Monitoring of rapid salt weathering in the central Namib Desert using limestone blocks

Pre-weighed blocks of a Jurassic Limestone were exposed on the ground surface in the coastal Namib Desert for a period of 2 years. The environment is both salty and foggy. Some of the blocks suffered extensive disintegration, and laboratory analyses (including geochemistry, XRD and SEM) indicate that the weathered samples have a high halite (sodium chloride) content. Cycles of wetting and drying associated with the frequent fog events of the area cause cycles of halite crystallization. Rocks exposed at the surface absorb salts from the surrounding desert surface and then disintegrate, contributing to planation of the landscape.     

POLITICS AND MEDIA RICHNESS IN WORLD WIDE WEB REPRESENTATIONS OF THE FORMER YUGOSLAVIA

ABSTRACT. Media-richness theory is applied to World Wide Web sites to demonstrate how Web-page designers are using hypertext markup language to shape conceptions of place and to provide competing visions of the events that have transpired in the former Yugoslavia. We argue that Web sites vary in their interactivity, strategically, to reduce equivocality surrounding conceptions of space and territory. Media-richness theory as applied here allows the development of a heuristic to understand how Web pages communicate information about geographical entities and help to shape perceptions of place.     

Periodicity of magnetic intensities in magnetic anomaly profiles: the Cenozoic of the South Atlantic

Spectral analyses of several published magnetic anomaly profiles from Candé & Kent (1992a) were undertaken prior to analysing, in the same way, raw magnetic anomaly data from similar parts of the South Atlantic. It was found that similar and distinct medium and short wavelengths were present in both the published and raw data. When these are converted into the time domain using the average rate of spreading for each profile, these periodicities appear similar, possibly identical, to those expected from the long-term eccentricity orbital parameters (Fischer, DeBoer & Premoli Silva 1990). While such correlations are not necessarily causative, they suggest that magnetohydro-dynamical processes near the core-mantle boundary may be affected by gravitational changes due to planetary orbital perturbations.     

Three-dimensional Rayleigh hysteresis of oriented core samples from the German Continental Deep Drilling Program: susceptibility tensor, Rayleigh tensor, three-dimensional Rayleigh law

Rayleigh hysteresis, as defined by the well-known Rayleigh relations, has been observed not only when magnetization of pyrrhotite-bearing KTB-samples is measured in parallel to a weak dc magnetic field, but also in experiments where field and measuring directions have been adjusted strictly perpendicularly to each other. Nine-tupels of independent Rayleigh hysteresis loops could thus be compiled. Their characteristic coefficients X ^ijk of initial susceptibility together with the Rayleigh loss coefficients α^jk have been proved to determine completely the samples' weak-field magnetic anisotropy. Interpreting the coefficient matrices ( X ^ijk) and (α^jk) as the tensor of initial susceptibility and the Rayleigh tensor, respectively, generalization of the isotropic Rayleigh relations in terms of corresponding tensor relationships has been suggested for the anisotropic case. Application to the KTB samples showed 3-D Rayleigh hysteresis measurements to be an excellent tool for rock magnetic analysis in terms of ore content and crystalline texture. In particular, a magnetocrystalline double texture of the basal planes of pyrrhotite precipitates and their [1120] directions of easy magnetization have been clearly detected. Surprisingly, the welt-known theorem α= const. X ²_I, formulated by Néel (1942) for the isotropic case, has been found to hold true even in tensor generalization (α_jk) = const ( X ²_jk). To reach sufficient resolution for the measurements performed, a sensitive vibrating coil magnetometer (VCM) has been developed.     

Updating the Vegetation Layer of the NZLRI National Database Using Satellite Imagery

This paper describes four methods of rapidly mapping vegetation using satellite imagery, for use in updating the vegetation layer of the New Zealand Land Resource Inventory (NZLRI). The visual interpretation method was tested in a 500 km² study area on the North Island west coast where 3 6 NZLRI vegetation classes occurred. Sixteen distinct groups of NZLRI vegetation classes were identified on Landsat Thematic Mapper imagery, and named “image classes”. Classes were identified by correlating ground data with image colour and texture and by using recognisable landform and cultural features. A limited number of vegetation changes have occurred since NZLRI mapping was first carried out. Updating the vegetation layer of the NZLRI requires recognising and mapping changes and modifying the database. Identifying distinctive groups of NZLRI vegetation classes on satellite imagery will facilitate this.     

