Elemental dispersion and stable isotope fractionation during reactive fluid-flow and fluid immiscibility in the Bufa del Diente aureole,NE-Mexico: evidence from radiographies and Li,B, Sr,Nd, and Pb isotope systematics

The 31.6±0.3 Ma old Bufa del Diente alkali-syenite (NE Mexico) intruded a sequence of Cretaceous limestones with intercalated sub-horizontal chert layers. The cherts acted as aquifers that facilitated transport of brines and pegmatitic melts within the shallow-level (<1 kbar) contact-metamorphic aureole. Fluid-driven reactions between chert and marble wallrock, and the influx of late melts and various fluids gave rise to distinct chemical and isotopic signatures within the aquifer and across the zones of infiltration and fluid-driven reaction. Aqueous brines of magmatic origin produced thick wollastonite mantles around the chert layers. Wollastonite formation occurred at the expense of limestone and chert and generated CO₂. This CO₂-induced fluid unmixing into an aqueous brine and a low-density CO₂-rich fluid, which was lost to the overlying marble where it oxidized organic matter and caused ¹³C and ¹⁸O shifts in a zone some 5–10 cm wide. After wollastonite formation, the chert aquifers were locally intruded by pegmatite veins carrying alkali feldspar, quartz, aegirine-augite, eudialyte, zircon, and apatite. Aqueous fluids that exsolved during crystallization of the pegmatite veins escaped along late cross-fractures and migrated along the inner and outer borders of the wollastonite margins. Chemical dispersion patterns of U, Al, Na + K, P, S, Fe, and REE across the chert-to-marble boundary and its metasomatic rims are shown by autoradiography and neutron-induced radiography. Scavenging of cations at mineralogical contacts and cation transport into the marbles occurred only on the mm to cm scale. Isotopic data for Pb and Sr across a simple metachert-marble boundary and for Pb, Sr, Nd, B, and Li across a metachert-pegmatite-marble sequence demonstrate the following: (1) The Pb and Sr isotopic signature of early fluids was buffered by the carbonate wallrock. Only late fluids, shielded from wallrock interaction by a wollastonite mantle, variably preserved a memory of their initial magmatic signature. (2) Since the Nd isotope signature of marble and chert is bound to calcite and clay minerals, systematic shifts to unradiogenic Nd in marble reflect loss of carbonate-bound Nd as the wollastonite margin is approached. Nd in the wollastonite margin is dominated by Nd originally bound to clay minerals. The later emplacement of the pegmatite, which carried the Nd isotope signature of its alkali-syenite source, had little effect on the Nd isotopic composition of the wollastonite rim. (3) Although the Li and B isotopic compositions reflect the alkali-syenite source, they are also affected by isotopic fractionation and partitioning between melt, fluid, and solids.Editorial responsibility J. Hoefs     

In situ measurements of heating parameters in the auroral ionosphere

Four sounding rocket payloads were launched in early 1977 to measure heating parameters in the auroral oval. Geophysical conditions were different for the four flights: auroral arc substorm main phase diffuse aurora, and auroral arc with negative bay. The conductivity tensor and the heating rates of particle and Joule heating are determined. The heating rates range in the order of a few tens of mWm^?2. These magnitudes accord with those determined with the aid of backscatter facilities and other sounding rocket observations.     

Superficial geochemical dispersion around sulphide deposits: Some examples in France

