changes, have been sampled at intervals of 5–6 cm from sections at Luochuan and Xifeng. The grain size and magnetic susceptibility was measured for all samples. The grain-size results (a proxy of winter monsoon strength) indicate that the winter monsoon strength fluctuated on a millennial timescale during cold climatic extremes, with climatic events of a few hundred to a few thousand years. However, the winter monsoon was relatively stable during warm periods. The magnetic susceptibility signal (a proxy of summer monsoon intensity) is practically constant over the same period. This is tentatively explained by the assumption that the summer monsoon intensity was too low to be recorded in the magnetic susceptibility signal. The intensified winter monsoon events show periodicities in a range of 1000 to 2770 yr, with a dominant cycle of approximately 1450 yr. The detection of this oscillation in older glacial stages strongly suggests that it may be a pervasive cycle of the cold climatic phases of the Quaternary. Millennial-scale variations of the winter monsoon may be caused by instability of the westerly jet, which is determined by temperature differences between the polar and the equatorial regions.     

Faulting, fracturing and in situ stress prediction in the Ahnet Basin, Algeria — a finite element approach

Many low-efficiency hydrocarbon reservoirs are productive largely because effective reservoir permeability is controlled by faults and natural fractures. Accurate and low-cost information on basic fault and fracture properties, orientation in particular, is critical in reducing well costs and increasing well recoveries. This paper describes how we used an advanced numerical modelling technique, the finite element method (FEM), to compute site-specific in situ stresses and rock deformation and to predict fracture attributes as a function of material properties, structural position and tectonic stress. Presented are the numerical results of two-dimensional, plane-strain end-member FEM models of a hydrocarbon-bearing fault-propagation-fold structure. Interpretation of the modelling results remains qualitative because of the intrinsic limitations of numerical modelling; however, it still allows comparisons with (the little available) geological and geophysical data.

In all models, the weak mechanical strength and flow properties of a thick shale layer (the main seal) leads to a decoupling of the structural deformation of the shallower sediments from the underlying sediments and basement, and results in flexural slip across the shale layer. All models predict rock fracturing to initiate at the surface and to expand with depth under increasing horizontal tectonic compression. The stress regime for the formation of new fractures changes from compressional to shear with depth. If pre-existing fractures exist, only (sub)horizontal fractures are predicted to open, thus defining the principal orientation of effective reservoir permeability. In models that do not include a blind thrust fault in the basement, flexural amplification of the initial fold structure generates additional fracturing in the crest of the anticline controlled by the material properties of the rocks. The folding-induced fracturing expands laterally along the stratigraphic boundaries under enhanced tectonic loading. Models incorporating a blind thrust fault correctly predict the formation of secondary syn- and anti-thetic mesoscale faults in the basement and sediments of the hanging wall. Some of these faults cut reservoir and/or seal layers, and thus may influence effective reservoir permeability and affect seal integrity. The predicted faults divide the sediments across the anticline in several compartments with different stress levels and different rock failure (and proximity to failure). These numerical model outcomes can assist classic interpretation of seismic and well bore data in search of fractured and overpressured hydrocarbon reservoirs.     

大别山双河和碧溪岭超高压变质岩流体包裹体研究

对大别山双河和碧溪岭含柯石英榴岩和硬玉石英岩进行了详细的流体包裹体研究。根据流体包裹体的成分和盐度的不同,可以划分出至少五种类型不同的气液包裹体;（１）Ｎ２包裹体;（３）高盐度流体包裹体;（３）ＣＯ２包裹体;（４）ＣＯ２－Ｈ２Ｏ包裹体;（５）低盐度流体包裹体。本仅见于含柯石英榴辉岩,而高盐度流体包裹体则几乎存在于所有的榴辉岩和硬玉石英岩中。ＣＯ２包裹体沿榴辉岩中微剪切带分布,或存在于强变形的硬玉石     

Comparative petrology and metamorphic evolution of the Limpopo (South Africa) and Lapland (Fennoscandia) high-grade terrains

Summary ?Detailed studies of rocks from the Limpopo (South Africa) and Lapland (Kola-Fennoscandia) high-grade terrains were carried out in order to reveal similar geological and thermodynamic conditions in their formation. Both complexes (1) are situated between Archean greenstone belts, (2) are younger than the belts, (3) are bounded by crustal-scale shear zones, (4) have a similar intrusive-like (harpolith) geometry, and (5) show similar reaction textures that reflect both breakdown and growth of garnet in each high-grade terrain. Local mineral equilibria within the textures indicate their successive formation with cooling of the granulite facies rocks. Some of the textures in the metapelites must have resulted from the following reverse reactions: Grt + Qtz ⇌Opx + Crd and/or Grt + Sil + Qtz ⇌ Crd. Based on these data, both the decompression cooling P-T path and the near-isobaric cooling P-T path were deduced for each HGT. However the near-isobaric cooling P-T path is not a characteristic of the central zones of both complexes studied. Similar structural framework of the high-grade terrains, similar morphologies (shapes of granulitic bodies), similar reaction textures developed in metapelites, and similar shapes of P-T paths suggest similarity in geodynamic history of both complexes.