Experimental determination of the solubility of Au in silicate melts

Summary We report here new data on the solubility of Au in silicate melts of anorthite-diopside eutectic composition at a wide range of oxygen fugacities, from pure oxygen to 10^–8 atm, and at a temperature range of 1300 °C to 1480 °C. Because experiments were done with metal loops at temperatures above the Au-melting temperature, PdAu-metal-alloys had to be used. Pd-solubility data derived from the same set of experiments agree with earlier data obtained from experiments with pure Pd-metal (Borisov et al., 1994a). The results of the present experiments show that Pd-solubilities are by a factor of 2 to 6 higher than Au-solubilities. Both, Au and Pd solubilities decrease with decreasing oxygen fugacity. At oxygen fugacities below the iron-wiistite buffer (IW) Au solubility increases with decreasing fO₂ probably reflecting formation of Au-silicides at such reducing conditions. Compared to Pd, Au has higher activity coefficients in Fe-metal and lower solubility in silicate melts. This leads to similar metal-silicate partition coefficients for both elements. At a temperature of 1350 °C and an oxygen fugacity corresponding to IW-2 D^Au (met/sil) is about 2.5 · 10⁷ and D^Pd (met/sil) about 1.6 · 10⁷. Thus similar behavior is expected during metal separation in planetary bodies including core formation in the Earth. The metal/silicate partition coefficient of Ir is, however, by several orders of magnitudes higher (Borisov and Palme, 1995a). Equilibration with chondritic metal will therefore lead to grossly non-chondritic Pd/Ir or Au/Ir ratios in coexisting silicate phases. Chondritic ratios are thus indicative of the presence of unfractionated meteoritic components. Samples from the upper mantle of the Earth, for example, reflect the admixture of a late unfractionated (chondritic) veneer (e.g.,Kimura et al., 1974;Jagoutz et al., 1979).Solubilities of Pd and Au in silicate melts are much higher than the contents in terrestrial basalts implying that the abundances of these two elements are not buffered by residual PGE- and Au-containing alloys. The most likely process for fractionating PGEs in terrestrial magmas are mineral-melt (e.g., olivine/melt) equilibria.

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Experimentelle Bestimmung der Löslichkeit von Au in Silikatschmelzen