Examination of geochemical exploration data accumulated in France over a period of almost 20 years has led to detailed consideration of the relationship between anomalies and mineral deposits.Geochemical anomalies associated with sulphide mineralizations seem to follow several general rules i.e. Ag and Pb are almost systematically present, As is very frequent, while Cu and Sb may also be present. The behaviour of Zn seems to be controlled by the lithological character of the host rock with anomalies in carbonate media even for occurrences without economic interest and an absence of anomalies in non-carbonate media, even for Zn deposits of considerable size. Moreover, the contents of this element are greatly amplified by contamination from old workings. The abundance (or scarcity) of supergene minerals in the occurrences directly influences the nature of the anomalies. Lead anomalies frequently correspond to the presence of Pb oxides such as pyromorphite while Zn anomalies derive from Zn oxides which are much more common in carbonate media (smithsonite-hydrozincite) than in schistose host rock. High Cu and As contents are most frequently linked to the presence of “ferruginous fragments” (gossans) derived from sulphides. As a general rule, the various geochemical peculiarities or associations encountered in soils are very much similar to those of the oxidized part of the deposits.We are therefore led to assume that, contrary to the usual conceptions, superficial dispersion is basically mechanical and that the anomalies detected near mineralizations are due to the presence in the soils of supergene minerals, gossan microfragments and more or less mineralised rock debris, etc. Mechanical migration of this kind can be explained by the existence in France during the Pleistocene of cold climates of the tundra type with associated permafrost. The thawing in summer of the superficial part of the frozen overburden resulted in massive downslope solifluction favoring mechanical migration over distances in the order of 100 m and contributing to the surface dispersal of the mineralizations. These phenomena are extremely important for geochemical exploration because they allow and justify the use of large grids (200 m × 200 m or even 400 m × 400 m) during soil surveys at least at the initial stage of prospecting on areas of several square kilometers.These conclusions are valid not only for France but for all regions with the same paleoclimate. These form a belt bounded to the north by glacial formations linked to ancient inland ice and to the south by the maximum extension of Pleistocene permafrost, including, in particular, the north of the United States and Central Europe. This paleo-climate belt characterized by traces of solifluction constitutes a well-defined unit in which geochemical prospecting has proved itself extremely effective.     

2-D SIMULATION OF CHANNEL FLOWS WITH MOVEABLE BED

1 INTRODUCTIONFor many hydraulic engineering problems, the analysis of flow and bed level variations in openchannels is a fundamental prerequisite. forcal methOds fOr alluvial rivers are well develoPednowadays as far as onediInensional descriPtions are concemed. A cOmPrhensive analysis of Ihe wellknown models is Presented by Habersack(l998). HOwever, for a number of Problems such as channelwidening, flow pattem close to sPuds and etc. a more deailed knowledge of the bed level behavio…     

Organic geochemistry and petrography of Tertiary coals and carbonaceous shales from Argentina

A series of eight Tertiary coal and carbonaceous shale samples with vitrinite reflectance values between 0.50 and 0.58% were extracted, fractionated and the saturated and aromatic hydrocarbons analysed for characteristic components by GC and GC-MS. Additionally, a microscopical study was undertaken in order to obtain a more precise picture of the samples under investigation.The saturated hydrocarbon fractions displayed the typical n-alkane distribution for coals of this rank, with CPI values between 2.0 and 3.1. Among the branched/cyclic compounds, pristane and α, β-homohopane were recognised as relevant components pointing to an oxic depositional environment. Detection of benzohopanes (C₃₂–C₃₅) in the aromatic hydrocarbon fractions suggests that bacteriohopanetetrol was a significant constituent of the coal biomass. Taking into consideration the Pr/Ph ratios, ash contents and microscopical characteristics of the samples, aspects of the possible degradation of hopanetetrol to homohopane are discussed. Resin-derived diterpenoids with the phyllocladane and kaurane skeleton were tentatively identified and, although minor compounds, they are interpreted to be a sign of the contribution of Podocarpaceae and Araucareaceae to the coal swamp.Aromatic compounds were dominated by alkylnaphthalene derivatives, presumably formed by C-ring cleavage and aromatisation of higher plant-derived pentacyclic triterpenois, which were main components in the high-boiling range of the fractions investigated. Angiosperms (especially Fagaceae) are postulated as source for these polycyclic compounds and, hence, for some of the polyalkylated aromatic bicyclics detected.     