---

Zusammenfassung ?Vergleichende Petrologie und metamorphe Entwicklung der hochgradig metamorphen Terrains von Limpopo (Südafrika) und Lappland (Fennoscandia) Eingehende Untersuchungen an Gesteinen aus den hochgradig metamorphen Terrains von Limpopo (Südafrika) und Lappland (Kola-Fennoscania) sollen m?gliche ?hnlichkeiten in den geologischen und thermodynamischen Bildungsbedingungen aufzeigen. Beide Komplexe sind Lokale Mineralgleichgewichte innerhalb der Texturen weisen auf ihre schrittweise Bildung w?hrend der Abkühlung der granulitfaziellen Gesteine hin. Einige der Texturen in den Metapeliten gehen auf folgende reversible Reaktionen zurück: Grt + Qtz ⇌Opx + Crd und/oder Grt + Sil + Qtz ⇌ Crd. Diese Daten erm?glichten es, sowohl den P-T Pfad der Abkühlung bei Druckentlastung sowie den fast-isobaren P-T Pfad der Abkühlung für jedes HGT zu ermitteln. Der fast-isobare P-T Pfad der Abkühlung ist jedoch kein Charakteristikum der Zentralzonen beider Komplexe. Ein ?hnlicher struktureller Rahmen der hochgradigen Terrains, ?hnliche Morphologien (Ausbildung der Granulitk?rper), ?hnliche Reaktionstexturen in Metapeliten und ?hnliche P-T Pfade weisen auf ?hnlichkeiten der geodynamischen Entwicklungsgeschichte beider Komplexe hin.

---

---

Received March 8, 1999; revised version accepted September 17, 1999     

In situ bioremediation of 1,2-dichloroethane under anaerobic conditions

Historic spillages of chlorinated hydrocarbons at a vinyl chloride plant in the Rotterdam–Botlek area in The Netherlands has lead to deep-seated pollution of the underlying aquifer. The principal pollutant is 1,2-dichloroethane (1,2-DCA). As a temporary measure, the contamination is being contained using a pump and treat system. In the long term, in-situ bioremediation has been proposed using a biologically active zone where pollutants would be dechlorinated by microorganisms that simultaneously degrade other carbon sources. In order to investigate the suitability of this new technology, a programme of laboratory tests was carried out. The laboratory programme involved a series of anaerobic soil column tests, where the selection and delivery of different carbon substrates that stimulated 1,2-DCA dechlorination were investigated. The soil columns were prepared using soil and groundwater samples from boreholes. Groundwater was flushed through the columns under anaerobic conditions. A comparison was made between the transformation of 1,2-DCA without a carbon substrate and in the presence of sugars (molasses) and alcohol (methanol) respectively. In addition, different modes of delivery were investigated. In the case of molasses, the material was injected into the column as a plug to simulate grout injection in the field, whereas methanol was delivered as a constant flow dissolved in the influent. Both carbon substrates resulted in the biotransformation of 1,2-DCA. However, fermentation of molasses produced secondary effects that led to a drop in pH and an excessive production of carbon dioxide, which temporarily blocked the flow of groundwater.     

Recent Climate Warming: Surface Air Temperature Series and Geothermal Evidence

Long-term (1961 – 1996) meteorological air temperature series together with the reconstructed ground surface temperature histories, obtained by inverting borehole temperature-depth profiles, were used to project regional patterns of the recent (climate) warming rate on the territory of the Czech Republic. The characteristic magnitude of the warming rate of 0.02 –0.03 K/yr was confirmed by the results of several years of monitoring the temperature in two experimental boreholes. The monitoring of shallow temperatures at depths of about 30 –40 m, i.e. below the reach of the seasonal surface temperature variations, can serve as an alternative tool of direct quantitative assessment of the present warming rate. The data also seem to sustain a potential man-made component contributing to the more pronounced recent warming rate observed in the areas of large agglomeration.     

Contribution of Local Seismic Networks To The Regional Velocity Model of The Bohemian Massif

The routine location of regional seismic events using data from the Czech National Seismological Network (CNSN) is based on Pn, Pg, Sn, Sg phases. A simple velocity model derived from Kárník's (1953) interpretation of an earthquake in Northern Hungary in 1951 has hitherto been used. At present, numerous local seismic networks record and locate local events, which are occasionally recorded at regional distances as well. Due to the relatively small dimensions of local networks, hypocenters (and origin times) determined by a local network might be considered as nearly exact from the point of view of regional-scale CNSN. The comparison of common locations performed by CNSN and by a local network enables us to estimate the accuracy of CNSN locations, as well as to optimize a simple velocity model. The joint interpretation of the CNSN bulletin and the catalogues of four local seismic networks WEBNET, OSTRAVA, KLADNO and LUBIN produced a new ID velocity model. The most frequent epicentral error in this model is less than 5 km, and most foci lie up to 15 km from the true position. The performed analysis indicates bimodal distribution of Sn residuals.