Zusammenfassung In der vorliegenden Arbeit wird über die Ergebnisse der Bestimmung der Löslichkeit von Au in Silikatschmelzen mit der Zusammensetzung des Anorthit-Diopsid Eutektikums berichtet. Die Versuche wurden mittels Metallschlaufe über einen weiten Sauerstoffpartialdruckbereich, von reinem Sauerstoff bis zu 10^–8 atm und in einem Temperaturbereich von 1300 °C bis 1480 °C, durchgeführt. Da diese Temperaturen jedoch den Au-Schmelzpunkt überschreiten, wurde mit AuPd-Legierungen gearbeitet. Die Ergebnisse der dadurch zusätzlich erhaltenen Pd-Versuche stimmen mit früher bestimmten, mit reinen Pd-Schlaufen durchgeführten Pd-Löslichkkeiten überein (Borisov et al., 1994a). Die auf reine Metalle zurückgerechneten Löslichkeiten von Pd sind um einen Faktor 2 bis 6 mal höher als die entsprechenden Au-Löslichkeiten. Die Löslichkeiten beider Metalle nehmen mit abnehmendem Sauerstoffpartialdruck ab. Unter noch stärker reduzierenden Bedingungen (Eisen-Wüstit Gleichgewicht) nimmt die Löslichkeit von Au jedoch zu. Dies könnte auf die Bildung von Au-Siliziden zurückzuführen sein.Im Vergleich zu Pd sind die Aktivitätskoeffizienten von Au in metallischem Eisen höher, die Löslichkeiten in Silikatschmelzen jedoch niedriger. Das führt zu ähnlichen Metall/Silikat Verteilungskoeffizienten von Au und Pd. Bei einer Temperatur von 1350 °C und einer Sauerstoffugazität von IW-2 ergeben sich für D^Au (met/sil) 2.5 · 10⁷ und für D^Pd (met/sil) 1.6* 10⁷. Der Metall/Silikat-Verteilungskoeffizient von Ir ist jedoch unter den gleichen Bedingungen um mehrere Größenordnungen höher (Borisov andPalme, 1995a). Ein chondritisches Pd/Ir- oder Au/Ir-Verhältnis kann also auf die Anwesenheit einer unfraktionierten chondritischen Komponente zurückgeführt werden. Dies gilt beispielsweise für Proben aus dem oberen Erdmantel. Hier handelt es sich vermutlich um Zumischung einer späten chondritischen Akkretionskomponente, die sich nicht mehr mit einer metallischen Phase (Kern) ins Gleichgewicht gesetzt hat (z.B.Kimura et al., 1974,Jagoutz et al., 1979).Die Löslichkeiten von Pd und Au in Silikatschmelzen sind wesentlich höher als ihre Gehalte in basaltischen und komatiitischen Laven. Dies bedeutet, daß Au und Pd in Schmelzen aus dem Erdmantel nicht durch residuale Au- und/oder Pd-haltige Metall phasen bestimmt sind. Gleichgewichte zwischen Schmelze und Mineralen (z.B. Olivin) sind die wahrscheinlichsten Fraktionierungsmechanismen für Platingruppenelemente in terrestrischen Magmen.

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With 5 Figures     

New palaeomagnetic data from Central Iran and a Triassic palaeoreconstruction

New pole positions for Triassic and Cretaceous times have been obtained from volcanic and sedimentary sequences in Central Iran. These new results confirm the general trend of the Apparent Polar Wander Path (APWP) of the Central-East-Iran microplate (CEIM) from the Triassic through the Tertiary as published by Soffel and Förster (1983, 1984). Two new palaeopoles for the Triassic of the CEIM have been obtained; limestones and tuffs from the Nakhlak region yield a mean direction of 094.0°/25.0°, N=12, k=4.1,α ₉₅=24.7°, after bedding correction, corresponding to a palaeopole position of 310.8°E; 3.9°S, and volcanic rocks from the Sirjan regions yield a mean direction of 114.5°/35.1°, N=44, k=45.9,α ₉₅=3.2° after bedding correction and a palaeopole position of 295.8°E; 10.3°N. Combining these with the two previously published results yields a new palaeopole position of 317.5°E; 12.7°N, for the Triassic of the CEIM, thus confirming that large counterclockwise rotations of the CEIM have occurred since the Triassic time. New results have also been obtained from Cretaceous limestones from the Saghand region of the CEIM. The mean direction of 340.7°/26.3°, N=33, k=44.3,α ₉₅=3.8°, and the corresponding palaeopole position of 283.1°E; 64.4°N, is in agreement with previously determined Cretaceous palaeopole positions of the CEIM. Furthermore, results have also been obtained from Triassic dolomite, limestone, sandstone and siltstone from the Natanz region, which is located to the west of the CEIM. A total of 161 specimens from 44 cores taken at five sites gave a mean direction of the five sites at 033.3°/25.1°, N=5, k=69.0,α ₉₅=9.3° and a palaeopole position of 167.2°E; 53.7°N. They pass the positive fold test of McElhinny (1964) on the level of 99% confidence. This pole position is in fairly good agreement with the mean Triassic pole position of the Turan Plate (149°E; 49°N). It indicates that the area of Natanz has not undergone the large counterclockwise rotation relative to the Turan plate since the Triassic, which has been shown for the CEIM. A Triassic palaeogeographic reconstruction of Iran, Arabia (Gondwana) and the Turan Plate (Eurasia) is also presented.