Behaviour of lead and zinc in calcrete-bearing soils around Bou Grine, Tunisia — its application to geochemical exploration

The behaviour of Pb and Zn was studied in soils containing calcrete to determine the effect of calcrete formation on the surface expression of mineralization. Earlier investigations had shown that: (1) calcrete develops epigenetically within the soil from other minerals present in the horizon that is being encrusted; and (2) this epigenetic replacement is isovolumetric.One consequence of this process for geochemical exploration is that the development of calcrete can be compared to installing a screen that weakens the contrast of the anomaly. Another geochemical consequence is that the metals that were initially associated with various mineralogical phases in the parent rock will associate mainly with HCl (1N) soluble components inthe calcrete horizons. In noncalcreted horizons, and in particular in the surficial layers, the metals are divided between carbonate and oxide phases, the latter being more or less well-crystallized.     

Eo-Alpine Metamorphism in the Uppermost Unit of the Cretan Nappe System — Petrology and Geochronology

The uppermost unit of the Cretan nappe system contains a variegated series of high-grade metamorphic rocks. In the Léndas area, amphibolites are present characterized by the assemblage

$$\text{brown}\;\text{hornblende}\;+\;\text{diopside}\;+\;\text{plagioclase}\;\text{(An 50)}$$

while associated metapelitic gneisses consist of

$$\text{garnet}\;+\;\text{cordierite}\;+\;\text{biotite}\;+\;\text{sillimanite (andalusite)}\;\pm\;\text{K-feldspar}\;+\;\text{plagioclase (An 40-50)}\;+\;\text{quartz}.$$

Judging from relevant experimental data for the gneiss assemblage including the Fe/Mg distribution on coexisting garnet and cordierite, the P-T conditions of metamorphism are estimated at about 700° C and 5 kb water vapour pressure.K/Ar determinations on hornblendes from three amphibolites yielded cooling ages of 71.3, 71.2, and 71.1 (±1.7) m.y. respectively; biotites from three paragneisses gave 70.2 ± 1.4, 69.7 ± 1.2, and 67.9 ± 1.4 m.y. respectively. Assuming a sealing temperature against argon diffusion of 300° C, for biotite, and 500° C, for hornblende, a cooling rate of 100–200° C/m.y. is calculated. Thus a late Cretaceous (eo-Alpine) metamorphic event is established in the post-Cretaceous nappes of Crete.     

Sparse matrix algorithms applied to dem generation

A method for generating a digital elevation model (DEM) was suggested byEbner. The heights of grid points are interpolated from arbitrarily distributed reference points using the finite element method. The requested grid heights of the DEM are defined as unknowns and estimated from the available reference points and a general curvature minimization of the interpolation surface with filtering at the reference points. This problem can be interpreted as an adjustment of indirect observations and can be solved using the least squares method. This leads to a banded structured system of normal equations in case of numbering the unknowns in a row-wise order. This ordering may not be the best strategy for the solution of the normal equations, because the band contains many zero elements. Therefore algorithms which exploit the nonzero/zero structure to better advantage have been applied in combination with the pivot strategies of “Nested Dissection” and “Minimum Degree Ordering” (OPTORD). The operational characteristics of these algorithms are compared with those of the band-algorithm for several DEM problems that vary in size to over 4000 unknowns. The result shows that the number of nonzeros and the number of essential operations can be reduced drastically when using sparse algorithms, but that because of the very high bookkeeping expenses, band algorithms are to be preferred.     

Morphologie und Infraciliatur einiger limnischer Ciliaten (Protozoa: Ciliophora)

The morphology, the infraciliature, and the silverline system of the fresh-water ciliatesUrotricha armata Kahl, 1927,Longifragma obliqua (Kahl, 1926) nov. gen., andTrochilioides fimbriatus nov. spec. are described. The new genusLongifragma is characterized by aProrodon-like “brosse” and an incompleteUrotricha-like somatic infraciliature. A new species,Urotricha dragescoi nov. spec., is suggested for theU. armata Kahl, 1927 of Dragesco et al. (1974).Trochilioides fimbriatus differs by its complex cyrtos from the other members of the genus.